Route Redistribution - Part 1

My goal in the following lab is to test the options and features while redistributing routing protocols.

 

Here is  the current topology:

1.       Virtual-link between area 0 and area 68, we need to configure the virtual link over area 36 because it is the transit between 68 and 0, also note that the virtual link is done using the router-id and not the IP.

R3: router ospf 1  area 36 virtual-link 6.6.6.6 R6: router ospf 1  area 36 virtual-link 6.6.6.6

2.       Area 17 has been configured as a totally stub area which means no LSA types other then 1 and 2

R7: router ospf 1  area 17 stub no-summary R1: router ospf 1  area 17 stub no-summary

3.       R4 is advertising 192.168.41.0/24 into RIP, R7 will redistribute it into OSPF Area 17

First we will create prefix-list and route-map to limit the exact networks we are going to redistribute:

R7: Ip prefix-list PL_NET41 permit 192.168.41.0/24 ! Route-map RM_RIP_TO_OSPF permit 10 Match ip address prefix-list PL_NET41 ! Route-map RM_RIP_TO_OSPF deny 100 !

Then we will using this route-map to redistribute RIP into OSPF:

R7: Router ospf 1 Redistribute rip subnets route-map RM_RIP_TO_OSPF !

And we got the following error message:

*Mar  3 11:43:48.353: %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area

The reason is that Area 17 is a stub area hence he can’t advertise LSA type 5 (external routes), so let’s convert area 17 into an NSSA area which eventually allow us LSA type 1,2 and convert type 5 into type 7:

R7: Router ospf 1  no area 17 stub no-summary  no area 17 stub  area 17 nssa no-summary ! R1: Router ospf 1  no area 17 stub no-summary  no area 17 stub  area 17 nssa no-summary !

Note that the first no command only remove the summary, the second no command remove the stub.

Now although area 17 is stub network he can advertise external routes into OSPF domain

R1: R1#sh ip route ospf      2.0.0.0/32 is subnetted, 1 subnets O       2.2.2.2 [110/129] via 10.1.13.3, 00:02:55, Serial0/0.13      3.0.0.0/32 is subnetted, 1 subnets O       3.3.3.3 [110/65] via 10.1.13.3, 00:02:55, Serial0/0.13      6.0.0.0/32 is subnetted, 1 subnets O IA    6.6.6.6 [110/75] via 10.1.13.3, 00:02:45, Serial0/0.13 O N2 192.168.41.0/24 [110/20] via 10.1.17.7, 00:02:45, FastEthernet0/1      7.0.0.0/32 is subnetted, 1 subnets O       7.7.7.7 [110/2] via 10.1.17.7, 00:02:45, FastEthernet0/1      8.0.0.0/32 is subnetted, 1 subnets O IA    8.8.8.8 [110/85] via 10.1.13.3, 00:02:45, Serial0/0.13      192.168.81.0/32 is subnetted, 1 subnets O IA    192.168.81.1 [110/85] via 10.1.13.3, 00:02:45, Serial0/0.13      192.168.83.0/32 is subnetted, 1 subnets O IA    192.168.83.1 [110/85] via 10.1.13.3, 00:02:45, Serial0/0.13      10.0.0.0/24 is subnetted, 6 subnets O       10.1.23.0 [110/128] via 10.1.13.3, 00:02:55, Serial0/0.13 O IA    10.1.36.0 [110/74] via 10.1.13.3, 00:02:45, Serial0/0.13 O IA    10.1.68.0 [110/84] via 10.1.13.3, 00:02:46, Serial0/0.13      192.168.82.0/32 is subnetted, 1 subnets O IA    192.168.82.1 [110/85] via 10.1.13.3, 00:02:46, Serial0/0.13      192.168.71.0/32 is subnetted, 1 subnets O       192.168.71.1 [110/2] via 10.1.17.7, 00:02:46, FastEthernet0/1

Note that the network 192.168.41.0/24 is marked with N2 which means it has learned via NSSA area external type 2.

External type 2 means a route with a calculated metric, to this network, from the advertised router perspective - R7, which is the ASBR in this case. This type along with metric 20 are defaults for OSPF and we can change them while issue the command redistribute under OSPF process.

4.       Now let’s redistribute network 192.168.61.0/24 into RIP so we could ping network 192.168.41.0 from 192.168.61.0, the first problem that we will encounter is the fact the area 17 is totally NSSA area and he doesn’t know network 192.168.61.0. Let’s convert area 17 into NSSA area only:

R7: Router ospf 1  no area 17 nssa no-summary ! R1: Router ospf 1  no area 17 nssa no-summary !

Configure prefix-list and route-map so we could control what we are advertising:

R7: Ip prefix-list PL_NET61 permit 192.168.61.0/24 ! Route-map RM_OSPF_TO_RIP permit 10 Match ip address prefix-list PL_NET61 ! Route-map RM_OSPF_TO_RIP deny 100 !

And redistribute OSPF into RIP:

R7: Router rip  redistribute ospf 1 route-map RM_OSPF_TO_RIP metric 5 !

Note that RIP doesn’t have default metric, which calculated based on hop counts, so if we wouldn’t specific the metric the network will be down.

While trying to redistribute 192.168.61.0/24 into RIP I have encounter a very common error,

In this lab I’m using loopback interfaces to simulate LAN segments, OSPF doesn’t advertise loopback with their correct prefix hence prefix-list PL_NET61 wouldn’t be able to catch this specific network and nothing will be advertised:

R7#sh ip prefix-list detail Prefix-list with the last deletion/insertion: PL_NET61 ip prefix-list PL_NET41:    count: 1, range entries: 0, sequences: 5 - 5, refcount: 2    seq 5 permit 192.168.41.0/24 (hit count: 3, refcount: 1) ip prefix-list PL_NET61:    count: 1, range entries: 0, sequences: 5 - 5, refcount: 3    seq 5 permit 192.168.61.0/24 (hit count: 0, refcount: 1) R7#sh ip route 192.168.61.0   Routing entry for 192.168.61.0/32, 1 known subnets   Redistributing via rip O IA    192.168.61.1 [110/76] via 10.1.17.1, 00:06:15, FastEthernet0/0

In order to fix that we need to configure under any loopback the following command:

R6: interface loopback 1  ip ospf network point-to-point !

This will allow OSPF to advertise the network with the correct prefix.

R7: R7#sh ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "ospf 1", distance 110, metric 76, type inter area   Redistributing via rip   Advertised by rip metric 5 route-map RM_OSPF_TO_RIP   Last update from 10.1.17.1 on FastEthernet0/0, 00:00:48 ago   Routing Descriptor Blocks:   * 10.1.17.1, from 1.1.1.1, 00:00:48 ago, via FastEthernet0/0       Route metric is 76, traffic share count is 1!

To verify the result:

R6#ping 192.168.41.1 source lo1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.41.1, timeout is 2 seconds: Packet sent with a source address of 192.168.61.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max =24/52/92 ms R6#ping 192.168.41.1           Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.41.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)

Only when ping network 192.168.41.0 with source 192.168.61.0 we are getting replies.

5.       Now let’s redistribute 192.168.41.0/24 into EIGRP AS100:

R4: Ip prefix-list PL_NET41 permit 192.168.41.0/24 ! Route-map RM_CONNECTED_TO_EIGRP permit 10 Match ip address prefix-list PL_NET41 ! Route-map RM_CONNECTED_TO_EIGRP deny 100 ! Router eigrp 100 redistribute connected route-map RM_CONNECTED_TO_EIGRP !

Note that we are redistributing connected and not rip as network 192.168.41.0/24 is connected in R4.

Now network 192.168.41.0/24 is known by R5 as external EIGRP route

R5#sh ip route eigrp      4.0.0.0/32 is subnetted, 1 subnets D       4.4.4.4 [90/156160] via 10.1.45.4, 01:18:26, FastEthernet0/1 D EX 192.168.41.0/24 [170/156160] via 10.1.45.4, 00:01:36, FastEthernet0/1      10.0.0.0/24 is subnetted, 4 subnets D       10.1.14.0 [90/284160] via 10.1.45.4, 01:18:26, FastEthernet0/1

So now let’s redistribute network 192.168.41.0/24 from EIGRP into OSPF domain on R2:

R2: Ip prefix-list PL_NET41 permit 192.168.41.0/24 ! Route-map RM_EIGRP_TO_OSPF permit 10 Match ip address prefix-list PL_NET41 ! Route-map RM_EIGRP_TO_OSPF deny 100 ! Router ospf 1 redistribute eigrp 100 subnets route-map RM_EIGRP_TO_OSPF !

Again always control what you are advertising.

Network 192.168.61.0/24 is learned via OSPF so we will need to redistribute it to EIGRP on R2:

R2: Ip prefix-list PL_NET61 permit 192.168.61.0/24 ! Route-map RM_OSPF_TO_EIGRP permit 10 Match ip address prefix-list PL_NET61 ! Route-map RM_OSPF_TO_EIGRP deny 100 ! Router eigrp 100 redistribute ospf 1 route-map RM_OSPF_TO_EIGRP metric 100000 10 255 1 1500 !

Very similar to RIP, EIGRP must be redistributed with metric.

On R4 EIGRP is learn network 192.168.41.0/24 from RIP so we need to redistribute network 192.168.61.0/24 into EIGRP so we can have complete return path:

R4: Ip prefix-list PL_NET61 permit 192.168.61.0/24 ! Route-map RM_EIGRP_TO_RIP permit 10 Match ip address prefix-list PL_NET61 ! Route-map RM_EIGRP_TO_RIP deny 100 ! Router rip redistribute eigrp 100 route-map RM_EIGRP_TO_RIP !

So now after advertising network 192.168.61.0/24 all over the network let’s examine who learned what and from where.

R6 is self-originate for this network

R3 is learning this network through OSPF from R6:

R3#show ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "ospf 1", distance 110, metric 11, type intra area   Last update from 10.1.36.6 on FastEthernet0/0, 05:42:37 ago   Routing Descriptor Blocks:   * 10.1.36.6, from 6.6.6.6, 05:42:37 ago, via FastEthernet0/0

R1 and R2 learn this network through OSPF from R3:

R1#show ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "ospf 1", distance 110, metric 75, type inter area   Last update from 10.1.13.3 on Serial0/0.13, 00:07:58 ago   Routing Descriptor Blocks:   * 10.1.13.3, from 3.3.3.3, 00:07:58 ago, via Serial0/0.13       Route metric is 75, traffic share count is 1

R2 is also redistributing this network to EIGRP.

R5 is learning this network through EIGRP from R2:

R5# show ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "eigrp 100", distance 170, metric 284160, type external   Redistributing via eigrp 100   Last update from 10.1.25.2 on FastEthernet0/0, 00:15:43 ago   Routing Descriptor Blocks:   * 10.1.25.2, from 10.1.25.2, 00:15:43 ago, via FastEthernet0/0       Route metric is 284160, traffic share count is 1       Total delay is 1100 microseconds, minimum bandwidth is 10000 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 1

R4 is learning this network from two sources: EIGRP through R5 and RIP through R7, but in the routing table the preferred route is through R7 as it got a better AD (RIP AD120, EIGRP External AD170):

R4#show ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "rip", distance 120, metric 5   Redistributing via rip   Last update from 10.1.47.7 on FastEthernet0/1, 00:00:25 ago   Routing Descriptor Blocks:   * 10.1.47.7, from 10.1.47.7, 00:00:25 ago, via FastEthernet0/1       Route metric is 5, traffic share count is 1

And last R7 which learn this network through OSPF from R1:

R7#sh ip route 192.168.61.0 Routing entry for 192.168.61.0/24   Known via "ospf 1", distance 110, metric 76, type inter area   Redistributing via rip   Advertised by rip metric 5 route-map RM_OSPF_TO_RIP   Last update from 10.1.17.1 on FastEthernet0/0, 00:16:40 ago   Routing Descriptor Blocks:   * 10.1.17.1, from 1.1.1.1, 00:16:40 ago, via FastEthernet0/0       Route metric is 76, traffic share count is 1

Now I like that R4 will prefer the route to 192.168.61.0/24 through R5 and not through R7:

R4: ip access-list standard 61 permit 192.168.61.0 0.0.0.255 ! Router rip distance 180 10.1.47.7 0.0.0.0 61 !

Using the distance command I can raise up the AD value of specific routes received from R7, in this case network 192.168.61.0/24 which will received from R7 will be with an AD of 180 which be higher then EIGRP external (AD 170).