For example I have
192.168.48.0/24 - 192.168.51.0/24 i used a 22- bit!
Under normal circumstances, you could simply summarize the four consecutive class-C networks with the ip summary address rip command on the R1 serial 0/0/0 interface. However, the RIP implementation in the Cisco IOS Software does not allow summarizing to a mask length that is less than the classful network prefix (in this case, 24 bits). This limitation does not affect other routing protocols. If you do try, you receive the following error message:
for example :
R1(config)# interface serial 0/0/0
R1(config-if)# ip summary-address rip 192.168.48.0 255.255.252.0
Summary mask must be greater or equal to major net
To get around the ip summary-address rip message error, create a static route on R1 to summarize the networks of loopbacks 48 through 51. Then redistribute the route on R1.
R1(config)# ip route 192.168.48.0 255.255.252.0 null0
R1(config)# router rip
R1(config-router)# redistribute static
This solution might seem unusual, but for RIPv2, it resembles many effects of summarization as performed in other routing protocols like EIGRP or OSPF. Again, this is not a limitation of RIPv2, but rather a Cisco IOS implementation issue.
Sometimes you might not want to advertise certain networks out a particular interface, or you might want to filter updates as they come in. This is possible with distance-vector routing protocols, such as RIP or EIGRP. However, link-state protocols are less flexible, because every router in an area is required to have a synchronized database as a condition for full adjacency.
Distribute lists can be used with either access lists or prefix lists to filter routes by network address. With prefix lists, they can also be configured to filter routes by subnet masks.
In this scenario, you want to filter updates from R1 to R2, allowing only the networks of Loopback 0 and Loopback 70 and the summary route to be advertised. You want to suppress the more specific prefixes so that routing tables are kept small, and CPU processor cycles on the routers are not wasted.
The 22-bit summary and the 24-bit major network address both have the same address, so access lists will not accomplish the filtering correctly. Therefore, it is necessary to use prefix lists.
To create a prefix list or add a prefix list entry, use the ip prefix-list command in global configuration mode.
ip prefix-list {list-name | list-number} {deny network/length | permit network/length} [ge ge-length] [le le-length]
The ge keyword represents the “greater than or equal to” operator. The le keyword represents the “less than or equal to” operator. If both the ge and le keywords are omitted, the prefix list is processed using an exact match.
On R1, use a prefix list as a distribution filter to prevent the more specific routes to loopbacks 48 through 51 from being advertised. Allow all other destination networks, including the summary route.
R1(config)# ip prefix-list RIP-OUT permit 192.168.48.0/22
R1(config)# ip prefix-list RIP-OUT deny 192.168.48.0/22 le 24
R1(config)# ip prefix-list RIP-OUT permit 0.0.0.0/0 le 32
Line 1 of the prefix list permits the summary route and nothing else, because no other route can match that network address with a mask of exactly 22 bits.
Line 2 denies all prefixes with a network address in the 192.168.48.0/22 block of addresses that have subnet masks from 22 bits to 24 bits. This removes exactly four network addresses matching the 22, 23, and 24 bits in length of the subnet mask. Line 2 would deny the 192.168.48.0/22 summary route you created if Line 1 did not explicitly permit the summary route.
Line 3 allows all IPv4 prefixes that are not explicitly denied in previous statements of the prefix list.
From the RIP configuration prompt on R1, apply this access list with the distribute-list command.
R1(config)# router rip
R1(config-router)# distribute-list prefix RIP-OUT out serial0/0/0
On R2, verify that the filtering has taken place using the show ip route rip and show ip rip database commands.
Message was edited by: roberto siu
