CCIE R&S Advanced Technologies Class v4.5

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 17m
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 42m

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	The next topic we're gonna look at here in OSPF
0:00:17	is how it does its path selection both for intra-area routes, inter-area routes,
0:00:23	and then external routes both the types 1 and 2 and the not-so-stubby types 1 and 2.
0:00:30	Now, at this point, we looked at...
0:00:31	how we enable the process, how we deal with the different issues of adjacencies
0:00:36	running over the different medias.
0:00:38	So, at this point, we should have the databases synchronized
0:00:41	throughout everyone in the topology.
0:00:45	Now, inside the router LSA, we also saw
0:00:48	the devices are gonna be describing all of their attached links,
0:00:51	and what are the cost attributes for those.
0:00:54	So, just like RIP of EIGRP, OSPF is gonna do the path selection
0:00:59	simply based on the lowest end-to-end cost to each of the paths.
0:01:04	Now, per the RFC, the cost value techically isn't defined.
0:01:09	It could be used as a hop count.
0:01:10	Or it could be used as a bandwidth value.
0:01:12	In the case of Cisco's implementation,
0:01:15	they use a reference bandwidth of a hundred megabits per second
0:01:18	to derive the cost of the link.
0:01:22	Now, in today's networks, since we're typically using links that are of a higher speed that a hundred megabits per second,
0:01:28	it would make sense to change the reference bandwidth
0:01:31	so that links that are above a hundred megabits per second all do not have a cost of 1.
0:01:37	So typically, the reference bandwidth would be set to 10 gigabits per second,
0:01:42	or a hundred gigabits per second so that we can tell the difference
0:01:46	between the higher speed links. Like the GigE, the OC48, OC192, etc,
0:01:51	Now, when we go to do the path selection,
0:01:55	we also need to take into account what type of route are we looking at
0:01:58	when we're comparing the different paths.
0:02:02	So, regardless of what the metric or the distance is to a particular destination,
0:02:06	OSPF is always gonna choose the intra-area routes over the inter-area routes,
0:02:12	then over external 1 and 2, and NSSA 1 and 2.
0:02:19	Now, of course, we still will use the longest match routing principle in the routing table
0:02:24	to figure out where the traffic is actually gonna go,
0:02:27	but if we have equal longest matches,
0:02:30	and one of them was an intra-area route, one of them was as external route or an inter-area route,
0:02:36	then, we would always choose the intra-area path.
0:02:40	So, regardless of the metric, regardless of the distance.
0:02:43	There's no way that we can modify this
0:02:45	because we need to make sure that all of the devices in the network
0:02:48	come up with the same shortest path tree
0:02:50	based on the input of the link state database.
0:02:55	Previously, in the examples I was showing,
0:02:57	we were using just a single area in OSPF in the entire diagram.
0:03:01	So now, I've modified this a little bit so that we have a multi-area design,
0:03:07	where over the frame-relay link between routers 1, 2 3, 4, and 5,
0:03:12	we're running area zero.
0:03:13	We also have area zero on the point-to-point serial links between 4 and 5, 1 and 3 and 2 and 3,
0:03:20	then, we see we have some other various areas 1, 2, and 51
0:03:25	that are going to... 51 down at the bottom is the connection between router 2 and BB2.
0:03:33	Then also, router 4 and 6, we have some external routing domains.
0:03:37	Router 4 is running RIP with BB3.
0:03:39	Router 6 is running EIGRP with BB1.
0:03:42	And on both of those points, we're also doing redistribution.
0:03:46	So, let's take a look at the network now from router 5's perspective.
0:03:51	And we'll see how the database has changed
0:03:53	and what principles router 5 is using in order to do its path selection.
0:03:59	So, we could see here on router 5 from the output of the...
0:04:02	Show IP OSPF Interface Brief.
0:04:05	Some of these links are running in area zero,
0:04:07	some of the links are running in area 2.
0:04:10	This means that router 5 is now an area border router
0:04:13	so it should be generating two separate router LSAs.
0:04:18	One for area 2 and one for area zero.
0:04:22	Additionally, any routes that are learned in the area 2 links,
0:04:26	those will be sent into area zero using the LSA type 3,
0:04:32	which is the network summary LSA.
0:04:35	Now, this is considered a summary,
0:04:37	because we are not summarizing the network layer reachability information.
0:04:42	So, the actual prefixes.
0:04:43	But instead what we are summarizing is the topology information.
0:04:49	So, we'll see that when we look at the difference between an intra-area lookup and an inter-area lookup,
0:04:55	for the intra-area lookups, we are gonna run SPF to every destination.
0:05:00	But for the inter-area lookups between the areas, we will use the SPF cost that we derived to the are border router
0:05:08	plus whatever value they are reporting in with the summary LSA.
0:05:14	So, on router 5, now, let's look at the Show IP OSPF Database.
0:05:19	We see, the first portion of the output defines what are the routers in area zero.
0:05:24	These are routers 1 through 5
0:05:27	with these respective number of links in area zero.
0:05:33	So, router 5, we see it has a large number of links,
0:05:37	because we have the frame-relay network configured as point-to-multipoint.
0:05:41	So, each of the adjacencies to routers 1, 2, 3, and 4 count as separate links in the database.
0:05:48	We'll also see that we have the loopback, the point-to-point link to router 4,
0:05:53	plus the actual address on the point-to-point link to router 4,
0:05:57	and our connection to the frame-relay network.
0:06:01	So again, OSPF does separate the difference between a link that is used to connect to an adjacency.
0:06:06	And then one that has the actual prefix assigned to it.
0:06:12	We see, next, we have the summary...
0:06:14	network link states.
0:06:16	These are the inter-area routes that are generated by the area border routers.
0:06:23	Now, notice between the first portion of the output and the second,
0:06:27	we no longer see any network link states.
0:06:31	This means that in area zero,
0:06:34	there are no designated routers on that in any segments.
0:06:38	Because the serial links are running as network type point-to-point.
0:06:41	The frame-relay is running as point-to-multipoint.
0:06:44	None of these are running the designated router or backup designated router election.
0:06:51	So normally, we would see another field between these two that said the network link states,
0:06:55	that would be the designated routers.
0:06:59	So, the summary network link states, this is LSA Type-3.
0:07:02	These are used to describe the inter-area routes.
0:07:06	Now, we see, some of them are duplicates, because we have multiple ABRs
0:07:10	that can reach the same portion of the network.
0:07:13	The first one we see, we have multiple paths to get to router 6's loopback.
0:07:19	One of these is via router 1, one of these is via router 3.
0:07:25	Now, when we actually do the lookup
0:07:27	to get towards router 6's loopback,
0:07:30	router 5 internally is gonna say Show IP OSPF Database.
0:07:34	And look for the summary LSA 150.42.6.6.
0:07:42	It says, inside area zero,
0:07:45	there are two possible paths to reach this destination.
0:07:49	One of them is being originated by...
0:07:52	router 1.
0:07:53	One of them is being originated by router 3.
0:07:56	We see the one that router 1 is advertising has a metric of 2.
0:08:00	Router 3 is advertising a metric of 3.
0:08:03	Based on this output, we really can't tell which path we're actually gonna choose.
0:08:07	We would then need to look at what is the SPF cost inside our own area
0:08:14	in order to reach these area border routers.
0:08:18	This is the behavior that sometimes is describes as OSPF being distance vector,
0:08:24	because this inter-area lookups
0:08:27	do not have full visibility of the end-to-end topology.
0:08:32	So, beyond router 1 and beyond router 3,
0:08:35	they are summarizing the topology information
0:08:38	where we don't know the exact path by path or link by link basis that they're going through
0:08:43	in order to reach this particular destination.
0:08:46	The only thing that router 5 knows
0:08:49	is that router 1 and router 3 are advertising these paths to me,
0:08:52	and they are ABRs that are part of my area zero.
0:08:56	So then, we would look at what is our SPF cost in order to reach 3 and 1.
0:09:02	Add this to whatever the metric that they area advertising,
0:09:07	this would then be the total cost that we would install in the routing table.
0:09:13	So, whichever one is gonna be the shorter
0:09:15	cost of what they are reporting plus our metric to them,
0:09:19	that's what's actually gonna get installed in the routing table.
0:09:23	There's a question here, "So, on this output, the 2 and 3 are the metric to get to the ABRs.
0:09:28	This is the metric that they are reporting in."
0:09:32	So, if I were to look at router 1's routing table,
0:09:35	and say...
0:09:36	Show IP Route 150.42.6.6.
0:09:43	Router 1 is using a metric of 2.
0:09:46	From router 3's perspective, if we Show IP Route....
0:09:51	150.42.6.6.
0:09:54	Router 3 is using a metric of 3.
0:09:56	So, this is what they have locally calculated in their topology
0:10:00	what their end-to-end cost is.
0:10:02	Then, they are advertising this to router 5.
0:10:06	So, from this output of the summary LSA, we don't actually know which path are we gonna choose.
0:10:11	We would then need to correlate this with the advertising router
0:10:14	to figure out what is our cost to get to router 1 and router 3,
0:10:19	add that to the metric that they are reporting,
0:10:23	that would then be out total cost to reach the neighbor.
0:10:28	Now, the way that router 5 does this...
0:10:31	is it first needs to know what is the graph of my network
0:10:34	in order to get to router 2, and...
0:10:38	or not router 2, router 1 and router 3.
0:10:40	So, router 5 is going to be the root of the shortest path tree.
0:10:45	So, it first need to know who are my neighbors that I'm adjacent with.
0:10:50	So internally, router 5 would say, Show...
0:10:54	IP OSPF Database...
0:10:57	for the router LSA that I am generating.
0:11:04	So here, 150.42.5.5, this is the router ID of router 5.
0:11:11	Router 5 says that it is adjacent with router 1.
0:11:16	And it is directly adjacent with router 3.
0:11:20	Since both of these connections are going over the same interface,
0:11:23	we see that they're both gonna have the same metric value of 64.
0:11:30	Since we're directly connected to these neighbors,
0:11:32	essentially, that's where the search is gonna end.
0:11:35	So, we don't need to go any further we know that we are directly connected to them.
0:11:38	This is gonna be the lowest cost to reach these neighbors.
0:11:41	With the metric of 64 on the connected serial link.
0:11:46	So, we would take the result of these.
0:11:48	Add this to what we previously saw in the summary LSA output.
0:11:54	So therefore, whichever is lower of 2 plus 64
0:12:00	versus 3 plus 64,
0:12:03	that's what router 5 should be installing in the table.
0:12:07	So, if we'll now look at the Show IP Route 150.42.6.6,
0:12:11	we should see that the total cost is 66.
0:12:15	And the neighbor we're using to route there is router 1.
0:12:20	So, if we Show...
0:12:24	Show IP Route 150.42.6.6,
0:12:29	it says, "The total metric is 66, and we're routing towards router 1 to get there."
0:12:35	Now, if there more diverse paths in the network,
0:12:38	we don't necessarily have to be using this directly connected link in order to get to router 1.
0:12:43	It's gonna be whatever from router 5's perspective is the shortest path to get to that particular neighbor.
0:12:50	So, we could have a potential case where maybe router 5 is routing to 2,
0:12:55	then, from 2 to 3, then, from 3 to 1 to get to the destination.
0:13:02	It really depends on what is our internal SPF cost to get to the neighbor.
0:13:07	Plus whatever value that they are reporting in.
0:13:12	Now, from a traffic engineering point of view, if we wanted to modify the path selection,
0:13:16	we essentially now have two different options.
0:13:19	We could change the value that router 1 is reporting in,
0:13:24	which right now, it's reporting a value of 2.
0:13:27	And router 3 is reporting a value of 3.
0:13:30	Or we could change our local SPF cost in order to reach them.
0:13:36	As long as the end-to-end metric when we add up all the paths along the way,
0:13:41	is the lowest end-to-end value, that's what we're gonna prefer.
0:13:46	Now, if for some reason, we had a directly connected link
0:13:48	between router 6 and router 5.
0:13:55	And this was running area zero.
0:13:58	We would then always prefer that path
0:14:01	over any other alternate path regardless of what the metric or what the distance is.
0:14:07	Because in the path selection state machine,
0:14:09	OSPF says, "We have to prefer the intra-area routes,
0:14:13	because we're actually doing a full SPF run on those."
0:14:17	For the inter-area routes, we run SPF to get to the ABR,
0:14:21	the, we're assuming that the ABR has run SPF to get to the destination,
0:14:25	or to some other intermediate neighbor that's advertising it to them.
0:14:31	Now, the case where that would be true,
0:14:33	if we were looking at this cost from the perspective of switch 4.
0:14:38	Switch 4 would say that "To get to router 6's loopback, I need to calculate what is my cost to router 5 as the ABR."
0:14:46	Then, router 5 says, "To get to 6, I need to calculate the cost to either 1 or 3.
0:14:52	Whichever is lower plus what they are advertising,
0:14:56	that's the neighbor that I'm gonna forward through."
0:15:00	But the key point being here that for the inter-area lookup,
0:15:03	the routers do not know what is going on in the topology beyond the ABR.
0:15:09	And this is one of the scalability features of OSPF
0:15:12	that the inter-area routing is going to terminate the flooding domain.
0:15:18	So, this means that if there is a change in the cost of router 6's link,
0:15:22	or an alternate path is going towards it,
0:15:26	it doesn't mean that router 5 needs to calculate SPF,
0:15:29	it would just need to take into account what are the new cost values that the ABRs are sending to me.
0:15:37	So, let's say for example that router 1 looses the link that goes to router 6.
0:15:43	This would the means that router 2 would have to withdraw this advertisement to get to 6,
0:15:48	and router 5 will ultimately prefer the only path that's left,
0:15:51	which is the one that's from router 3.
0:15:56	So, from router 5's perspective, let's look at the...
0:15:59	the change in the path.
0:16:02	So right now, if we were to trace 150.42.6.6,
0:16:08	right now, it's going through router 1.
0:16:10	If we look at the Debug IP OSPF SPF,
0:16:16	then, on router 1, I'm gonna shut my link down that is connecting to router 6.
0:16:22	Which is Fast Ethernet 0/0.
0:16:26	We'll see that on router 5,
0:16:30	and this is a very noisy debug. The Debug IP OSPF SPF.
0:16:34	Normally, you would wanna send this to the buffer. Not to the console.
0:16:41	So, this is the actual calculation that is going through for the SPF algorithm.
0:16:45	But if we look at what has changed,
0:16:49	it says that "Router 1 says that there's a change in this particular link state ID."
0:16:54	It says that "Now, to get to...
0:17:04	To get to this prefix, we now have a better path to get to router 3."
0:17:10	But the difference is that we're doing what's known as the "Partial Route Calculation", or a PRC.
0:17:16	We're not actually doing a full SPF run in order to reach them.
0:17:21	Because what has not changed here is the SPF cost to reach the ABRs.
0:17:27	Only what has changed is what is they're reporting into the area.
0:17:30	So, when router 5 looks at the Show IP OSPF Database and looks at its own router LSA,
0:17:37	the cost to 1 is still 64.
0:17:41	And the cost to router 3 is still 64.
0:17:44	The only thing that has changed is what they are reporting into the area.
0:17:47	So now, if we look at the Show IP OSPf Database for the summary LSA of router 6,
0:17:54	we could see that now, in area zero, there's only one possible path.
0:17:58	And this is going through router 3.
0:18:03	So, there is a recalculation that's done.
0:18:05	But it's different than a normal SPF run. It's called a "Partial Route Calculation", or a PRC.
0:18:11	So, if for any inter-area or external change, we're gonna do the partial recalculation.
0:18:17	for any change that is inside the area,
0:18:20	we would have to do a new SPF run.
0:18:23	So, from a scalability point of view,
0:18:25	it's better to design OSPF with the multiple areas,
0:18:29	instead of putting everyone just in a giant area zero.
0:18:35	So, let's now say we wanna change what the path is
0:18:39	that we're using to route to this particular destination.
0:18:43	So, with the previous default...
0:18:46	configuration, router 5 is saying, "I have a cost of 64 to get to router 1.
0:18:51	And router 1 is reporting a cost of 2 in.
0:18:54	Then likewise, I have a cost of 64 to reach router 3
0:18:57	and they're reporting a cost of 3."
0:19:00	Now, configuration wise, there's a lot of different ways that we can influence this.
0:19:04	The ultimate goal is that we're trying to change the cost of what the end-to-end path is between the neighbors.
0:19:10	Now, since the cost is based on the interface's bandwidth,
0:19:14	it implies that if we were to change the bandwidth,
0:19:16	it then would inherently change the cost.
0:19:19	The only disadvantage of doing this with the interface level bandwidth command
0:19:23	is that that statement is used to affect other operations in the network
0:19:27	like QoS calculations.
0:19:30	So typically, if we were doing traffic engineering for OSPF,
0:19:34	it would make more sense just to change the OSPF cost directly,
0:19:40	but technically, we have these other methods to do it.
0:19:43	We could change the bandwidth, which inherently changes the cost.
0:19:46	We could change the cost directly,
0:19:48	we could change the actual calculation,
0:19:51	where auto-cost is changing what the reference bandwidth is.
0:19:56	Where if the reference bandwidth is a hundred megabits per second,
0:19:59	and our interface bandwidth is a hundred megabits per second,
0:20:02	then, the result of that would be a cost of 1.
0:20:07	So, if we wanted to change the calculation to take into account the larger scale links,
0:20:11	let's say that we wanna change the cost so that a 20 GigE link
0:20:19	would be a cost of 1, or...
0:20:24	Right. Let's say, 20 gigabits per second is gonna be a cost of 1. Not 20 GigE, but 20 gigabits per second.
0:20:32	Now, we could just run through the actual calculation and figure out what value is gonna work.
0:20:37	Or we could just use the show commands on the router to kind of work backwards from there,
0:20:43	really, the preference is up to you.
0:20:45	Okay, the difference is that you would have to remember what the...
0:20:49	the calculation is, the cost equals the reference bandwidth over the interface bandwidth.
0:20:54	We could get the same result just by trial and error on the command line.
0:20:59	So, if I were to go to router 5,
0:21:02	and let's say that on...
0:21:04	our Fast Ethernet 0/0 link.
0:21:08	I'm gonna change this to a 20 gig link,
0:21:13	and we'll see what the result of the bandwidth is.
0:21:17	Or result of the cost is.
0:21:19	So, on this link, let's say the bandwidth...
0:21:22	is in kilobits per second. So, 2000 would be 2 megabits per seconds.
0:21:28	20 megs, 200 megs, 2 gigs and 20 gigs.
0:21:33	So, it actually...
0:21:36	It's not gonna support that then...
0:21:37	The...
0:21:40	maximum is 10 gigs then.
0:21:47	Alright. So, that's 1 megabit, 10 megs, 100 megs, 1 gig, and 10 gigs.
0:21:52	So, let's rework this calculation then. Let's say that we want...
0:21:57	a 10 gig link to be a cost of 5.
0:22:02	So then the...
0:22:05	the higher speeds would still have a value that we could go below that.
0:22:10	So first, I'll just change the bandwidth. So, this is now gonna be representative of a 10 gig link.
0:22:16	If we Show IP OSPF Interface Fast Ethernet 0/0 and include the cost.
0:22:24	The cost value right now should be 1.
0:22:26	Because anytime we go above a hundred megabits per second,
0:22:29	everything is gonna be a cost of 1.
0:22:32	So, OSPF doesn't use...
0:22:34	any fractions or decimals for the calculation. It has to be a whole number.
0:22:40	So, under the OSPF process, I'm now gonna change the auto-cost reference bandwidth.
0:22:45	Let's say I'm just gonna send it to be the maximum.
0:22:50	The result of this now...
0:22:52	is that the cost is 429.
0:22:57	So, we know that's gonna be too high. I wanna get it to be a cost of 5.
0:23:02	So, let's say I take the rest of the numbers at the end.
0:23:05	Let's make it...
0:23:09	4 million.
0:23:11	As the round numbers... 4 million? Right.
0:23:14	Okay, so 4 million then is a cost of 400.
0:23:20	So, if we were to cut this in half, let's say, 2 million.
0:23:25	It is a cost of 200.
0:23:27	So, you could work down from there.
0:23:28	You could just look at what is the actual calculation,
0:23:33	but if you see, basically just with trial and error, you can get to whatever value that you need.
0:23:38	Okay, so now, with the reference bandwidth of 50,000 megabits per second,
0:23:43	a 10 gig link would then be a cost of 5.
0:23:47	Now typically, you would want to change this everywhere,
0:23:50	because if you don't, they're not gonna agree on what the topology looks like,
0:23:54	and you could have failures in the SPF calculation.
0:23:59	So, I'll just send this to everyone.
0:24:03	We'll say that...
0:24:05	under the OSPF process,
0:24:08	I'm changing the reference bandwidth.
0:24:15	So typically, in today's networks, you would want to change this.
0:24:19	Because otherwise, anything over a hundred meg is gonna be the same as a cost of 1.
0:24:24	And ideally, you would wanna prefer your gig, or 10 gig links, or OC48
0:24:28	over Fast Ethernet, or over OC3.
0:24:35	So now, if we look at router 5,
0:24:37	and Show IP OSPF Interfaces,
0:24:40	we'll see that the other links that I did not change, these are gonna have much higher cost values now.
0:24:46	So, the point-to-point serial link now is a cost of 32,000.
0:24:50	Same with the frame-relay.
0:24:54	And the Fast Ethernet, this is a cost of 5.
0:24:57	Okay, that's because I changed the bandwidth value.
0:25:02	Now, if we look at the result of this in the database,
0:25:05	let's now say, what is...
0:25:08	the...
0:25:10	the path to the summary LSA?
0:25:14	We can see now that router 1 and 3 are reporting metrics of 501 and 551.
0:25:21	If I look at what my metric is to reach them, it's gonna be the same as before.
0:25:25	It's not gonna be the same value, but it's gonna be the same between the two of them,
0:25:28	because I'm using the same interface to reach both 1 and 3.
0:25:33	So, if I didn't want to route the traffic to 1,
0:25:36	if I wanted to route it to 3, I could simply go to 1,
0:25:39	and then, increase its metric to the path.
0:25:43	So on router 1's link to router 6, I'll say that this has a cost of...
0:25:50	a thousand.
0:25:51	Or we'll actually look what it is currently.
0:25:55	And we'll change it based on that. So, let's say Show IP OSPF Interface Fast Ethernet 0/0,
0:26:01	and look for the cost.
0:26:03	Okay, right now, it's 500.
0:26:04	So, if I say that the IP OSPF cost is a thousand.
0:26:10	Then, we should see now in the summary LSA.
0:26:14	This should change the value that router 1 is advertising.
0:26:18	It's not gonna change my metric to get to 1.
0:26:21	It's changing what's 1's metric is to get to the destination.
0:26:28	So now, we see that router 1 is reporting 1001.
0:26:34	So, if 1001 plus are local metric,
0:26:38	to reach router 1 is higher than 551 plus the local metric,
0:26:42	then, we should start to prefer the alternate path.
0:26:46	So now, if we look at the result in the routing table, let's Show IP Route for 150.42.6.6,
0:26:57	we see that now, we're using router 3.
0:26:59	So, if we trace to the destination,
0:27:07	we see that these packets now go to router 3 first
0:27:11	as opposed to going to router 1.
0:27:15	So, as compared to EIGRP, the path selection is pretty straightforward in OSPF,
0:27:20	because we're simply adding the value on a hop-by-hop basis.
0:27:23	The key point to remember though
0:27:26	is that if it is a different type of route,
0:27:29	there is no way that we can change that.
0:27:31	So, if router 5 had an intra-area route versus an inter-area route,
0:27:38	even if somehow the inter-area route's cost was lower,
0:27:43	then, we would have to still take the intra-area route.
0:27:48	So, it pick the O over the OIA.

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CCIE R&S Advanced Technologies Class v4.5
Title:	CCIE R&S Advanced Technologies Class v4.5
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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.
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OSPF Path Selection

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