CCIE Routing & Switching Advanced Technologies Class - OSPF Network Type Broadcast, OSPF DR/BDR Election, OSPF over NBMA, OSPF Network Type Non-Broadcast and Point-to-Multipoint

Transcript

1	Introduction	0h 43m
2	Using the Cisco Documentation	0h 52m
3	Ethernet Overview, Layer 2 Switchports, Trunking, ISL, 802.1q, DTP	0h 47m
4	VTP, VTP Authentication, VTP Pruning	1h 02m
5	VTP Prune Eligible List, VTP Transparent, VTP Troubleshooting, Trunk Allowed List, Extended VLANs	0h 48m
6	VLAN and VTP Review	0h 19m
7	SVIs, Native Routed Interfaces, Router-on-a-Stick	0h 46m
8	Layer 2 EtherChannel, EtherChannel Load Balancing, Layer 3 EtherChannel	0h 57m
9	802.1q Tunneling, Layer 2 Protocol Tunneling, EtherChannel over 802.1q Tunneling	1h 03m
10	STP Root Bridge Election, STP Root Port Election, STP Designated Port Election, STP Priority, STP Cost, STP Port-Priority	1h 46m
11	STP Review	0h 12m
12	STP Timers, STP PortFast	0h 34m
13	STP UplinkFast	0h 11m
14	STP BackboneFast	0h 12m
15	STP BPDU Filter	0h 13m
16	STP BPDU Guard	0h 11m
17	STP Root Guard	0h 11m
18	STP Loop Guard, Unidirectional Link Detection (UDLD)	0h 11m
19	Multiple Spanning-Tree Protocol (MST)	0h 40m
20	Rapid Spanning-Tree Protocol (RSTP), Rapid-PVST	0h 18m
21	MST with Multiple Regions	0h 33m
22	Flex Links	0h 16m
23	Frame Relay	0h 39m
24	Frame Relay Configuration Part 1	0h 34m
25	Frame Relay Configuration Part 2	0h 41m
26	Frame Relay Switching	0h 27m
27	Back-to-Back Frame Relay	0h 08m
28	Frame-Relay End-to-End Keepalives	0h 18m
29	PPP, PPP PAP Authentication, PPP CHAP Authentication	0h 59m
30	PPPoFR, PPPoE	0h 40m
31	Transparent Bridging, IRB	0h 32m
32	Fallback Bridging	0h 14m
33	IP Routing Overview, Switching Paths, Static Routing	0h 42m
34	Static Routing Examples	0h 25m
35	IP Default-Gateway, IP Default-Network	0h 14m
36	On-Demand Routing (ODR)	0h 07m
37	Floating Static Routes	0h 10m
38	Backup Interface	0h 29m
39	Enhanced Object Tracking, IP SLA, Reliable Static Routing	0h 21m
40	Policy Routing	0h 43m
41	GRE Tunneling, GRE Recursive Routing Errors	0h 27m
42	Reliable Backup Interface with GRE	0h 25m
43	RIP Overview, RIP Versions, RIP Auto-Summary	0h 48m
44	RIP Split-Horizon, RIP Timers	0h 22m
45	RIP Broadcast Updates, IP Directed Broadcast, IP Broadcast-Address, Smurf Attacks, Fraggle Attacks	0h 16m
46	RIP Unicast Updates	0h 03m
47	RIP Offset-List	0h 18m
48	RIP Authentication	0h 11m
49	RIP Summarization	0h 10m
50	Prefix-Lists, RIP Distribute-List Filtering, RIP Administrative Distance Filtering	0h 48m
51	RIP Default Routing, RIP Conditional Default Routing	0h 18m
52	RIP Triggered, RIP Validate Update Source,	0h 12m
53	EIGRP Overview	0h 29m
54	EIGRP Auto-Summary	0h 07m
55	EIGRP Split-Horizon	0h 11m
56	EIGRP Update Types, EIGRP Neighbor, EIGRP Passive-Interface	0h 24m
57	EIGRP Hello-Interval, EIGRP Hold-Time	0h 05m
58	EIGRP Authentication	0h 11m
59	EIGRP Time Based Authentication	0h 25m
60	EIGRP Path Selection, EIGRP Metric Weights, EIGRP Traffic Engineering	0h 47m
61	EIGRP Unequal Cost Load Balancing, EIGRP Variance	0h 19m
62	EIGRP QUERY, EIGRP Summarization, EIGRP Leak-Map	0h 28m
63	EIGRP Stub Router Advertisement	0h 22m
64	EIGRP Default Routing	0h 13m
65	EIGRP Route Filtering	0h 11m
66	EIGRP Router-ID	0h 07m
67	Miscellaneous EIGRP Features	0h 02m
68	OSPF Overview	0h 27m
69	Establishing OSPF Adjacencies, Understanding the OSPF Database	0h 43m
70	OSPF Network Type Broadcast, OSPF DR/BDR Election, OSPF over NBMA, OSPF Network Type Non-Broadcast and Point-to-Multipoint	0h 56m
71	OSPF Network Type Point-to-Point, OSPF Network Type Mismatch	0h 13m
72	OSPF Network Type Point-to-Multipoint Non-Broadcast, OSPF Per Neighbor Cost	0h 17m
73	OSPF Network Type Loopback	0h 02m
74	OSPF Path Selection	0h 27m
75	OSPF Convergence Timers	0h 09m
76	OSPF Authentication	0h 12m
77	OSPF Summarization	0h 32m
78	OSPF Stub Areas, OSPF Totally Stubby Areas, OSPF NSSAs, OSPF Totally NSSAs	0h 40m
79	Controlling OSPF NSSA Redistribution	0h 02m
80	OSPF Type 7 to 5 Translator Election, OSPF LSA Type 3 Filter, OSPF Forwarding Address Suppression	0h 24m
81	Route Redistribution Overview	0h 36m
82	Route Redistribution Configuration & Verification, Connected Redistribution	0h 22m
83	OSPF External Path Selection, TCL PING Scripting	0h 33m
84	Routing Loops Overview, EIGRP Route Loop Prevention	0h 55m
85	Metric Based Routing Loops, Route Tagging	0h 46m
86	Administrative Distance Based Routing Loops, Debug IP Routing, IP Route Profile	0h 58m
87	BGP Overview, BGP Peering Types	0h 51m
88	Establishing BGP Peerings, EBGP Multihop, BGP Neighbor Disable Connected Check, BGP TTL Security	0h 49m
89	BGP 4-Byte ASNs	0h 20m
90	BGP Local AS, BGP Peer Groups	0h 30m
91	BGP Next-Hop Self, BGP Next-Hop Processing	0h 30m
92	BGP Route Reflectors	0h 37m
93	Large Scale BGP Route Reflectors with Clusters	0h 34m
94	BGP Confederation	0h 37m
95	BGP NLRI Advertisements, BGP Network Statement, BGP Redistribution	0h 43m
96	BGP Aggregation, BGP Aggregation & Summary-Only, BGP Aggregation & Unsuppress-Map	0h 46m
97	BGP Origin, BGP Aggregation & AS-Set, BGP Aggregation & Advertise-Map	0h 22m
98	BGP Conditional Route Injection	0h 21m
99	BGP Bestpath Selection, BGP Multipath, BGP Weight	0h 53m
100	BGP Local Preference, BGP Local Preference and Communities, BGP AS-Path Prepending,	0h 30m
101	BGP MED, BGP MED Missing As Worst, BGP Deterministic MED	0h 34m
102	BGP Communites, BGP Regular Expressions	0h 43m
103	BGP Filtering, BGP Convergence Timers, BGP Default Routing, Miscellaneous BGP Features	0h 24m
104	MPLS Overview, MPLS Label Distribution Protocol (LDP)	1h 05m
105	MPLS Tunnels, MPLS Penultimate Hop Popping (PHP)	0h 40m
106	MPLS Layer 3 VPNs Overview, VRF Lite	0h 36m
107	MPLS Layer 3 VPNs and VPNv4 BGP	1h 08m
108	MPLS Layer 3 VPN Verification & Troubleshooting	0h 49m
109	MPLS Layer 3 VPN OSPF PE-CE Routing Overview	0h 06m
110	MPLS Layer 3 VPN OSPF PE-CE Routing Path Selection & Sham-Links	2h 01m
111	MPLS Layer 3 VPN OSPF PE-CE Routing Loop Prevention & Down-Bit, OSPF Capability VRF-Lite	0h 40m
112	IPv6 Overview, IPv6 ICMPv6 ND, IPv6 over NBMA	0h 44m
113	IPv6 Routing Overview, IPv6 Static Routing	0h 28m
114	IPv6 RIPng	0h 20m
115	IPv6 EIGRPv6	0h 09m
116	IPv6 OSPFv3	0h 31m
117	MP-BGP for IPv6	0h 26m
118	IPv6 Tunneling	1h 13m
119	Multicast Overview, IGMP	0h 34m
120	PIM Overview	0h 18m
121	PIM Dense Mode	1h 03m
122	PIM Sparse Mode	0h 30m
123	PIM Sparse Mode Configuration	0h 36m
124	PIM Sparse Mode RPF & RP Troubleshooting	0h 27m
125	Auto-RP, Multicast over GRE Tunnels	0h 53m
126	Bootstrap Router (BSR), Bidirectional PIM, Source Specific Multicast (SSM)	0h 40m
127	MSDP, Anycast RP	0h 27m
128	IPv6 Multicast Routing	0h 24m
129	QoS Overview, IntServ QoS, DiffServ QoS, IP Precedence, DSCP, MQC, HQF	0h 41m
130	MQC Classification	0h 44m
131	FIFO, FQ, WFQ, CBWFQ, HQF	0h 50m
132	LLQ	0h 23m
133	WRED	0h 31m
134	Traffic Shaping, Traffic Policing	0h 40m
135	Frame Relay Traffic Shaping (FRTS)	0h 33m
136	RSVP	0h 07m
137	Catalyst 3560 QoS	1h 00m
138	IOS Security Overview, Access Lists (ACLs)	0h 58m
139	Time Based ACLs, Dynamic ACLs	0h 33m
140	Reflexive ACLs	0h 21m
141	TCP Intercept, Content Based Access Control (CBAC)	0h 34m
142	Zone Based Policy Firewall (ZBPF)	1h 09m
143	AAA, Local Authentication, Local Authorization, Role Based CLI Access	0h 43m
144	Port Security, Static CAM Entries, Storm Control, 802.1X Authentication, PACLs, RACLs, VLAN Access Maps (VACLs)	0h 48m
145	DHCP Snooping, Dynamic ARP Inspection (DAI), IP Source Guard	0h 12m
146	Protected Ports, Private VLANs	0h 25m
147	IP Services Overview, HSRP	1h 10m
148	VRRP, GLBP	0h 28m
149	NAT	0h 42m
150	DHCP, DNS	0h 30m
151	NetFlow	0h 09m
152	WCCP, Miscellaneous IP Services	0h 06m
153	System Management Overview, NTP, NTP Authentication	0h 24m
154	SNMP, RMON	0h 38m
155	Syslog, Telnet, SSH, Banners	0h 21m
156	EEM Scripting, Miscellaneous System Management	0h 26m
Total Duration		81h 59m

Slides - Diagrams

Slides - Introduction

Slides - Ethernet Switching

Slides - Advanced Ethernet Switching

Slides - Frame Relay

Slides - PPP

Slides - Transport Bridging

Slides - Redistribution

Slides - IOS Security

Slides - Layer 2 Security

Slides - IP Services

Slides - System Management

0:00:13	In our next section here, we're gonna look at how OSPF behaves differently
0:00:17	depending the different media that it is running on.
0:00:20	So, we'll see that there are different behaviors for OSPF
0:00:24	whether we're configuring over an Ethernet interface versus a point-to-point link running PPP or HTLC.
0:00:32	versus frame-relay multipoint or point-to-point interfaces.
0:00:37	Now, we'll see the different behaviors of OSPF with the different network types
0:00:43	will control how updates are sent whether they are multicast or unicast.
0:00:48	Who exactly is going to form an adjacency?
0:00:51	And how the next hop value is gonna be calculated when updates are sent between the neighbors?
0:00:56	Now, the different network types that we have, we can break them down at 6 different categories
0:01:02	which are broadcast, non-broadcast, point-to-point, point-to-multipoint,
0:01:07	point-to-point, point-to-multipoint non-broadcast and loopback.
0:01:10	The first of them, the network-type broadcast
0:01:14	will be default in any type of multi-access broadcast media
0:01:18	like Ethernet, Token Ring or FDDI.
0:01:21	So, in our case, mainly this is gonna be for Fast Ethernet, gig Ethernet or 10 gig Ethernet links.
0:01:28	We'll see that the hellos and updates over these links
0:01:32	will use two different multicast addresses.
0:01:35	Going to 224.0.0.5 and 224.0.0.6
0:01:40	Again, both of these multicast addresses are going to use the OSPF transport protocol,
0:01:46	which is IP Protocol number 89.
0:01:49	So, like as we saw with EIGRP before, this means that within the scope of OSPF,
0:01:55	we need to make sure that no one is filtering out protocol number 89.
0:01:59	Otherwise, we would not be able to form an adjacency
0:02:02	and not actually exchange the updates between the neighbors.
0:02:06	Now, for the broadcast network-type, we do use the designated router and back-up designated router election.
0:02:13	Which is gonan be used for two things.
0:02:15	The first is to cutdown on the number of adjacencies and the LSA flooding.
0:02:22	the second is for that extra recursive step in the database
0:02:26	when we're trying to figure out how the graph of the network looks
0:02:29	inside an individual area.
0:02:32	For interfaces that are not Ethernet, we can configure this network-type manually
0:02:38	if we say IP OSPF Network Broadcast.
0:02:41	But typically, there's not many design cases where you would need to change the network to be broadcast manually.
0:02:46	Now, for the designated router election that we're using over broadcast,
0:02:51	again, this is gonna be used to minimize the adjacencies and the LSA replication.
0:02:56	The reason that this works this way, is that on the broadcast segment,
0:03:01	everyone forms an adjacency with the designated router and the back-up designated router.
0:03:07	The DR is the device that is in-charged of
0:03:11	receiving the link updates from the neighbors on the segment.
0:03:15	And then forwarding them back out to the other neighbors.
0:03:18	This is why we have the difference between those two different multicast addresses.
0:03:22	The 224.0.0.5 and 224.0.0.6
0:03:27	So, in the case of routers 1, 4, and 6 that are on this multi-access Ethernet network,
0:03:36	we saw previously that router 6 was elected the designated router.
0:03:42	This means that if router 1 wants to send an update,
0:03:46	let's say about the point-to-point link that goes to router 3,
0:03:50	router 1 will send the LSU, the Link State Update Packet to the designated router
0:03:57	using the multicast address 224.0.0.6
0:04:02	So, this is going to the designated router and the back-up designated router.
0:04:08	Now, the BDR techincally doesn't do anything with this update.
0:04:11	It simply listens for the updates coming in and it listens for the hellos coming from the designated router.
0:04:20	Once the DR receives the link state update in,
0:04:24	it generates its own update that sis going back to 224.0.0...
0:04:32	224.0.0.5,
0:04:35	which is for all of the other routers on the segment.
0:04:38	So, the DR and the BDR, they're listening for 224.0.0.6,
0:04:42	where everyone else is listening for 224.0.0.5.
0:04:47	So, this means that for devices that neither the DR nor the BDR,
0:04:53	we'll see that they do not actually establish adjacency.
0:04:57	So, the drothers will stop at the two-way state between each other.
0:05:03	So, if there are four or more devices on the broadcast network-type,
0:05:08	we would see everyone forms an adjacency with the DR and the BDR.
0:05:12	between the other routers they're gonna be stuck in the two-way state, which is normal.
0:05:18	Now, as I mentioned, the backup designated router,
0:05:19	it doesn't actually do the LSA replication.
0:05:22	It's just used for redundancy of the designated router.
0:05:26	So, if it hears that the DR is no longer reachable,
0:05:29	it promotes itself to the DR state,
0:05:31	then, there is a new election for the backup designated router.
0:05:38	The actual election is gonna be based on two fields in the hello packet.
0:05:43	The router's priority, and the router ID.
0:05:47	The priority is gonna be a range or 0 to 255, where higher is better.
0:05:52	And zero means that they will not participate in the election.
0:05:57	So, if we wanna configure a device,
0:05:58	and it would not be the DR, and not be the BDR,
0:06:02	we would set the IP OSPF priority to be zero at the link level.
0:06:07	If there is a tie in the priority, then,
0:06:10	we look at whoever has the highest router ID.
0:06:14	And again, the router ID is gonna come from the highest loopback interface on the device.
0:06:19	Or if there's no loopback,
0:06:20	it's gonna come from the highest interface address.
0:06:24	We can statically set this with the router ID command on the process.
0:06:27	And it is a good best practice
0:06:30	to get into the habit of doing this.
0:06:33	So, as I mentioned previously for EIGRP,
0:06:36	OSPF, BGP, and MPLS, and Label Distribution Protocol,
0:06:42	it's always a good idea to statically set the router ID.
0:06:47	So, if we know exactly what these assignments are,
0:06:49	we should never run into a case where there are duplicate addresses,
0:06:53	which is gonna cause some failures in the topology calculation.
0:06:57	Now, we'll see for the election in the DR,
0:07:00	there is no preemption unlike IS-IS,
0:07:04	the designated Intermediate System election,
0:07:07	but sometimes, we'll se the election can be unpredictable
0:07:11	based on the order that the OSPF process loads.
0:07:17	So, although technically, the higher priority value
0:07:20	and the higher router ID are gonna be chosen by default,
0:07:22	we can see that the router can actually elect itself the DR
0:07:27	without other devices being on the network.
0:07:31	So, let's take a look at this between routers 1, 4, and 6.
0:07:38	On the command line, let's look at router 1.
0:07:42	And we'll look at the Show IP OSPF Neighbors.
0:07:47	Right now, it says that router 6 is the designated router,
0:07:50	and router 4 is the backup designated router.
0:07:54	This is because all three of the devices had the same priority.
0:07:58	They're all configured with the default priority of 1.
0:08:01	And router 6 is the highest router ID of all three of these.
0:08:04	It's 150.42.6.6 versus 150.42.4.4 and .1.1.
0:08:12	Now, if we look at the Debug IP OSPF Adjacency,
0:08:17	and we were to clear the OSPF process,
0:08:22	we'll see in the detailed output,
0:08:25	for the Ethernet link, we'll see the actual election occurring.
0:08:29	So, if we go back to the detail of this,
0:08:32	It says that...
0:08:35	we are performing a DR and a BDR election on the Ethernet.
0:08:39	Okay, right now, we don't know who the DR is.
0:08:43	Previously, it was router 6.
0:08:47	So now, we're gonna listen with the wait timer
0:08:53	to figure out, is there someone already in the segment who has been elected?
0:08:59	So, when a new router joins in the segment
0:09:01	and it first initializes its OSPF process,
0:09:04	it's gonna go through this wait timer
0:09:06	to make sure it does not accidentally elect itself versus someone else.
0:09:12	If the wait timer expires,
0:09:13	and no one has seen the DR or BDR be elected,
0:09:19	then, the router is gonna automatically promote itself in the DR state.
0:09:24	Now, the potential issue that we could run into with this
0:09:27	is that the order that the process loads
0:09:31	will then be related to who is going to be elected the DR or the BDR.
0:09:38	So, even though router 6 has the highest address,
0:09:41	we can run into a case that if router 1's process loads before router 6's,
0:09:47	then, router 1 would automatically be elected the designated router.
0:09:53	Another case would be that if for some reason,
0:09:55	there's a lost of Layer 2 connectivity,
0:09:58	and router 1 promotes itself to the DR state,
0:10:03	technically, there can be preemption in the DR election.
0:10:09	So first, let's look at the case where we go to router 6,
0:10:13	and we clear the OSPF process.
0:10:18	Once router 6 declared itself down,
0:10:21	we should see that router 4, who was the BDR
0:10:25	promotes itself to the DR state.
0:10:28	Now, if we look at the Show IP OSPF Neighbors,
0:10:32	we see that...
0:10:34	router 6 is in the two-way state with router 1 and 4.
0:10:41	Once they go into the full state,
0:10:46	we should see which of them was elected the designated router
0:10:49	versus the backup designated router.
0:11:00	So now, we can see, router 4 was promoted from the BDR state to the DR.
0:11:06	Then, router 1 and router 6 had a new election for the backup designated router.
0:11:13	since router 6 has a higher router ID than router 1,
0:11:17	router 6 is winning.
0:11:20	But we could see that depending on who loads the process first.
0:11:24	So, if router 6's process loads after 4 or 1's,
0:11:28	then, router 6 is never gonna be elected as the designated router.
0:11:31	This would be true even if router 6 had a higher priority value.
0:11:37	So, the election behind the scenes,
0:11:38	it actually is very sensitive to the order of operations.
0:11:42	Now, one way we could see this
0:11:44	would be to go to the link that router 1 has connected to the Layer 2 switches.
0:11:50	And if we Show CDP Neighbors on router 1,
0:11:55	we see that this link is switch 1's interface Fast Ethernet 0/1.
0:12:03	So on switch 1's Fast Ethernet 0/1,
0:12:09	right now, this is configured in VLAN 146,
0:12:12	which is the segment between...
0:12:14	between those three neighbors.
0:12:16	Okay, also, the interface is running in port fast mode.
0:12:19	So, we don't have to wait for the forwarding delay.
0:12:22	Now, let's say that on this link, we misconfigured the VLAN.
0:12:27	So, we'll say Switch Port Access VLAN 145.
0:12:31	Okay, it's really supposed to be 146, but we typed the wrong number in there.
0:12:36	Now, we'll see, once router 1...
0:12:39	looses the adjacencies with 4 and 6,
0:12:44	if we Show IP OSPF Neighbors,
0:12:46	this is gonna be based on the dead timer.
0:12:49	So, on about 20 seconds from now,
0:12:52	router 1 is not gonna hear the hellos from 4 and 6 anymore.
0:12:56	It would assume that those neighbors are down.
0:12:59	So, it needs to run a new election process
0:13:03	to figure out who is still left on the link.
0:13:06	If we Debug IP OSPF Adjacency,
0:13:10	we would actually see that router 1 is going to elect itself.
0:13:16	So, if I Clear IP OSPF Process again,
0:13:32	we'll see that once the wait timer expires,
0:13:38	which is related to the OSPF dead interval,
0:13:41	then, router 1 is gonna promote itself to the DR state.
0:13:46	So, if we Show IP OSPF Interface Fast Ethernet 0/0,
0:13:51	it says, "Right now, the state is waiting."
0:13:55	The wait time before the DR election is 5 or more seconds.
0:13:59	So, we see now, router 1 did not hear about a DR or a BDR.
0:14:04	So it elects itself as the DR.
0:14:08	Now, at this point, if we were to add router 1 back to the segment,
0:14:13	we would see that there is not a new election for the designated router.
0:14:18	Router 1 is actually gonna preempt the other routers.
0:14:22	So, on 4 and 6, if we look at the Debug IP OSPF Adjacency,
0:14:29	and the same thing on router 6, Debug IP OSPF Adjacency.
0:14:34	Then, on the switch, I'm gonna out this port back into the correct VLAN.
0:14:40	If we look at router 4 and 6,
0:14:52	then, back at router 1 here. Let's see, router 1 now says...
0:15:00	that 4 is the DR and 6 is the BDR.
0:15:05	So, if we Show IP OSPF Neighbors.
0:15:08	Okay, right now, router 1 is then demoted from the DR state to the drother.
0:15:13	But ultimately, the final result of this we'll see
0:15:15	is based on the order of operations.
0:15:18	So, although technically, it's specified that there is not preemption
0:15:22	in the designated router election,
0:15:25	technically there is, and it depends on the order that the process loads.
0:15:31	So, if the routers have previously elected themselves as the DR
0:15:34	before they join the segment,
0:15:36	if their hello is sent out before the other routers,
0:15:42	then, whoever announces themselves as the DR first
0:15:45	is gonna be the one that takes that state.
0:15:48	So, if you're trying to change the election,
0:15:51	really, the only way to be a 100% sure about it is to set the routers to be zero
0:15:56	that you do not want the election to occur.
0:16:01	Now, on an Ethernet segment, it's really not gonna matter who is the DR.
0:16:05	Because assuming that there's full Layer 2 connectivity everywhere,
0:16:09	it really doesn't matter if 6 is doing the replication versus 4 or 1.
0:16:12	As long as the LSAs get there, it's gonna be fine.
0:16:17	Now, where this will become a problem
0:16:19	is for any Layer 2 topology that the devices do not have
0:16:24	full Layer 2 connectivity to each other.
0:16:28	Which in this particular case is going to be the frame-relay segment
0:16:32	between routers 1, 2, 3, 4, and 5.
0:16:36	If we were to use either the network type broadcast or non-broadcast,
0:16:43	which means that we do use the designated router.
0:16:47	And if one device is not...
0:16:50	having full Layer 2 connectivity
0:16:53	to the rest of the network,
0:16:55	which in this case would be one of the spokes.
0:16:58	If router 4, or router 3 for example was elected the designated router,
0:17:02	we would end up in a case where LSA replication can only happen between 3 and 5,
0:17:09	but not between 3 and 1, or 3 and 4, or 3 and 2.
0:17:16	Because the OSPF unicasts and the OSPF multicasts
0:17:20	are link local packets.
0:17:23	Where link local means that they should not be routed between interfaces,
0:17:27	and they have a Time to Live of 1.
0:17:31	We'll see that the end result of this is that if router 3 elected the DR,
0:17:35	when it sends a link state update out towards router 5,
0:17:41	5 is not able to pass this back down to the other spokes.
0:17:47	So, we can end up in some strange result of the topology,
0:17:50	where some portions of the network have reachability to each other
0:17:53	and other portions do not.
0:17:57	So, for the Ethernet segments again,
0:17:59	it really doesn't matter who wins the election.
0:18:01	But in any case where there is only a
0:18:03	partial mesh of Layer 2 connectivity,
0:18:06	we need to guarantee that the device
0:18:08	that has the full Layer 2 connectivity
0:18:11	is elected the DR,
0:18:14	which in this case, it's router 5.
0:18:16	So, router 5 must be the DR.
0:18:19	Otherwise replication down to the spokes is not gonna be complete.
0:18:25	Now, if there was some sort of other more complex design here,
0:18:28	maybe we have...
0:18:28	multiple hubs for the network,
0:18:31	where we have router 1 and router 2
0:18:35	that connects to spoke 1, spoke 2, and spoke 3.
0:18:42	Where we have a full mesh of circuits
0:18:44	from router 1 to spoke 1, spoke 2, spoke 3.
0:18:48	And then again, from router 2 down to them.
0:18:51	Between router 1 and 2,
0:18:54	one of them can be the DR, one of the can be the BDR.
0:18:58	Or in any combination of this.
0:18:59	Router 2 could be the DR, router could be the BDR.
0:19:02	As long as any of the spokes are no elected either the designated router
0:19:07	or the backup designated router,
0:19:09	then, the topology is gonna work fine.
0:19:13	Now, we'll also see that there are some Layer 2 designs
0:19:17	that the broadcast and non-broadcast network types simply will not work across.
0:19:24	So, if we have a case where router 1...
0:19:27	has a frame-relay PVC that goes to router 2,
0:19:32	and router 2 has a PVC that goes to router 3,
0:19:35	and 3 has a PVC that goes to router 4,
0:19:41	in this type of design, there is no one on the network
0:19:45	that has full Layer 2 connectivity everywhere.
0:19:48	So, regardless who was elected the DR or the BDR,
0:19:52	the full LSA replication is not going to occur.
0:19:57	So, we'll see, there are some workarounds with this
0:19:59	using the network type point-to-multipoint
0:20:02	that eliminates the need for the designated router
0:20:05	over these partial meshed non-broadcast designs.
0:20:10	But again, in our case where this is gonna matter is
0:20:12	when it's running over the frame-relay partial meshed networks.
0:20:16	Whoever is the hub of the network,
0:20:19	if we're running network type broadcast or non-broadcast,
0:20:23	that device will have to be the designated router.
0:20:26	Now, the main difference between the broadcast
0:20:28	and the non-broadcast network types
0:20:30	is that broadcast is sending its updates as multicasts
0:20:34	going to 224.0.0.5 and 224.0.0.6.
0:20:38	Where the non-broadcast OSPF network type is sending them as unicasts.
0:20:45	We'll see that this is the default
0:20:46	for multipoint non-broadcast interfaces
0:20:50	like the main interface in frame-relay,
0:20:52	or a multipoint interface in frame-relay,
0:20:54	main interface in ATM, multipoint sub-interface in ATM.
0:20:57	Those are gonna be network type non-broadcast by default.
0:21:03	This means that we need to ensure
0:21:05	that whoever has full Layer 2 connectivity is the designated router
0:21:10	and under their OSPF process,
0:21:12	tell them who I actually want to send the unicast updates to.
0:21:17	This is the reason why in our topology here
0:21:21	that with the default options,
0:21:23	we look at Show IP OSPF Neighbors.
0:21:26	We see that router 1 does not have any adjacencies over serial 0/0.
0:21:33	When we look at the Show IP OSPF Interface Serial 0/0,
0:21:38	this link is running network type non-broadcast,
0:21:43	but we have zero neighbors on the interface.
0:21:45	Now, for the non-broadcast configuration,
0:21:49	we're gonna have to do two things:
0:21:51	specify who is the designated router,
0:21:55	which in this case, we needed to be router 5.
0:21:58	And specify who are the neighbors that we need to send the updates to.
0:22:01	So first, to influence the designated router election,
0:22:05	we basically need to tell the spokes of the network
0:22:07	not to become the DR and not to become the BDR.
0:22:11	Again, the way that we can do this is by setting the priority to be zero.
0:22:16	So, the priority is zero, they do not participate in the election.
0:22:20	For any other value,
0:22:22	if we leave it as the default of 1,
0:22:24	or we change it to any other number, could be 255 as the highest.
0:22:28	Ultimately, who is elected the DR depends on who's process loads first.
0:22:35	So, if one of the spokes of the network, let's say router 3
0:22:39	is booting up faster than router 5 is,
0:22:43	3 could elect itself as the DR,
0:22:45	which would then mean, the replication cannot go to router 1,
0:22:49	to router 4, or to router 2.
0:22:54	So, there's very limited options for this type of design.
0:22:57	Either we specify that router 5 must be the DR,
0:23:00	or we use some sort of network type
0:23:01	that doesn't need the designated router election.
0:23:05	Now, implementation wise,
0:23:07	our first step would be to go to the interfaces
0:23:10	that the spokes are running frame-relay,
0:23:14	and set the IP OSPF priority to be zero.
0:23:17	So, this is gonna ensure that they are never
0:23:21	elected either the DR nor the BDR.
0:23:26	So on router 1,
0:23:29	on router 2,
0:23:34	on router 3,
0:23:39	and on router 4.
0:23:44	So, their priorities are now set as zero.
0:23:47	On router 5, if we Show IP OSPF Interface Serial 0/0/0,
0:23:54	likewise, this network type is non-broadcast.
0:23:57	It's priority is 1.
0:23:59	We could see that its state is the DR.
0:24:03	Or if we were to compare this to router 1's output previously,
0:24:07	router 1 likewise thought it was the DR.
0:24:11	So, it means that if router 1 were to load first,
0:24:13	and send the hellos to router 5 before 5 generated its own,
0:24:18	then, router 1 would take the DR state.
0:24:21	Okay, that's not what we want there.
0:24:23	We need to make sure that all the spokes are drothers,
0:24:26	they're neither the DR nor the BDR.
0:24:28	So, once we have the priority set to zero on the spokes,
0:24:31	then, on the hub,
0:24:33	who again is the DR,
0:24:35	under the OSPF process,
0:24:36	we're gonna specify who are the unicast neighbors.
0:24:41	So, we need to send updates to router 1,
0:24:46	to router 2, 3, and 4.
0:24:52	This neighbors command is only required on the designated router
0:24:56	and the backup designated router,
0:24:58	the devices on the other side, once they hear the unicast hellos come in,
0:25:02	they will automatically respond back to the neighbor that initiated them.
0:25:08	So, if we look at the Debug IP Packet Detail,
0:25:13	we'll see that on the LAN interfaces of router 5,
0:25:17	we are sending our hellos to the multicast address, which is the 224.0.0.5.
0:25:25	But on the frame-relay interface,
0:25:30	these are gonna go to the unicast address.
0:25:35	We could see, for both of these,
0:25:37	the IP protocol number is 89. So, they're both OSPF.
0:25:41	But the broadcast network type is using the 224.0.0.5 and 224.0.0.6,
0:25:47	where non-broadcast is gonna use the unicast.
0:25:51	So, with the unicast, we need to manually tell it...
0:25:54	who are the neighbors on the link.
0:25:57	Now, if we'd look at the Show IP OSPF Neighbors,
0:26:01	it says we have a full adjacency with all of the spokes, routers 1, 2, 3, and 4,
0:26:06	and they are not the DR and they are not the BDR.
0:26:09	So, they are the drothers.
0:26:13	On our other two links, the one that goes to router 4,
0:26:16	and the one that goes to switch 2,
0:26:18	the null field here means that they're not running network type broadcast,
0:26:23	and they're not running network type non-broadcast.
0:26:27	So, they have some sort of network type configured
0:26:29	that is not using the DR or BDR election.
0:26:33	Which would be point-to-point,
0:26:34	point-to-multipoint, or point-to-multipoint non-broadcast.
0:26:38	So now, let's look at the result of this. Let's go to one of the spokes.
0:26:41	Let's say, router 2.
0:26:43	And look at the Show IP Route OSPF.
0:26:50	We see that we have multiple routes
0:26:53	installed in the table.
0:26:54	Some of them are via the point-to-point link that goes over to router 3,
0:26:59	some of them are via the frame-relay interface that goes to router 5.
0:27:05	Now, to simplify the topology a little bit here,
0:27:08	I'm going to disable the point-to-point link that is between...
0:27:13	3 and 2, and the point-to-point link that's between 3 and 1.
0:27:18	So, for the spokes of the network to reach each other through...
0:27:22	OSPF, they're gonna have to route over the frame-relay network.
0:27:25	So next, on router 3, I'm gonna go to interface serial 1/2, and shut this down.
0:27:31	And interface serial 1/3.
0:27:34	Both of those are disabled.
0:27:37	If we look back on router 2 and look back the routing table now,
0:27:42	we should see that the only possible way we have to reach...
0:27:46	the remote networks is via the frame-relay interface.
0:27:55	So, for example for the...
0:27:58	VLAN 7 interface of switch 1,
0:28:01	it says, "This traffic needs to be routed towards router 3."
0:28:05	For the VLAN 8 interface of switch 2,
0:28:09	"This traffic needs to be routed towards router 5"
0:28:11	So, the key point to note about this output
0:28:15	is that when the designated router receives the link state update in from the drothers,
0:28:20	and it forwards it out to the other neighbors,
0:28:24	it is not modifying the next hop value.
0:28:29	Now, on Ethernet segments, this would be the ideal design.
0:28:33	So, if we look at the case between router 1, 4, and 6,
0:28:36	let's say that router 6 is the DR,
0:28:40	and router 4 sends an update to 6 about this LAN segment Fast Ethernet 0/0.
0:28:49	6 is the DR, which means it replicates the update back down to router 1.
0:28:55	If router 6 were to update the next hop value to point to its own interface,
0:29:02	it would mean that the traffic from router 1
0:29:05	trying to reach BB3,
0:29:08	would go from 1 to 6, 6 to 4, and 4 to BB3,
0:29:13	which would not be ideal in this case, because router 1 and 4,
0:29:16	they already have direct Layer 2 connectivity.
0:29:19	Ideally, we would want the actual data plane flow
0:29:22	to go from 1 directly to 4,
0:29:25	then, from 4 to BB3.
0:29:30	By default, the designated router will try to do this...
0:29:34	by inserting itself in the control plane
0:29:38	for the actual link state updates,
0:29:41	but not inserting itself into the data plane.
0:29:45	And the way it does, this is by not changing the next hop value
0:29:49	for the updates that are received on the segment.
0:29:53	So, when 4 sends the update to 6,
0:29:55	4 saying, "Use my interface as the next hop."
0:29:59	When this update is replicated down to router 1,
0:30:02	router 1 then knows we need to go to router 4 to reach that.
0:30:07	We can see this on router 1, if we look at the...
0:30:10	Show IP Route 204.12.42.0,
0:30:17	which is router 4's link to BB3.
0:30:21	So, even though the update is technically coming from router 6 who was the DR,
0:30:27	actually now, in this case, router 4 is the DR.
0:30:31	If we were to look at the other way around, let's say from the...
0:30:36	the serial interface of router 6.
0:30:40	If we look at Show IP Route 54,
0:30:47	it says, "The specific match is 54.42.1.0.
0:30:55	The next hop is router 6,
0:31:00	and router 6 is the person who is originating it."
0:31:04	So really, regardless of who the designated router is here,
0:31:07	whether it was router 4 or router 6,
0:31:09	the next hop value is the same value
0:31:12	as it was when it was originated onto the segment.
0:31:16	If we Show IP OSPF Neighbors,
0:31:19	router 4 is the DR.
0:31:22	So, we can tell it's not changing the next hop value that 6 is reporting.
0:31:25	So, the key point here being that over broadcast segments, this is the ideal design.
0:31:30	We don't want the traffic to have to go from 1 to 4,
0:31:33	to 4 to 6 to reach the serial link.
0:31:37	Now, over the non-broadcast networks,
0:31:41	this can be a problem,
0:31:43	because when router 2 is forwarding traffic to get to 3
0:31:48	we know that this traffic has no option but to physically go through 5.
0:31:55	So again, assuming that we disabled the link that's between...
0:31:59	2 and 3.
0:32:03	So then, if the only physical path for 2 to get to 3 is from router 5,
0:32:10	we could potentially have a failure in the routing lookup
0:32:15	when router 2 goes to send traffic towards
0:32:18	the next hop of 3 instead of the next hop of 5.
0:32:24	So, let's look at what the routing table on router 2 says.
0:32:28	Router 2 says that if I were to go to the loopback of router 3.
0:32:36	150.42.3.3.
0:32:40	It says, "This is reachable via serial 0/0.
0:32:44	The next hop is router 3.
0:32:46	The person that is advertising the route is the router who has this router ID.
0:32:53	If we were to compare this to something that is behind router 5,
0:32:57	let's say, the loopback of switch 4,
0:33:02	the next hop value of this is router 5.
0:33:06	So, the difference between the two advertisements is one of them is coming from...
0:33:11	3...
0:33:13	to 5, then from 5 to 2.
0:33:18	The other advertisement is coming from switch 4 to switch 2,
0:33:24	switch 2 to router 5, then, from 5 to 2.
0:33:30	The first update
0:33:32	is not having its next hop value changed.
0:33:35	So, router 2 sees this as .3.
0:33:40	The second is getting the next hop changed,
0:33:44	because the update did not come from someone on that segment.
0:33:47	It came from a different interface Fast Ethernet 0/0.
0:33:51	So, when the update goes out here,
0:33:52	the next hop value is changed to router 5.
0:33:55	This would now mean that it's to the Layer 2 frame-relay process
0:34:00	to figure out what is the Layer 2 circuit number
0:34:04	that would be needed to reach either the .3 or the .5 address.
0:34:11	If we look at the result of this on router 2,
0:34:14	and we try to send traffic towards 150.42.10.10,
0:34:21	we see, we're able to reach that destination.
0:34:24	But if we try to send traffic towards router 3,
0:34:29	which we do know how to reach them. Let's look at the...
0:34:33	Show Frame-Relay Map.
0:34:36	I have a bunch of dynamic mappings here.
0:34:39	Let's look at the configuration of frame-relay here.
0:34:43	The only thing I have is the IP address,
0:34:45	the encapsulation, and then the mapping to the...
0:34:48	mapping to the hub.
0:34:50	What I should have done here before was to...
0:34:54	say No Frame-Relay Inverse ARP.
0:34:59	Because if we look at the frame-relay mappings,
0:35:02	we can see there's basically a full mesh of the PVCs being used.
0:35:06	But based on the way the diagram looks,
0:35:08	we really should only have circuit 205.
0:35:11	So I need to remove all those other mappings.
0:35:14	So on router 1,
0:35:19	we'll say that on router 3,
0:35:26	and on router 4.
0:35:32	Then, if we Clear Frame-Relay In ARP,
0:35:36	this should delete those dynamic mappings.
0:35:53	Now, on router 2, let's look at the Show Frame-Relay Map.
0:35:57	We see now, we just have our single static mapping that's going to router 5,
0:36:01	which is what I want.
0:36:03	So again, now, if we try to send traffic to switch 4,
0:36:07	there's no problem with that.
0:36:09	But if we try to send it to router 3,
0:36:12	now, the Layer 2 frame-relay process
0:36:16	doesn't understand...
0:36:18	what circuit number we need to use to reach that particular next hop.
0:36:23	So, remember what we talked about, the difference between routing to...
0:36:26	a point-to-point interface versus a multipoint interface.
0:36:32	When we route towards a point-to-point interface
0:36:34	there's only one possible destination, which is the other end of the link.
0:36:39	So, on medias like PPP, or HTLC, or point-to-point GRE tunnel,
0:36:45	there's no Layer 2 addressing that's needed,
0:36:48	because there's only one possible device that could actually receive the packet.
0:36:53	But in the case of a multipoint network like Ethernet or multipoint frame-relay,
0:36:58	the Layer 2 process needs to know,
0:37:01	what is the MAC addressing for Ethernet, or what is the DLCI number for frame-relay
0:37:06	that actually needs to go on the packet when we forward it out the link.
0:37:10	In this case, router 2 is saying, "To get to this particular destination,
0:37:15	I need to go towards this next hop."
0:37:19	If we then do a recursive lookup on the next hop,
0:37:22	router 2 says, "I'm directly connected with this destination."
0:37:29	But based on the topology, we know that this is actually not the case.
0:37:32	Router 2 and router 3 are not directly connected over a Layer 2 circuit.
0:37:37	They actually have to route the traffic up to router 5 first.
0:37:43	So, the problem now becomes,
0:37:44	there's a disconnect between the Layer 3 OSPF process,
0:37:48	and the underlying frame-relay process.
0:37:52	If we were to look at the Debug Frame-Relay Packets,
0:37:57	or the Debug IP Packets,
0:37:59	when router 2 goes to send the packets towards the loopback of 3,
0:38:05	the frame-relay process doesn't now what to do with these.
0:38:09	We see, it says encapsulation is failed,
0:38:11	because there's no mapping for that particular destination.
0:38:19	Now, there's a couple different solutions to this design.
0:38:23	One way to fix this would be to simply tell the frame-relay process
0:38:28	that when I am trying to reach any of these next hop values,
0:38:32	whether they'd be router 4, router 1, router 3, or router 5,
0:38:39	I need to tell them,
0:38:40	"For any of these, I'm gonna use the same circuit number, which is 205."
0:38:45	The only problem with this configuration is that it's difficult to maintain.
0:38:50	If we have new devices that are added on to the network,
0:38:53	it means that we would have to keep adding frame-relay map statements over and over and over
0:38:57	on all of the spokes.
0:39:00	We would essentially need a full mesh of the mapping statements.
0:39:05	Another solution would be to change the underlying Layer 2 network.
0:39:10	So, if I were to go to all the spokes of the frame-relay network,
0:39:14	and change these to point-to-point sub-interfaces,
0:39:19	the point-to-point sub-interfaces would already know
0:39:22	what the circuit value is supposed to be,
0:39:25	because there's only one possible number.
0:39:29	This is why when we talked about this before in frame-relay,
0:39:33	that would be the preferred Layer 2 design
0:39:36	to always use point-to-point sub-interfaces,
0:39:39	where the Layer 3 network maps directly to the Layer 2 network.
0:39:46	So, we could just go to the spokes
0:39:47	and change them to point-to-point sub-interfaces, that's gonna be fine.
0:39:50	So then, regardless what the next hop value points to,
0:39:54	it's always gonna point it that particular Layer 2 circuit
0:39:58	that's assigned on the sub-interface.
0:40:02	So again, on router 2, I could just go to this link and say, Frame-Relay Map.
0:40:06	To get to router 5, I'm going out 205.
0:40:15	So, this is the mapping I already have there.
0:40:17	But then, for the other neighbors,
0:40:18	if I were to go to router 1, to router 3, or to router 4,
0:40:24	I would need to say Mapping Values.
0:40:28	Likewise on router 3,
0:40:30	I would need to say...
0:40:32	"To get to router 2,
0:40:36	I need to go to router 5.
0:40:38	To get to router 1, I need to go to 5.
0:40:40	To get to router 4, I need to go to 5."
0:40:44	At this point, when router 2 and 3 performed the route recursion
0:40:48	towards the final destination,
0:40:51	when the packet is passed on to the frame-relay process,
0:40:54	frame-relay now knows what is the Layer 2 address
0:40:58	that I need to use in the packet.
0:41:01	Because again, the final route...
0:41:04	says, "The next hop...
0:41:06	for 150.42.3.3...
0:41:10	is 155.42.0.3."
0:41:16	This is a directly connected neighbor,
0:41:19	so now, we switch the packet to the interface
0:41:21	and set it for encapsulation.
0:41:25	The Layer 2 encapsulation now needs to know
0:41:28	what is the circuit number for this destination.
0:41:32	So internally, the frame-relay process is gonna say, Show Frame-Relay Map,
0:41:37	and tell me what is associated with 155.42.0.3.
0:41:45	Which is this case is circuit number 205.
0:41:50	Which is fine because I manually did all the mappings.
0:41:54	So, I basically gave it every possible option.
0:41:57	If I had 10 more spokes in the network,
0:41:59	then I would have to add 10 more mappings.
0:42:05	Now, the issue with this is that within the scope of the lab exam,
0:42:09	if they say you can't use static mappings,
0:42:12	or you can't change the interface type to a sub-interface,
0:42:16	then, that's gonna limit the possible options that we have.
0:42:20	But the key point being here, when we look at the default behavior
0:42:23	of the network type broadcast or non-broadcast,
0:42:27	the designated router is not updating the next hop value
0:42:32	for any link state updates coming from the drothers.
0:42:38	So, in this particular case, router 3 is sending the update
0:42:42	about 150.42.3.3 to router 5.
0:42:49	5 turns around and passes it back to 2,
0:42:52	but it didn't change the next hop value.
0:42:55	So now, router 2 needs to do a Layer 2 lookup on 155.42.0.3.
0:43:03	Now, the other way that we could fix this
0:43:05	would be to tell the OSPF process
0:43:08	that there's really not a full mesh of Layer 2 connectivity.
0:43:13	And this is specifically what the network type point-to-multipoint is designed to do.
0:43:19	In point-to-multipoint,
0:43:20	instead of treating the interface as just one multipoint link,
0:43:25	we treat it as an underlying connection of point-to-point links.
0:43:30	Which in reality, that's what out topology looks like here.
0:43:33	Router 5 doesn't have one link that goes to the frame-relay network.
0:43:37	It has multiple logical point-to-point links.
0:43:40	One that's going out circuit 405,
0:43:42	one that's going out 501, one that's going out 503, one that's goes out 502.
0:43:48	They're separate Layer 2 connections to different physical portions of the topology.
0:43:54	So, with the network type point-to-multipoint,
0:43:57	based on the neighbors that we form adjacencies with,
0:44:01	router 5 will form an adjacency with 2,
0:44:05	5 and 3, 5 and 1, and 5 and 4,
0:44:10	but 1 and 4, and 1 and 3, and 3 and 2, they would not form adjacency.
0:44:17	Because there's no direct Layer 2 circuit between them.
0:44:21	So, this means that when 4 sends an update to 5,
0:44:26	and 5 replicated it to the other neighbors,
0:44:29	it's gonna tell them to use me to reach that destination.
0:44:34	If it so happens that router 4 and 1 do have a directly Layer 2 circuit,
0:44:41	then, it's not gonna matter, because 4 is gonna directly send this update to 1.
0:44:47	So, in point-to-multipoint,
0:44:50	you would always choose your closest Layer 2 neighbor on the circuit
0:44:55	for any routing information that is received.
0:44:58	But the key point in the operational distance we'll see...
0:45:03	is that the next hop values with be changed.
0:45:07	So, when router 5 receives an update from 3 and passes it down to 2,
0:45:11	the next hop value is gonna change to router 5's address, not stay as router 3's.
0:45:17	We'll also see that when this link is described in the database,
0:45:21	it's not gonna be listed as a transit network.
0:45:25	It is gonna be listed as a collection of point-to-point adjacencies.
0:45:30	So, if we were to look at router 2,
0:45:33	and look at the Show IP OSPF Database,
0:45:36	for my router LSA, which is 150.42.2.2,
0:45:44	router 2 says that "I am connected to a transit network,
0:45:50	where I'm adjacent with the designated router 155.42.0.5."
0:45:57	This means that from an SPF search algorithm point of view,
0:46:02	the other routers would then need to ask the designated router,
0:46:05	who are your other adjacencies on the link?
0:46:09	Based on this, they can figure out
0:46:10	what does the rest of the graph of the network look like?
0:46:14	However, in point-to-multipoint,
0:46:17	router 2 will understand that this is really not a transit network.
0:46:20	It's simply a point-to-point adjacency to 5.
0:46:24	If 5 has other adjacencies, that's out of my control.
0:46:27	All that I know is that I have a direct Layer 2 circuit to router 5.
0:46:31	So, for any traffic that points to that interface,
0:46:35	I'm gonna use router 5 as the next hop.
0:46:39	Now, we'll see that there's an additional behavior of this link
0:46:43	that is gonna automatically solve of Layer 2 frame-relay problem.
0:46:49	So right now, from router 2's perspective,
0:46:52	we would not be able to reach the loopback of router 1.
0:46:57	Because I didn't configure a frame-relay mapping between router 1 and router 2.
0:47:02	I did configure the mappings between 2 and 3.
0:47:06	So, 2 is able to send traffic to router 3's loopback.
0:47:12	So now, instead of adding the additional mappings,
0:47:15	we're changing the frame-relay interfaces to point-to-point.
0:47:18	I'm gonna change the frame-relay network
0:47:21	to IP OSPF network point-to-multipoint.
0:47:26	So, on all 5 of these neighbors,
0:47:29	they're all gonna need to agree on this.
0:47:33	On router 1's link, this is OSPF network point-to-multipoint.
0:47:39	On router 3,
0:47:44	on router 4,
0:47:49	and then lastly, on router 5.
0:47:58	Now, on router 5, if we look at the Show IP OSPF Neighbors,
0:48:03	We should see that eventually, we have adjacencies with 1, 2, 3, and 4.
0:48:12	Which they're now all in the full state.
0:48:13	And notice that none of our adjacencies now either specified that they are the DR or BDR.
0:48:20	So, it means that all of these interfaces,
0:48:23	they are not running network type broadcast,
0:48:25	and they are not running network type non-broadcast.
0:48:30	Just from this output, I don't necessarily know what they running,
0:48:33	but it's gonna be either point-to-point,
0:48:35	point-to-multipoint, or point-to-multipoint non-broadcast.
0:48:39	Because those three are not using the designated router election.
0:48:43	If we look at the difference in the database now,
0:48:47	if we Show IP OSPF Database,
0:48:50	and look at the router LSA that router 2 is generating,
0:48:55	previously, it said it had a transit network that was going to router 5.
0:49:05	In this case, it says now, it has a point-to-point adjacency
0:49:06	that goes exactly to this neighbor.
0:49:10	So, between router 2 and router 5, there are no other routers. It's a point-to-point link between them.
0:49:16	If we look at router 5's advertisement,
0:49:20	router 5 now says, "I have four point-to-point adjacencies."
0:49:26	One of them goes to router 1,
0:49:29	one of them to 2, one of them to 3, one of them to 4.
0:49:34	Each of these adjacencies have separate values associated with them.
0:49:39	So, router 5 knows that there's a difference from the Layer 2 point of view
0:49:44	when I route traffic to router 1 versus routing it to 2, 3, or 4.
0:49:51	Also, well see that the actual endpoint of the network
0:49:58	is advertised as a stub host.
0:50:02	This 155.42.0.5,
0:50:06	this is the actual address that router 5 has configured on the frame-relay interface.
0:50:12	But notice that the mask is now /32.
0:50:16	So, in point-to-multipoint, the actual underlying transit network,
0:50:20	which in this case is the frame-relay between...
0:50:23	these five routers.
0:50:25	It's no longer gonna be advertised as...
0:50:30	155.42.0.0/24.
0:50:35	It's now gonna be advertised as the end host of the topology.
0:50:41	So, router 4 is announcing itself as 155.42.0.4/32.
0:50:47	Router 1 says, "I'm 155.42.0.1/32."
0:50:53	Likewise for 2, 3, and 5.
0:50:59	This is the normal desired behavior for point-to-multipoint.
0:51:03	Because the link itself is not transit.
0:51:07	We know that there's no host that's actually in the frame-relay cloud that's 155.42.0.100.
0:51:15	So, there's no reason we should actually be advertising a /24,
0:51:19	because we know that the network really is just made up of 5 end points.
0:51:23	Routers 1, 2, 3, 4, and 5.
0:51:26	So, we'll advertise reachability exactly to them.
0:51:29	There's no other host that you can reach on that network.
0:51:32	Now, if we look at the result of this from router 2's perspective,
0:51:36	and look at the change in the routing table,
0:51:40	we'll see now that all routes that are learned from the frame-relay network
0:51:45	have had their next hop modified to be router 5.
0:51:52	Also, we see the endpoints of the network, router 5, router 4, router 3, and router 1
0:51:59	as being reachable via router 5.
0:52:02	So now, what do you think this is gonna change when we look at the Layer 2 frame-relay process?
0:52:09	When router 2 goes to send a packet, let's say to router 1's loopback.
0:52:14	How is this gonna be different now
0:52:17	versus when it was running the network type non-broadcast?
0:52:20	Previously, router 2 was saying,
0:52:23	"For me to get to...
0:52:27	150.42.1.1/32, so that's router 1's loopback.
0:52:36	It was reachable via 145.42.0.1",
0:52:42	which was the next hop address of router 1's frame-relay.
0:52:47	We then did an another recursive lookup on that next hop.
0:52:52	And we said that 155.42.0.1 is via connected on serial 0/0.
0:53:02	So it means we needed to know what is the Layer 2 address of the 0.1 network.
0:53:07	But now, in this case, the lookup is changed.
0:53:09	Router 2 is gonna say that this destination on router 1,
0:53:14	is actually reachable via router 5.
0:53:19	Then, when we do the lookup of router 5, it says "This is via...
0:53:23	connected on serial 0/0",
0:53:28	which means that the end result is that we only need to know the Layer 2 address of the hub.
0:53:36	In this case, we would no longer need the frame-relay mappings between the spokes,
0:53:40	because now, the Layer 3 routing table is fixing what was previously
0:53:45	a problem in a Layer 3 to Layer 2 resolution.
0:53:50	So, if we'd look at router 2,
0:53:53	and try to send traffic towards the loopback of router 1,
0:53:57	we see that we can reach this
0:54:00	without needing the frame-relay mapping to them.
0:54:03	Likewise if I were to send traffic directly to router 1's endpoint,
0:54:08	or...
0:54:10	155.
0:54:13	Router 1's endpoint, or router 4's endpoint.
0:54:16	Even though those neighbors do not have mappings back to router 2,
0:54:23	when we look at the routes to the endpoints,
0:54:27	router 4 says, "To get to router 2,
0:54:30	this is no longer directly connected."
0:54:33	Instead, this is reachable via router 5,
0:54:37	and we could either reach this by going out the point-to-point link to router 5,
0:54:41	or we could get there by going directly over the frame-relay.
0:54:51	So again, looking at the final result of 2's routing table,
0:54:55	it's gonna solve both the issue of needing, or not needing, I should say.
0:55:00	Not needing the frame-relay mapping between the spokes,
0:55:03	and also, we will be able to send traffic directly to the spoke's interfaces
0:55:08	by doing a Layer 3 lookup towards the hub.
0:55:14	So in this case, router 2,
0:55:17	we could go to the frame-relay interface,
0:55:20	and remove the mapping to 1,
0:55:24	the mapping to 3,
0:55:26	and the mapping to 4,
0:55:31	but it's not going to affect our connectivity to these endpoints.
0:55:36	Because instead of needing the Layer 2 frame-relay lookup,
0:55:40	we're now using OSPF to say,
0:55:43	"I know that all of this is gonna be reachable via the hub via router 5."

Now Viewing

CCIE Routing & Switching Advanced Technologies Class
Title:	CCIE Routing & Switching Advanced Technologies Class
Duration:	81h 59m
The CCIE Routing & Switching Advanced Technologies Class is the first step in understanding CCIE level technologies and is a companion to the Advanced Technologies Lab Workbook. Each technology you need to know for the CCIE Routing & Switching lab will be described in detail using an instructor led hands on demonstration. The class consists of over 80 hours of in depth explanations and examples.
Get instant access to our entire library!
$159/month	Add to Cart
Download this Course
$299.00	Add to Cart

OSPF Network Type Broadcast, OSPF DR/BDR Elec...

Create Bookmark