RIPv2 - Offset-list, distribute-list and AD

Here is the network diagram:

R4#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router rip
R4(config-router)#network 172.16.10.0
R4(config-router)#network 172.16.20.0
R4(config-router)#network 172.16.30.0
R4(config-router)#network 172.16.40.0
R4(config-router)#network 172.16.50.0


RIPv2 will summarize contiguous networks to their classful boundaries although no auto-summary is configured.


the result:
R4#show running-config | s rip
router rip
 version 2
 network 10.0.0.0
 network 172.16.0.0
 no auto-summary


Also the RIP database shows the following:
R4#show ip rip database
10.0.0.0/8    auto-summary
10.0.15.0/24
    [1] via 10.0.45.5, 00:00:20, FastEthernet1/0
10.0.24.0/24    directly connected, FastEthernet0/0
10.0.34.0/24    directly connected, FastEthernet0/1
10.0.45.0/24    directly connected, FastEthernet1/0
10.0.123.0/24
    [1] via 10.0.34.3, 00:00:13, FastEthernet0/1
    [1] via 10.0.24.2, 00:00:06, FastEthernet0/0
172.16.0.0/16    auto-summary
172.16.10.0/24    directly connected, Loopback1
172.16.20.0/24    directly connected, Loopback2
172.16.30.0/24    directly connected, Loopback3
172.16.40.0/24    directly connected, Loopback4
172.16.50.0/24    directly connected, Loopback5
192.168.10.0/24    auto-summary
192.168.10.0/24
    [2] via 10.0.45.5, 00:00:20, FastEthernet1/0
    [2] via 10.0.34.3, 00:00:13, FastEthernet0/1
    [2] via 10.0.24.2, 00:00:06, FastEthernet0/0
192.168.20.0/24    auto-summary
192.168.20.0/24
    [2] via 10.0.45.5, 00:00:20, FastEthernet1/0
<Output omitted>

So now we will use distribute-list in order to decide what and to whom we will advertise networks, for example:

R4(config)#ip access-list standard 10 

R4(config)#permit 172.16.10.0 0.0.0.255

R4(config)#permit 172.16.20.0 0.0.0.255

then add a distribute-list on router configuration, outbound interface toward R5 (Fa1/0):

router rip
 version 2
 network 10.0.0.0
 network 172.16.0.0
 distribute-list 10 out FastEthernet1/0
 no auto-summary

the result:

R5#sh ip route     
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 2 subnets
R       172.16.20.0 [120/1] via 10.0.45.4, 00:00:00, FastEthernet0/0
R       172.16.10.0 [120/1] via 10.0.45.4, 00:00:00, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.45.0 is directly connected, FastEthernet0/0

R5 will have only 172.16.10.0/24 and 172.16.20.0/24 from R4.

Now with extended ACL the things are little bit different,  first RIP will accept only numeric extended ACL's and the treatment to extended ACL is as follow:
1. The first host/network is reference as the next-hop
2. The second host/network is referene as the advertised network

For example:
R1#show ip access-lists 111
Extended IP access list 111
    10 permit ip host 10.0.123.2 172.16.10.0 0.0.0.255
    20 permit ip host 10.0.123.2 172.16.20.0 0.0.0.255
    30 permit ip host 10.0.123.3 172.16.30.0 0.0.0.255
    40 permit ip host 10.0.123.3 172.16.40.0 0.0.0.255
    50 permit ip any 172.16.50.0 0.0.0.255

this ACL instruct to receive 172.16.10.0/24 and 172.16.20.0/24 only if the next-hop is 10.0.123.2 and 172.16.30.0/24 and 172.16.40.0/24 only if the next-hop is 10.0.123.3.
Network 172.16.50.0/24 will be accepted from any source and all other RIP updates will deny.

Configuring this extended ACL as R1 inbound filter:

R1#show running-config | s rip
router rip
 version 2
 network 10.0.0.0
 network 192.168.10.0
 network 192.168.20.0
 network 192.168.30.0
 network 192.168.40.0
 network 192.168.50.0
 distribute-list 111 in
 no auto-summary

the result:

R1#show ip route rip
     172.16.0.0/24 is subnetted, 5 subnets
R       172.16.50.0 [120/7] via 10.0.123.3, 00:00:25, FastEthernet0/0
                    [120/7] via 10.0.123.2, 00:00:22, FastEthernet0/0
                    [120/7] via 10.0.15.5, 00:00:17, FastEthernet0/1
R       172.16.40.0 [120/7] via 10.0.123.3, 00:00:25, FastEthernet0/0
R       172.16.30.0 [120/7] via 10.0.123.3, 00:00:25, FastEthernet0/0
R       172.16.20.0 [120/7] via 10.0.123.2, 00:00:22, FastEthernet0/0
R       172.16.10.0 [120/7] via 10.0.123.2, 00:00:22, FastEthernet0/0

Now while RIP allows up to 16 equal-cost paths he install by default only 4, the load-balance between the paths is based on the switching method and on most routers using fast-swicthing or CEF it will use per-destination basis.

Note that network 172.16.50.0/24 is seen from both R2, R3 and R5, now let's say we want to prefer R5 as the next-hop to this network. For this case we will use offset-list.

First configure standard ACL for identify the network:
R1#show ip access-lists 50
Standard IP access list 50
    10 permit 172.16.50.0, wildcard bits 0.0.0.255

Then configure the offset-list in RIP:
R1(config)#do sh run | s rip
router rip
 version 2
 offset-list 50 in 5 FastEthernet0/0
 network 10.0.0.0
 network 192.168.10.0
 network 192.168.20.0
 network 192.168.30.0
 network 192.168.40.0
 network 192.168.50.0
 distribute-list 111 in
 no auto-summary


I have configured offset-list 50, which refer to ACL 50, with offset of 5 - hence add 5 to the metric received from interface FastEthernet 0/0 for network 172.16.50.0/24

the result:
R1#show ip route rip
     172.16.0.0/24 is subnetted, 5 subnets
R       172.16.50.0 [120/2] via 10.0.15.5, 00:00:08, FastEthernet0/1
R       172.16.40.0 [120/2] via 10.0.123.3, 00:00:12, FastEthernet0/0
R       172.16.30.0 [120/2] via 10.0.123.3, 00:00:12, FastEthernet0/0
R       172.16.20.0 [120/2] via 10.0.123.2, 00:00:14, FastEthernet0/0
R       172.16.10.0 [120/2] via 10.0.123.2, 00:00:14, FastEthernet0/0

We can also use the offset-list to add hop metric for advertised networks as well.

Another method for controlling RIP in/out routes is using the administrative-distance, note that i have removed the offset-list from the previous example:
R1(config-router)#distance 90 10.0.15.5 0.0.0.0 50

this will change the administrative distance for network 172.16.50.0/24, which defined by ACL 50, and received by source host 10.0.15.5 which is R5

the result:
R1#sh ip route rip
     172.16.0.0/24 is subnetted, 5 subnets
R       172.16.50.0 [90/2] via 10.0.15.5, 00:00:02, FastEthernet0/1
R       172.16.40.0 [120/2] via 10.0.123.3, 00:00:01, FastEthernet0/0
R       172.16.30.0 [120/2] via 10.0.123.3, 00:00:01, FastEthernet0/0
R       172.16.20.0 [120/2] via 10.0.123.2, 00:00:01, FastEthernet0/0
R       172.16.10.0 [120/2] via 10.0.123.2, 00:00:01, FastEthernet0/0

RIPv2 is simple and easy to configure dynamic routing protocol which can be used even today in small networks or when there is demand to support customer's CPE like D-Link, Linksys etc.