CCIE R&S Advanced Technologies Class v4.5 - Administrative Distance Based Routing Loops, Debug IP Routing, IP Route Profile

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 06m
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 02m

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

0:00:14	In our final example for redistribution today, we're gonna look at a topology
0:00:19	that a loop is caused based on the administrative distance,
0:00:23	not based on the metric, as we previously saw
0:00:26	that causes route feedback and the route is deleted
0:00:30	and added from the routing table's network over and over and over.
0:00:34	Now, this type of problem sometimes can be a little bit harder to track down
0:00:38	than the previous one we looked at,
0:00:40	because when we look at the traceroute output
0:00:44	for traffic coming from switch 2 that was supposed to leave router 6 towards BB1,
0:00:51	it was fairly obvious that when the packets got to router 6,
0:00:55	and they're supposed to be routes out that particular interface,
0:00:58	they were routed towards the wrong neighbor,
0:01:01	and then ultimately, the loop occurs.
0:01:05	So, for this type of problem that is an actual traffic loop in the data plane,
0:01:09	usually, this is easier to see because the traceroute is gonna show us exactly where
0:01:14	the packet is looping between.
0:01:16	But for cases where the routing control plane itself is the problem,
0:01:21	where the advertisement is sent out,
0:01:25	and then, immediately withdrawn and then advertised and withdrawn over and over and over.
0:01:29	This can be a little bit harder to figure out who is the actual source of the problem to begin with.
0:01:35	So, to illustrate this problem in this topology,
0:01:38	we have on router 4
0:01:39	three different protocols, OSPF, EIGRP, and RIP.
0:01:43	And on router 3, we have EIGRP and OSPF meeting as well.
0:01:48	We're gonna do redistribution between RIP and EIGRP on router 4,
0:01:55	and EIGRP and OSPF on router 3.
0:02:00	Now, before we actually do this,
0:02:03	let's think about what would be the advertisement path on a hop-by-hop basis
0:02:08	as the routes come in from RIP to router 4,
0:02:12	then are redistributed to EIGRP, then, from EIGRP back to OSPF.
0:02:20	So, to start,
0:02:22	router 4 is gonna be learning these prefixes in from RIP.
0:02:28	These are from RIP
0:02:30	via BB3
0:02:33	with an administrative distance of 120.
0:02:37	Router 4 is then redistributing these prefixes into EIGRP.
0:02:42	They're gonna go down to router 5 via both of these connections.
0:02:46	Router 5 is gonna say that these are EIGRP external routes.
0:02:52	They're learned from router 4,
0:02:55	and they have a distance of 170.
0:02:59	5 is then gonna advertise this down to switch 2 and to switch 4,
0:03:05	and back out to frame-relay to router 2 and router 3.
0:03:10	Router 3 is then gonna have this installed likewise as an EIGRP route
0:03:15	with the distance of 170,
0:03:17	and this is via router 5.
0:03:21	3 then performs redistribution of EIGRP into OSPF.
0:03:26	Now, switch 1 learns the prefix.
0:03:29	It says this is OSPF
0:03:32	via router 3 with a distance of 110.
0:03:37	This is advertised to switch 3,
0:03:39	and switch 1 to router 6,
0:03:42	from 6 back to router 4.
0:03:46	So now, we see a potential problem in the topology
0:03:49	where router 4 is learning the same prefix from multiple protocols.
0:03:54	At which point, router 4 is gonna look at whichever one has the lower administrative distance.
0:04:00	Now, router 4 has two options.
0:04:02	The RIP route that came from BB3 originally with the administrative distance of 120,
0:04:07	and the OSPF route via router 6 with the distance of 110.
0:04:17	Since the OSPF...
0:04:20	distance is now lower than RIP,
0:04:23	what's gonna happen on router 4?
0:04:33	If the distance of OSPF is lower,
0:04:35	it means the RIP route is gonna be removed from the routing table and replaced by OSPF.
0:04:42	If the RIP route is then removed,
0:04:45	what does this imply about the redistribution into EIGRP?
0:04:53	Remember, since the redistribution occurs from the routing table, not the routing database.
0:05:00	If router 4 chooses the OSPF route over RIP,
0:05:04	it means that RIP is not installed in the routing table,
0:05:08	which means it cannot be redistributed into EIGRP.
0:05:13	So, what we would see in a topology design like this
0:05:16	is that once the route is learned back through the incorrect protocol,
0:05:25	so the route goes from RIP into EIGRP
0:05:31	into OSPF.
0:05:33	This essentially is the wrong path.
0:05:36	The RIP route is the right path.
0:05:39	But the RIP route is withdrawn,
0:05:42	which means there is a withdrawn message sent into EIGRP,
0:05:48	which means that router 3 needs to send a withdraw message into OSPF,
0:05:57	which means that router 4 would then delete the prefix from the routing table.
0:06:04	Now, the issue with this type of design
0:06:06	is that it's sometime hard to track down
0:06:08	because we would have to look at the routing table of router 4 over and over and over
0:06:13	in order to see that that's actually the problem.
0:06:17	So, there's a couple different methods that we're gonna look at here
0:06:20	that are going to make it a little bit easier for us to track down where the problem is coming from in the first place.
0:06:27	And these two are gonna be the Debug IP Routing Output,
0:06:31	and the IP Route Profile feature.
0:06:34	Now, of course, we'll see with the connectivity testing with ICMP pings, or with traceroutes.
0:06:39	If the route is looping, then we're not gonna have reachability.
0:06:43	The problem is that the pings in the traceroute,
0:06:46	it might not actually tell us what is the source of the problem to begin with.
0:06:50	This is what the IP Route Profile and the actual Debug Output
0:06:56	can help us to figure out.
0:06:59	So first, let's actually do the redistribution and see what the result is.
0:07:04	We see, from the command line of router 4, we have the three different protocols, EIGRP, OSPF, and RIP.
0:07:11	At this point, there's no redistribution going on.
0:07:15	So first, let's go to the EIGRP process.
0:07:19	We'll redistribute RIP, and then give it a metric value.
0:07:25	We'll say, a hundred megabits per second,
0:07:30	a thousand microsecond delay,
0:07:32	100% reliable,
0:07:35	minimum load,
0:07:36	and then an MTU of 1500.
0:07:39	Again, these values here are fairly arbitrary because it is the only physical point in the network
0:07:44	that these routes are reachable via.
0:07:49	Then, likewise, under the RIP process, we're gonna redistribute EIGRP 1,
0:07:55	and give it a metric.
0:07:59	This should now mean, if I were to go to anywhere inside the EIGRP topology,
0:08:05	let's say on switch 2,
0:08:08	I should be able to see all of the routes that came from the RIP process.
0:08:17	On switch 2, if we Show IP Route EIGRP,
0:08:22	I see these external routes that are the 204 network that's the transit link between router 4 and BB3,
0:08:30	then the 30 and the 31 prefixes.
0:08:36	So first, let's test, do we actually have reachability to these?
0:08:39	We'll ping 30.0.0.1.
0:08:43	We are able to reach it, and if we look at the traceroute,
0:08:47	we should see that these packets are gonna go through the EIGRP network up to router 4,
0:08:52	then, from router 4 out.
0:08:54	Which is what we would expect here because router 4 is the only device doing the redistribution,
0:08:59	it's the only possible way to get to those destinations.
0:09:05	Next, we're gonna go to router 3,
0:09:08	and then, do it for the redistribution from OSPF and EIGRP.
0:09:14	Because at this point, switch 2 will have reachability to the RIP routes,
0:09:23	but it's not gonna have reachability to any of the OSPF prefixes,
0:09:26	because we're not doing redistribution between those two domains.
0:09:34	So on router 3, let's go under the OSPF process.
0:09:39	And say Redistribute EIGRP 1 Subnets.
0:09:45	Under EIGRP 1, we'll Redistribute OSPF 1.
0:09:53	And then, basically any arbitrary metric value here.
0:10:01	If we look at the result of switch 2,
0:10:04	look in the routing table for EIGRP,
0:10:07	we see, there's some additional prefixes in there
0:10:11	like the 155.28.7.0 network.
0:10:18	So, we're now able to reach the OSPF domain.
0:10:24	But notice that some of the previous routes
0:10:28	are now withdrawn.
0:10:33	Now, I know about the...
0:10:36	The 204 network, let's just say Show IP Route Include D EX.
0:10:45	So, these are my external EIGRP routes.
0:10:51	So, it looks like some of these, I have a route to, but not all of them.
0:11:04	So ,if we ping 30.0.0.1,
0:11:10	which previously, we did have reachability to on switch 2.
0:11:13	We can see, there's...
0:11:16	no response from them.
0:11:18	If we look in the routing table to see, do we actually have a route to that,
0:11:23	we see, it's not in the table.
0:11:25	Now, the problem we'll see in this specific topology
0:11:28	is that sometimes, this route will be installed,
0:11:32	sometimes the route won't be installed.
0:11:35	And you can see, it's kind of hard to catch there, because for a very, very specific amount of time,
0:11:41	the route is installed via EIGRP, but then, almost immediately afterward, it's withdrawn.
0:11:48	So, unless I knew to constantly look at the routing table for this prefix to begin with,
0:11:53	it's nearly impossible to figure out that this is the problem
0:11:57	just by looking at the Show IP Route Output.
0:12:01	Now, what I could do instead
0:12:03	is that once the redistribution is done,
0:12:06	I could tell the router to start collecting statistics on the table
0:12:10	to see if the network is stable or if there are changes in the routing table.
0:12:15	And this is what the IP Route Profile feature is designed to do.
0:12:20	So, on all of the routers,
0:12:22	and I'll simply send this on the access server.
0:12:25	I'm going to say, IP Route Profile.
0:12:29	So, it's just one command in global config, IP Route Profile.
0:12:37	Now, the feature's on.
0:12:39	And we can look at the Show IP Route Profile.
0:12:51	So here, this output says, we're looking at the routing table change statistics.
0:12:56	The frequency of changes in a 5-second sampling interval,
0:13:01	and we're looking at how many changes happened in that interval,
0:13:06	where the interval is 5 seconds.
0:13:09	So essentially, we're looking at the routing table over time,
0:13:12	and we're taking samples every 5 seconds.
0:13:15	We're looking at, did the number of routes go up, did the number of routes go down?
0:13:20	Did the next hop values change?
0:13:23	Did I get a normal prefix refreshed,
0:13:26	Which would be like in RIP where we're receiving periodic updates that I'm resetting the timer value?
0:13:33	Or in OSPF, if we have the periodic flooding for the paranoid update.
0:13:38	That would be considered a prefix refresh.
0:13:44	Now, the way that we read this output
0:13:46	is that the first row says that there were zero changes over a 5-second interval.
0:13:55	The number of intervals of which that occurred,
0:14:00	or two intervals...
0:14:03	of sample.
0:14:04	Okay, this doesn't exactly tell us when it happened,
0:14:09	but it's just that there were zero changes per 5 seconds twice so far.
0:14:16	If we saw a value in the row for 10,
0:14:21	it would mean that in some 5-second sampling interval,
0:14:25	there were 5 changes in the forwarding path, or the next hops,
0:14:30	or the number of paths have been changing.
0:14:34	So essentially, what we're looking for here is any values that are somewhere around here.
0:14:43	If we see these numbers count,
0:14:47	that's definitely bad.
0:14:48	It's saying that there's 25 changes over a 5-second period
0:14:53	for the number of routes being added, or the number of next hops values being changed.
0:14:59	So, in general, these values should all be zero.
0:15:02	And the values towards that hop should be counting up.
0:15:08	So, if we now look at this over time,
0:15:11	and check on switch 2 again if anything changed,
0:15:17	we can see, there are some problems here.
0:15:22	So, six times,
0:15:25	there were 5 changes over a 5-second sampling interval
0:15:30	for routes being added and forwarding paths changing.
0:15:37	If we look at this on the other routers, let's say on router 4,
0:15:41	and do the identical thing. Show IP Route Profile.
0:15:46	We could see likewise. A number of routes are changing fairly often in the routing table.
0:15:54	Now, it doesn't tell us what exactly is going on,
0:15:57	but just that there is something that suspect about the routing table stability,
0:16:03	which would lead us to look further into the Debug Output to figure out
0:16:06	what exactly are the routes that are having the problems.
0:16:10	This is what the Debug IP Routing Output is gonna show us.
0:16:14	But the key point here with the IP Route Profile
0:16:17	is that we can turn this feature on,
0:16:20	and let it run as we continue to work on the network.
0:16:24	If we come back an hour later and look at the Show IP Route Profile,
0:16:27	ideally, all of these values should be zero.
0:16:32	If we see that down here, it says that there's 5 changes, or 10 changes here, 7 changes here,
0:16:39	this is gonna be an indication of some sort of instability.
0:16:43	And in most cases, it's not valid to have that many changes in the routing table
0:16:49	over that period of time.
0:16:51	So either it means we're going through the initial convergence of the network,
0:16:56	or there's some sort of flapping topology information that's going back and forth between different protocols.
0:17:04	Now, have we looked at this in out previous example
0:17:07	with the loop based on the metric?
0:17:10	Technically, that was not an in-stable state of the routing topology.
0:17:16	It was simply an incorrect state.
0:17:19	So, it's not that the routes are flapping between the interfaces,
0:17:22	this is that the router shows the wrong path to begin with.
0:17:27	But in our case here, there will be a flapping in the routing table,
0:17:30	because once the new information in installed on router 4,
0:17:35	it's gonna force it to withdraw the old information.
0:17:39	And the way that we can see exactly what this is
0:17:41	is to look at the Debug IP Routing.
0:17:47	Now, depending on how many prefixes that we have in the network.
0:17:51	It may make more sense to send this Debug Output to the buffer
0:17:55	instead of sending it to the console.
0:18:00	But here, there's only about 10 prefixes or so that are going to be affected by the change.
0:18:05	So, it's not that big of an issue.
0:18:09	But we can see, between RIP and OSPF,
0:18:13	there's now a flapping of the topology.
0:18:18	So, let's let this run for...
0:18:22	for one more flap and then we'll look at the result of it.
0:18:45	So, our first potion of the output
0:18:48	says that we received this update
0:18:51	from BB3
0:18:55	via RIP with the distance of 120 and a metric of 1.
0:18:59	So now, we're installing it into the routing table.
0:19:05	As we keep going down, it says that there's now a new path
0:19:11	for this particular prefix.
0:19:15	It says that for...
0:19:17	30.0.0.0/16, there's now a closer administrative distance.
0:19:24	The new route is now via OSPF with the distance of 110 and a metric of 20.
0:19:32	This is then overriding the previous RIP distance and metric,
0:19:37	where the distance is 120, now, the distance is 110.
0:19:44	So, if we let this debug continue to run,
0:19:46	we'll see it over and over and over,
0:19:49	router 4 receives the correct information,
0:19:53	receives the incorrect information,
0:19:56	then, has to delete the route out of the table.
0:20:00	Now, another potential way that we can see this problem
0:20:04	is that if we were to send a ping to any of the destinations.
0:20:10	We might possibly see on router 4
0:20:13	that sometimes, the traffic goes through, and sometimes it doesn't.
0:20:17	It really depends on how fast the network is re-converging with the wrong information.
0:20:25	So, we let this run a little bit, we see if we get any responses back in here.
0:20:31	And we may try this, let's say, a time-out of 1.
0:20:43	So, we're able to send traffic to the destination.
0:20:46	Then, we're getting ICMP unreachable,
0:20:49	then, the packets are simply getting dropped.
0:20:54	So, if we let this ping continue, we should see the pattern happening over and over and over.
0:20:58	If the packets get through,
0:21:00	we get ICMP unreachable,
0:21:03	and then, there's a period where the routes are deleted from the routing table.
0:21:07	Now, the actual amount of time that it's gonna take is based on the convergence timers for RIP.
0:21:13	So, with the default 30-second update timer,
0:21:16	we should see, it's somewhere around 30 seconds that it's gonna flap back and forth.
0:21:22	Now, if we wanted to get more information on this exactly where the packets are going,
0:21:27	we could also look at the Debug IP ICMP
0:21:31	to see who is sending us the unreachable message.
0:21:36	So, under the console, Line Console 0.
0:21:40	I'll say, No Logging Synchronize,
0:21:43	because I wanna see the ICMP...
0:21:46	messages for each of these pings that...
0:21:55	I'm sending out.
0:22:07	It says, "ICMP host unreachable received from 155.28.37.3."
0:22:15	So, we now need to know in the top, who is 155.28.37.3.
0:22:23	This is...
0:22:26	router 3's interface that is connected to switch 1.
0:22:32	So, from router 4, if we're trying to send packets that direction,
0:22:38	why would router 3 have sent us an unreachable?
0:22:43	This is now an indication that the packets are being routed in the wrong direction.
0:22:48	So, for some reason, that packet is getting to 3,
0:22:50	and then, 3 doesn't actually have the route,
0:22:55	because someone has the wrong information in the network.
0:23:02	So, based on the output of the Show IP Route Profile,
0:23:07	that's first off telling us that there are changes in the topology.
0:23:10	Not necessarily what the changes are, but just that the changes are happening.
0:23:14	So, we Show IP Route Profile again,
0:23:18	we see that these numbers here keep incrementing.
0:23:20	So, the problem is still there, it hasn't been solved.
0:23:26	This would then lead us to look at the Debug IP Routing.
0:23:30	So, if we were to look at this on everyone. Say, Debug IP Routing.
0:23:41	We would then wanna know, what are the particular routes that are flapping?
0:23:46	So, what are the actual prefix values?
0:23:48	And where are those coming from in the first place?
0:23:52	So, if we look at let's say, on router 5,
0:23:56	router 5 says, "I have no route to that destination.
0:24:01	Delete the route via 155.28.0.4."
0:24:10	So, what does this message here tell us?
0:24:19	This means that router 4 is withdrawing that particular prefix into EIGRP.
0:24:26	So, this would then lead us to look at router 4's output to see what it's debug said.
0:24:32	Likewise, if we were to look at this on router 3,
0:24:35	router 3 is gonna say that "Router 2 withdrew the route."
0:24:41	Or depending on who we're routing through.
0:24:43	Router 3 says that "I have the route through router 2, but then it was withdrawn."
0:24:49	Router 2 is gonna say,
0:24:52	"I have the route from router 5, but it was withdrawn."
0:24:56	5 says, "It was withdrawn from router 4",
0:24:59	which leads us back to router 4, we can see now, the problem here is the administrative distance.
0:25:05	It says, "Closer administrative distance for 31.2.0.0."
0:25:10	We're deleting the RIP route and installing the OSPF route.
0:25:14	Which is an issue here, because once the RIP routes in deleted,
0:25:19	if this route's not in the table,
0:25:22	again, we can't advertise it.
0:25:24	Which means it can't be redistributed into EIGRP,
0:25:27	which means it can't be redistributed into OSPF,
0:25:30	which means that this withdrawal is gonna happen over and over and over.
0:25:40	So, we know what the problem is now
0:25:42	that router 4 is choosing
0:25:45	the wrong route based on the administrative distance.
0:25:55	It's choosing the OSPF route with a lower distance of 110
0:25:58	as opposed to the RIP route with the distance of 120.
0:26:03	So, if we were to change the distance so that RIP was lower,
0:26:08	this is gonna prevent OSPF from being installed in the first place.
0:26:12	Now, the problem with this type of scenario
0:26:15	is that filtering and redistribution is not really gonna help us.
0:26:19	So, let's say theoretically, when we...
0:26:22	redistribute from RIP into EIGRP, we set a tag value of 4.
0:26:29	Then, on router 3, as we go from EIGRP to OSPF,
0:26:34	we say, "Don't redistribute routes with a tag of 4."
0:26:40	This would prevent the loop from happening,
0:26:43	but then, it's also gonna prevent what?
0:26:49	It's gonna prevent any of these routers from being able to reach those destinations.
0:26:55	Because if the only redistribution point if on router 4,
0:26:58	then, the traffic has to route this direction.
0:27:04	Now, another possible solution
0:27:06	would be just to do the redistribution on router 4 to begin with.
0:27:11	So, if router 4 is the originator of the prefix into both EIGRP and OSPF,
0:27:19	when the prefix gets to router 3, 3 has two different choices.
0:27:24	It has OSPF at 110,
0:27:27	and it has external EIGRP at 170.
0:27:34	Router 3 says, "I'm gonna choose the lower distance, which is via OSPF."
0:27:40	Which then means that EIGRP route cannot get installed,
0:27:44	which then means it cannot be advertised.
0:27:50	So, if router 4 performs the redistribution itself into both domains,
0:27:55	this is gonna prevent the problem to begin with.
0:27:59	Because remember again, the redistribution happens from the routing table, not from the database.
0:28:04	If router 3 does not have this path installed in the routing table,
0:28:10	it's not gonna be able to redistribute it.
0:28:13	So, by forcing it to use the OSPF path as opposed to EIGRP,
0:28:18	it means that this route will ever be advertised unless router 4 looses that link.
0:28:30	So in reality, this would be the simplest solution, just to go to router 4,
0:28:34	and then, do the redistribution both ways.
0:28:39	So, if we go to the OSPF process and say, Redistribute EIGRP 1 Subnets,
0:28:44	then, under EIGRP,
0:28:46	say, Redistribute OSPF 1, and give it a metric. Okay, let's say, any arbitrary metric. I'll say all 1s.
0:28:59	The end result of this, if we look on router 3,
0:29:04	once the routing table converges everywhere and stabilizes,
0:29:09	if we were to Show IP Route for 30.0.0.1,
0:29:14	right now, router 3 says, "My path is via EIGRP.
0:29:19	I have a distance of 170 for this."
0:29:24	Since I have this installed in the routing table, it means that I can do the redistribution.
0:29:30	So, if I Show IP OSPF Database,
0:29:37	router 3 is performing this redistribution.
0:29:43	But only because the network is temporarily converging.
0:29:48	Okay, this is what sometimes is referred to as transient loop in the network.
0:29:52	It's a temporarily loop why the network is converging,
0:29:55	but once everyone again agrees on the topology, then, the loop is gonna go away.
0:30:00	So really, the easiest way to fix this would just be to...
0:30:06	to clear the routing table on all of the devices.
0:30:17	So eventually, once the network is done converging,
0:30:21	we should see on router 3 that this route is gonna be installed via OSPF and not via EIGRP.
0:30:31	So, let's look at router 4, let's say, Show IP Route...
0:30:37	Show IP Route RIP.
0:30:39	Right now, we don't have anything installed via RIP.
0:30:43	If we Show IP Route...
0:30:47	Let's say Include E2 or D EX,
0:30:53	looking at the external routes, it says that we're installing...
0:30:57	these prefixes via OSPF from router 6.
0:31:03	So now, we essentially have a traffic loop.
0:31:06	If we were to trace any of these destinations,
0:31:10	we see, the traffic is...
0:31:13	bouncing between router 4.
0:31:20	4 is sending it to 6
0:31:22	instead of sending it to BB3.
0:31:32	So really, to get this to work, if we're gonna do redistribution on all of the points here,
0:31:37	the order of operations is gonna be significant.
0:31:41	So, what we would need to do here, let's go to router 3,
0:31:45	and shutdown these links into the EIGRP network.
0:31:54	So, we'll say on serial 1/0, that's shutdown
0:31:58	as with serial 1/3.
0:32:04	So, we no longer have the adjacencies with router 5 or router 2,
0:32:10	which should be that we withdraw our redistribution into OSPF,
0:32:18	which then means that router 4
0:32:21	should start installing these routes as RIP,
0:32:25	which it is.
0:32:27	Which then means that router 4 can redistribute these into OSPF.
0:32:31	So, if we Show IP OSPF Database,
0:32:34	look at the external routes.
0:32:37	We see, now, router 4 is the originator.
0:32:40	Actually, only for some of them. So let's try clearing the routing table.
0:32:51	Then, look at the database.
0:32:57	And I may be missing... Let's say Show Run Begin Router. I may be missing one redistribute statement.
0:33:05	Which I am. I'm not redistributing RIP into OSPF, that's what the problem is.
0:33:10	So, under OSPF, let's say Redistribute RIP Subnets.
0:33:15	Under RIP, Redistribute OSPF 1 Metric 1.
0:33:21	Okay, now, router 4 should be the originator. If we look at the database under Type-5 LSAs,
0:33:30	we see now, router 4 is originating all of these RIP routes.
0:33:35	From router 3's perspective, if we now Show IP Route OSPF,
0:33:42	these destinations, the 30 and the 31s,
0:33:46	notice, they're now being installed as OSPF external.
0:33:50	Distance of 110, and a metric of 20.
0:33:55	This should now mean, once my EIGRP links come back,
0:34:02	it doesn't matter if I learn these alternate paths via EIGRP.
0:34:07	I'm never gonna use them,
0:34:10	because the default external distance is higher than OSPF.
0:34:17	So, if we Show IP Route EIGRP,
0:34:20	EIGRP is used for internal routes,
0:34:24	but not at the external routes.
0:34:28	If we look at the end result of this,
0:34:32	now, the trace is working.
0:34:35	The problem however with this type of solution
0:34:38	is that it's gonna be sensitive to the order of operations.
0:34:42	We could potentially end up in a case
0:34:44	where if router 4 fails,
0:34:48	and this link goes down,
0:34:51	the OSPF route is withdrawn,
0:34:53	which means that router 3 installs
0:34:57	the route via EIGRP.
0:35:01	If the EIGRP route is then installed, it means 3 can be redistributed OSPF.
0:35:09	Now, it's being advertised from 6 to 4.
0:35:13	Once router 4's interface comes back,
0:35:17	depending on the order that the routing process loads
0:35:21	is possible that it would learn this OSPF route before it learns the RIP route,
0:35:29	which means that this path would be preferred,
0:35:33	this path would be withdrawn,
0:35:36	and then, we would have the same process over and over.
0:35:45	So really, to fix this problem a 100% at the time, what do we need to do then?
0:35:55	We need to tell router 4 that at least, for those specific routes,
0:36:01	we have to use RIP for them.
0:36:05	Now, the problem with this design though which can make it a little bit more difficult to understand,
0:36:12	route tagging is not gonna help us.
0:36:15	Because if...
0:36:19	If the goal is to get the reachability from the...
0:36:25	OSPF network to the RIP domain,
0:36:29	unless the redistribution is happening directly on router 4,
0:36:33	and never on router 3,
0:36:39	then, we're not gonna have a problem.
0:36:42	Because in that case, the routing domains are not meeting on multiple points.
0:36:46	If we're not doing multiple redistribution.
0:36:49	What we could potentially do is to say...
0:36:53	that when 4 redistributes RIP into EIGRP,
0:36:57	we could set the tag to be 4.
0:36:59	Then, when we go from...
0:37:01	EIGRP to OSPF, say, Deny 4.
0:37:07	So, these routes never get advertised to OSPF.
0:37:10	But the problem with that though
0:37:12	is that if 4 is not redistributing in that direction, from RIP to OSPF,
0:37:16	then, switch 1 and switch 3, they're never gonna get reachability to those RIP prefixes.
0:37:32	Okay, there's a comment tag filter on router 4, carry the tag into OSPF.
0:37:42	So, redistributing from RIP to OSPF, you're saying?
0:37:49	Then, filter tag four routes on router 4 from OSPF.
0:37:57	So, you're saying, set the tag in this direction?
0:38:09	Okay, so the...
0:38:13	The comment here is to go from... On router 4, from RIP into EIGRP correct?
0:38:20	From RIP into EIGRP, and set the tag to be 4.
0:38:25	Then, on router 3, from EIGRP to OSPF, say Match 4...
0:38:33	And Set 4. Correct?
0:38:39	Then, when router 4 receives this back in, say, "Deny any routes that have a tag of 4."
0:38:50	Theoretically, that would work.
0:38:53	The problem is, there's nowhere to apply this filter.
0:38:58	So, the tag is applied as a filter to the redistribution process,
0:39:03	not as a distribute list.
0:39:06	And this is why in this design, you can't use tagging. You have to use administrative distance.
0:39:12	So, the idea behind the tagging is that when...
0:39:15	the route is originated to begin with.
0:39:17	Router 4 is gonna set some value
0:39:19	so that it knows if this route comes back, it's not valid to begin with.
0:39:24	So, we'll set the tag of 4 into EIGRP.
0:39:29	Then, when EIGRP redistributes into OSPF, we wanna continue the pass list.
0:39:33	So, let's actually try this up.
0:39:35	Okay, on router 4, let's look at the... Let's say Show Run Include Router or Redistribute.
0:39:43	And this is why this example gets
0:39:46	really confusing for a lot of people because there's a limited
0:39:48	number of ways that you can actually solve this problem.
0:39:51	So, let's remove all of the...
0:39:54	the previous redistribute statements.
0:40:09	And we'll start again from scratch. So, we'll say, Show Run Include Router or Redistribute.
0:40:16	Okay, and notice here, this is an important point I forgot to mention.
0:40:20	If you're trying to remove your redistribution configuration,
0:40:25	notice that under the OSPF process, I said, No Redistribute EIGRP 1 Subnets.
0:40:30	No Redistribute RIP Subnets.
0:40:33	The result of this on the command line was to remove the argument subnets,
0:40:38	not to remove the redistribution.
0:40:42	So, when you do this, it's kind of an odd configuration, you actually have to do it twice.
0:40:47	So, remove the redistribution to get rid of the arguments, and then remove it again
0:40:51	to get rid of the entire redistribution process. So, actually, I have to do it again
0:40:57	with that exact syntax.
0:40:58	So, No Redistribute EIGRP 1.
0:41:02	No Redistribute RIP.
0:41:08	So now, there's no redistribution between any of the processes.
0:41:11	Okay, on router 4.
0:41:14	So, router 4 is gonna have a route map that is gonna set the tag.
0:41:18	It says, Set Tag 4.
0:41:22	This is gonna be from RIP into EIGRP.
0:41:27	So, in EIGRP 1,
0:41:29	we'll say, Redistribute RIP.
0:41:33	Use the Route Map Set Tag.
0:41:35	And I'll use any arbitrary metric.
0:41:43	So now, on the devices in the EIGRP domain, if I were to go to router 5,
0:41:50	and look at the EIGRP topology,
0:41:53	I should see that that particular external route,
0:41:55	basically, anything coming from RIP,
0:41:57	it should now have tag number 4 associated with it.
0:42:03	We Show IP EIGRP Topology.
0:42:06	Let's say for 30.0.0.0/16.
0:42:13	Now, this is... Right now, it's not in the topology because router 3...
0:42:21	is still doing redistribution.
0:42:32	So on router 3, let's remove...
0:42:35	redistribution of OSPF and EIGRP.
0:42:38	And No Redistribute EIGRP 1 under OSPF.
0:42:48	Okay, so now, the only point of redistribution is only from RIP into EIGRP on router 4.
0:42:54	The result of this is that in the EIGRP topology,
0:42:58	once the network converges here, we should see that the...
0:43:03	administrative tag is 4.
0:43:05	Okay, so, this is what we're saying the route tag value.
0:43:08	We can also see this in the routing table itself, if we Show IP Route for 30.0.0.0/16,
0:43:19	it says that the route tag is 4.
0:43:25	So, this tells us that the setting of the tag on router 4 was correct.
0:43:29	And it is being maintained as it goes throughout the EIGRP network.
0:43:34	Next, let's go to router 3,
0:43:37	and redistribute from EIGRP to OSPF.
0:43:47	Under OSPF 1, we'll say, Redistribute EIGRP 1 Subnets.
0:43:53	If we look now in the OSPF database
0:43:56	for the external LSA 30.0.0.0,
0:44:03	we'll see that the prefix will be there sometimes, but not all the time, because now, it's looping.
0:44:21	So, to stop this temporarily, what I'm gonna do is...
0:44:24	shutdown router 4's link to OSPF.
0:44:31	So, this will stop the route flapping if there's now way for it to feed back.
0:45:01	So now, router 4 has the route from RIP which is correct.
0:45:05	It should've sent it to EIGRP,
0:45:07	and then, router 3 should send it to OSPF, which it did.
0:45:10	Okay, notice that the tag value is automatically maintained.
0:45:16	So, router 4 is the only one who is setting the tag.
0:45:19	Router 3, under its redistribution...
0:45:26	is not matching or setting any tag values, it's just saying, Redistribute EIGRP into OSPF.
0:45:34	So now, on router 4, we need to figure out, is there some way for the inbound updates,
0:45:39	can we match the tag and then apply it?
0:45:44	The problem with this though
0:45:46	is that under the OSPF process,
0:45:50	let's say, Distribute List.
0:45:53	This one says, "To filter based on a route map", let's see if this version is going to allow this.
0:45:58	So, let's say, Route Map No Tag 4, Deny 10. Match Tag 4.
0:46:08	Router Map No Tag 4 Permit 20.
0:46:13	So, this should theoretically filter out all routes from the routing table
0:46:17	that have a tag of 4, but then allow everything else.
0:46:22	So now, under OSPF, let's say...
0:46:25	Distribute List Route Map No Tag 4 In.
0:46:34	Now, before I re-enable this link, we're gonna look at the Debug IP Routing
0:46:39	to see what's the actual result when OSPF re-converges.
0:46:44	Let's see if we received any of those 30 routes in the...
0:46:51	in the routing table.
0:46:55	So, the adjacency is up. Now, we're starting to do our exchange.
0:47:12	So, it look like the prefix says...
0:47:15	Everything is learned from router 6, now, let's look at the...
0:47:19	Show IP OSPF Database.
0:47:29	So, we are learning the prefixes
0:47:34	but we're being stopped from being installed in the routing table.
0:47:39	So, for this particular version, this is supported.
0:47:42	Let's look at the final configuration for this. Let's Show Run...
0:47:47	Begin Router OSPF.
0:47:49	And I think this is a relatively new feature that you're allowed to apply the route map as the distribute list.
0:47:55	Let's take a look at the...
0:47:57	the Command Reference here for OSPF.
0:48:01	So, under... Let's look at the full documentation path.
0:48:06	So, let's go to the Main Documentation.
0:48:10	Products, IOS.
0:48:13	Regular IOS.
0:48:17	12.4T.
0:48:21	Then, under Reference Guides,
0:48:24	Command References.
0:48:29	IP Routing OSPF.
0:48:38	Then...
0:48:43	Distribute List is not under OSPF. This is gonna be then under Routing Protocol Independent.
0:48:48	So, let's try Protocol Independent Command Reference.
0:48:54	Distribute List In IP.
0:49:12	It says, "OSPF routes cannot be filtered from the database.
0:49:14	If you use this command it filters routes from the routing table,
0:49:17	it does not prevent link-state packets from being propagated." Okay, which is fine.
0:49:20	The route map can match on the tag.
0:49:31	And this is 12.0(24)S, I'm wondering if this is applying to the tag value or not.
0:49:38	Let's look at...
0:49:43	Real quick before I show another solution for this. Let's look at one of the switches.
0:49:48	And this is running... This is gonna be 12.2 Mainline, because this is the Catalyst IOS.
0:49:53	Under the OSPF process here,
0:49:58	so this is on switch 1. Distribute List...
0:50:04	So, this one has the route map as well.
0:50:12	So, it is gonna work now with OSPF then.
0:50:16	The problem with this though like the documentation says
0:50:19	is that the distribute list in OSPF, it's filtering the routing table, not the database.
0:50:25	In this particular topology, it's not gonna make a difference,
0:50:28	but you could run into a case where if router 1 is learning prefixes from 2 and 3.
0:50:40	Then, for some reason router 1 learns a route,
0:50:43	where let's say it's route A,
0:50:46	we're stopping it from entering the routing table.
0:50:50	This does not stop the LSA that actually contains the link state for A from continuing to be advertised on.
0:51:00	So, you can end up in a case where you cause a traffic black hole,
0:51:04	where router 3 thinks that 1 is in the shortest path to get to A,
0:51:09	which technically is it based on the OSPF database,
0:51:12	but then, when you get to the routing information based on router 1,
0:51:16	which is the routing table,
0:51:18	then, that particularly gonna is not installed.
0:51:21	So, you can do this type of filtering.
0:51:23	The key point is that if you do it on one router,
0:51:26	you basically need to do it on all of them.
0:51:31	And it's kind of hard to predict
0:51:33	unless you look at the overall topology and figure out what
0:51:38	portions of the shortest path tree is that router in from different people's perspective.
0:51:45	So, it's not to say that it's a general rule, "Don't ever use it."
0:51:50	It's just that there's a lot of planning that would have to go into whether that's a valid implementation or not.
0:51:55	Now really, in this scenario, there's a much easier way to solve this to begin with.
0:51:59	Okay, the problem is that router 4 is picking OSPF when it really should be picking RIP.
0:52:07	So, the only thing we need to do is tell router 4 not to do that.
0:52:12	Don't choose OSPF over RIP at least for these particular prefixes.
0:52:16	So, instead of doing the tag filtering,
0:52:20	so, let's remove this from the process on router 4.
0:52:31	Instead, we'll say...
0:52:34	under the RIP process, "Just change the distance to 109."
0:52:40	So now, if RIP is more preferred than OSPF,
0:52:45	when we look at the Show IP Route RIP,
0:52:49	even if we do learn these routes back in via OSPF,
0:52:53	since the distance of OSPF is 110, and we've modified RIP now to be 109,
0:52:58	then, this would be the preferred exit point.
0:53:03	The disadvantage of doing this solution though
0:53:06	is that it's applying into all prefixes of the RIP network.
0:53:10	So, if we wanted to be very accurate about the configuration,
0:53:13	we could get more specific here and say, Access List 1 Permit 30.0.0.0.
0:53:20	Permit 30.1, 30.2, 30.3.
0:53:27	So, match all of the routes in question.
0:53:29	Then, under the routing process,
0:53:32	say, we wanna set the distance
0:53:35	to let's say 105.
0:53:39	The route source, I don't necessarily care who it came from.
0:53:46	Now, if I wanted to, I could specifically set it based on that address that I'm learning it from.
0:53:52	But match it based on Access List 1.
0:53:57	So now, when the routing table is refreshed,
0:54:02	and we Show IP Route RIP,
0:54:04	we see that just those for prefixes that match in the access list,
0:54:09	those now have their distance set to 105.
0:54:17	So, in this very specific case, we could technically use the...
0:54:21	the tag to accomplish the filter.
0:54:24	But in the vast majority of cases,
0:54:26	if the loop in the network is based on the...
0:54:31	the distance to begin with,
0:54:33	generally, the easiest fix is to change the distance.
0:54:37	If the loop is based on the metric, that's generally where the route filtering is gonna be used,
0:54:43	and mainly the route tagging.
0:54:46	So, with the previous example of the loop just between OSPF when I did the route tagging,
0:54:52	it would have had the same effect if I said, "Match all of the individual prefixes,
0:54:57	and then selectively choose which entry point and exit point they're redistributed on."
0:55:02	The problem is that's less scalable router to maintain.
0:55:06	With the tagging,
0:55:08	it's more of a general solution similar to BGP communities
0:55:12	that are grouping the prefixes into the same forwarding pattern.
0:55:20	So really, the overall key of all of these redistribution
0:55:24	is that there's not really general rules again that you can apply on to this.
0:55:30	What you need to do is to...
0:55:33	to look at the topology to begin with.
0:55:42	Here we go...
0:55:46	So, we need to visually trace the route advertisement
0:55:50	to figure out what are the cases the router's gonna learn prefix from multiple protocols,
0:55:55	or learn it from the same protocol in multiple interfaces.
0:56:00	If it's learned via the same protocol in multiple interfaces,
0:56:05	Then, we need to look to see if there's gonna be a problem with the metric values.
0:56:10	If it's learned from multiple protocols, then, we need to look at if there's problems with the distance.
0:56:19	Now again, with the second problem,
0:56:22	with the distance, usually, this is a little bit harder to track down.
0:56:26	This is where Debug IP Routing and the IP Route Profile is gonna help you.
0:56:30	With loops that are based on the metric, usually, the traceroute is enough,
0:56:34	because it's gonna show you what particular hops that the traffic is looping between.
0:56:41	But in a network that may have a hundred or more prefixes
0:56:44	that we're trying to troubleshoot,
0:56:46	then, a lot of the times, it's gonna be hard to test all of them one-by-one
0:56:51	unless we use some sort of automation for that,
0:56:53	which is what we would want the TCL scripting for.

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CCIE R&S Advanced Technologies Class v4.5
Title:	CCIE R&S Advanced Technologies Class v4.5
Duration:	81h 02m
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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