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adminPost at 24-1-2010 14:56 | This Author's Posts Only

RIP Summary Address and Default Routes

Summary, RIP, Address, Default, Routes
i  ran into this strange problem while doing Mock Lab 6, and I thought it warranted an article because it highlights why the order of operations that IOS performs certain tasks is important. Let’s say you have this scenario:

mocklab6.gif
24-1-2010 14:56

R4 is a border router between two routing domains, OSPF and RIPv2. We are doing mutual redistribution between OSPF and RIPv2. Simple enough.
R4:

  2. router ospf 1
  3. version 2
  4. router-id 150.1.4.4
  5. log-adjacency-changes
  6. redistribute rip subnets
  7. network 150.1.4.4 0.0.0.0 area 0
  8. network 176.1.45.4 0.0.0.0 area 45
  9. network 176.1.145.4 0.0.0.0 area 0
  10. !
  11. router rip
  12. version 2
  13. redistribute ospf 1 metric 1
  14. passive-interface default
  15. no passive-interface Ethernet0/0.46
  16. network 176.1.0.0
  17. no auto-summary
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Let’s have a look at the route table of both R4 and R6 to verify that we have connectivity
R4:
  1. R4#sh ip route
  2. Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  3.        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  4.        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  5.        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  6.        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  7.        ia - IS-IS inter area, * - candidate default, U - per-user static route
  8.        o - ODR, P - periodic downloaded static route

  10. Gateway of last resort is not set

  12.      54.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  13. R       54.1.7.0/24 [120/1] via 176.1.46.6, 00:00:01, Ethernet0/0.46
  14. R       54.1.7.254/32 [120/1] via 176.1.46.6, 00:00:01, Ethernet0/0.46
  15. R    212.18.1.0/24 [120/2] via 176.1.46.6, 00:00:01, Ethernet0/0.46
  16. R    212.18.0.0/24 [120/2] via 176.1.46.6, 00:00:01, Ethernet0/0.46
  17. R    212.18.3.0/24 [120/2] via 176.1.46.6, 00:00:01, Ethernet0/0.46
  18.      176.1.0.0/16 is variably subnetted, 5 subnets, 2 masks
  19. C       176.1.145.0/24 is directly connected, Serial1/0
  20. C       176.1.45.5/32 is directly connected, Serial1/1
  21. C       176.1.45.0/24 is directly connected, Serial1/1
  22. C       176.1.46.0/24 is directly connected, Ethernet0/0.46
  23. C       176.1.4.0/24 is directly connected, Ethernet0/0.4
  24. R    212.18.2.0/24 [120/2] via 176.1.46.6, 00:00:03, Ethernet0/0.46
  25.      150.1.0.0/16 is variably subnetted, 3 subnets, 2 masks
  26. C       150.1.4.0/24 is directly connected, Loopback0
  27. O       150.1.5.5/32 [110/782] via 176.1.145.5, 00:00:04, Serial1/0
  28. O       150.1.1.1/32 [110/782] via 176.1.145.1, 00:00:04, Serial1/0

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R6:

  2. R6#sh ip route
  3. Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  4.        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  5.        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  6.        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  7.        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  8.        ia - IS-IS inter area, * - candidate default, U - per-user static route
  9.        o - ODR, P - periodic downloaded static route

  11. Gateway of last resort is not set

  13.      54.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  14. C       54.1.7.0/24 is directly connected, Loopback1
  15. C       54.1.7.254/32 is directly connected, Virtual-Access1
  16. R    212.18.1.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  17. R    212.18.0.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  18. R    212.18.3.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  19.      176.1.0.0/16 is variably subnetted, 5 subnets, 2 masks
  20. R       176.1.145.0/24 [120/1] via 176.1.46.4, 00:00:05, Ethernet0/0
  21. R       176.1.45.5/32 [120/1] via 176.1.46.4, 00:00:05, Ethernet0/0
  22. R       176.1.45.0/24 [120/1] via 176.1.46.4, 00:00:05, Ethernet0/0
  23. C       176.1.46.0/24 is directly connected, Ethernet0/0
  24. R       176.1.4.0/24 [120/1] via 176.1.46.4, 00:00:05, Ethernet0/0
  25. R    212.18.2.0/24 [120/1] via 54.1.7.254, 00:00:02, Virtual-Access1
  26.      150.1.0.0/16 is variably subnetted, 4 subnets, 2 masks
  27. C       150.1.6.0/24 is directly connected, Loopback0
  28. R       150.1.4.0/24 [120/1] via 176.1.46.4, 00:00:06, Ethernet0/0
  29. R       150.1.5.5/32 [120/1] via 176.1.46.4, 00:00:06, Ethernet0/0
  30. R       150.1.1.1/32 [120/1] via 176.1.46.4, 00:00:06, Ethernet0/0
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The routing table looks good, and it looks like all routes from OSPF on R4 are being passed into RIP and are seen by R6. We should now have connectivity from R4 to BB1 (54.1.7.254).

  2. R4#ping 54.1.7.254

  4. Type escape sequence to abort.
  5. Sending 5, 100-byte ICMP Echos to 54.1.7.254, timeout is 2 seconds:
  6. !!!!!
  7. Success rate is 100 percent (5/5), round-trip min/avg/max = 48/48/52 ms
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Let’s do some summarization. We want to summarize at the 176.1.0.0/16 and 150.1.0.0/16 network boundaries on R4 before we send them to R6.
R4
  1. interface Ethernet0/0.46
  2. encapsulation dot1Q 46
  3. ip address 176.1.46.4 255.255.255.0
  4. ip summary-address rip 176.1.0.0 255.255.0.0
  5. ip summary-address rip 150.1.0.0 255.255.0.0

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R6:

  2. R6#clear ip route *
  3. R6#sh ip route
  4. Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  5.        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  6.        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  7.        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  8.        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  9.        ia - IS-IS inter area, * - candidate default, U - per-user static route
  10.        o - ODR, P - periodic downloaded static route

  12. Gateway of last resort is not set

  14.      54.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  15. C       54.1.7.0/24 is directly connected, Loopback1
  16. C       54.1.7.254/32 is directly connected, Virtual-Access1
  17. R    212.18.1.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  18. R    212.18.0.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  19. R    212.18.3.0/24 [120/1] via 54.1.7.254, 00:00:01, Virtual-Access1
  20.      176.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
  21. C       176.1.46.0/24 is directly connected, Ethernet0/0
  22. R       176.1.0.0/16 [120/1] via 176.1.46.4, 00:00:02, Ethernet0/0
  23. R    212.18.2.0/24 [120/1] via 54.1.7.254, 00:00:00, Virtual-Access1
  24.      150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
  25. C       150.1.6.0/24 is directly connected, Loopback0
  26. R       150.1.0.0/16 [120/2] via 176.1.46.4, 00:00:02, Ethernet0/0
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You can see above that the routes have been summarized correctly on R6. This summarization will in turn be passed onto BB1 and connectivity from R4 will be maintained because of this.
So far so good. Okay now for the fun part. R6’s only connection to the rest of the network is via R4. Let’s send a default-route to R6 from R4. But this isn’t an ordinary default route. We want to make sure that R6 doesn’t pass on that default-route it learns from R4 onto BB1. BB1 should still get the 176.1.0.0/16 and 150.1.0.0/16 summaries though. Sounds simple. Oh and we can only do the configuration on R4 (Evil mock lab writers!).
Let’s try this:
R4:

  2. router rip
  3. default-information originate route-map DEFAULT
  4. !
  5. route-map DEFAULT permit 10
  6. set metric 15
  7. !
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On R4, we are generating a default route with a route-map attached. The route-map forces a metric of 15 on the default route. This way when R6 gets it, it should have a metric of 15. RIP’s maximum metric is 16 so when BB1, gets this route it will be marked as invalid (ie not accept the route).
Let’s verify this on R6:

  2. R6#sh ip route
  3. Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  4.        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  5.        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  6.        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  7.        i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
  8.        ia - IS-IS inter area, * - candidate default, U - per-user static route
  9.        o - ODR, P - periodic downloaded static route

  11. Gateway of last resort is 176.1.46.4 to network 0.0.0.0

  13.      54.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  14. C       54.1.7.0/24 is directly connected, Loopback1
  15. C       54.1.7.254/32 is directly connected, Virtual-Access1
  16. R    212.18.1.0/24 [120/1] via 54.1.7.254, 00:00:10, Virtual-Access1
  17. R    212.18.0.0/24 [120/1] via 54.1.7.254, 00:00:10, Virtual-Access1
  18. R    212.18.3.0/24 [120/1] via 54.1.7.254, 00:00:10, Virtual-Access1
  19.      176.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
  20. C       176.1.46.0/24 is directly connected, Ethernet0/0
  21. R       176.1.0.0/16 [120/15] via 176.1.46.4, 00:00:13, Ethernet0/0
  22. R    212.18.2.0/24 [120/1] via 54.1.7.254, 00:00:11, Virtual-Access1
  23.      150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
  24. C       150.1.6.0/24 is directly connected, Loopback0
  25. R       150.1.0.0/16 [120/15] via 176.1.46.4, 00:00:13, Ethernet0/0
  26. R*   0.0.0.0/0 [120/15] via 176.1.46.4, 00:00:15, Ethernet0/0
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  1. R6#debug ip rip
  2. RIP protocol debugging is on
  3. R6#cle
  4. 5d20h: RIP: received v2 update from 176.1.46.4 on Ethernet0/0
  5. 5d20h:      0.0.0.0/0 via 0.0.0.0 in 15 hops
  6. 5d20h:      150.1.0.0/16 via 0.0.0.0 in 15 hops
  7. 5d20h:      176.1.0.0/16 via 0.0.0.0 in 15 hops
  8. 5d20h: RIP: sending v2 flash update to 224.0.0.9 via Virtual-Access1 (54.1.7.6)
  9. 5d20h: RIP: build flash update entries
  10. 5d20h:  0.0.0.0/0 via 0.0.0.0, metric 16, tag 0
  11. 5d20h:  54.1.7.0/24 via 0.0.0.0, metric 1, tag 0
  12. 5d20h:  150.1.0.0/16 via 0.0.0.0, metric 16, tag 0
  13. 5d20h:  176.1.0.0/16 via 0.0.0.0, metric 16, tag 0
  14. 5d20h:  176.1.46.0/24 via 0.0.0.0, metric 1, tag 0
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Looks like we got the default route with a metric of 15, and we are sending it on the BB1 with a metric of 16. Looks great, but take a look closer.
The Mock Lab 6 solution guide, presents the answer above. Now, I don’t really like this solution because check out the metrics for the 176.1.0.0/16 and the 150.1.0.0/16 routes. They are 15 on R6 as well! Why? Well think about how rip chooses a metric when it summarizes a route. The Cisco documentation states “RIPv2 route summarization requires that the lowest metric of the “best route” of an aggregated entry, or the lowest metric of all current child routes, be advertised. The best metric for aggregated summarized routes is calculated at route initialization or when there are metric modifications of specific routes at advertisement time, and not at the time the aggregated routes are advertised.”
So when RIPv2 summarizes a route, it choses the lowest metric of all the routes that it is summarizing and then advertises that as the metric. So why do we have the 176.1.0.0/16 and 150.1.0.0/16 networks advertised as 15? Well, what I think is happening is for some crazy reason IOS considers the 0.0.0.0/0 route a child of these summaries, so they get its metric. Crazy! I don’t think that should happen, but okay. When I change the metric on the default route, these summaries will get that as well. The only reason I can think of for this is because the default route is generated first, then the summary-addresses are calculated.
So how do we fix this? Well let’s try this:
R4:
  1. router rip
  2. default-information originate
  3. offset-list DEFAULT out 14 Ethernet0/0.46
  4. !
  5. ip access-list standard DEFAULT
  6. permit 0.0.0.0

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We’ve changed the default-information originate to one with a route-map and then we’ve applied an offset-list to change the metric to 14 of that default route. “Hang on a sec Arden, didn’t you just say that whatever you change the default route metric it will change the summary route metric as well?”. Let’s take a look at R4:

  2. R4#debug ip rip
  3. RIP protocol debugging is on
  4. R4#clear ip route *
  5. 5d20h: RIP: sending v2 flash update to 224.0.0.9 via Ethernet0/0.46 (176.1.46.4)
  6. 5d20h: RIP: build flash update entries
  7. 5d20h:  0.0.0.0/0 via 0.0.0.0, metric 15, tag 0
  8. 5d20h:  150.1.0.0/16 via 0.0.0.0, metric 2, tag 0
  9. 5d20h:  176.1.0.0/16 via 0.0.0.0, metric 1, tag 0
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Cisco Network Engineering
Email: khmerccie@gmail.com
CCIE Routing&Switching #26884
CCNA,CCNP,JNCIS-ER,JNCIA-ER
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