• By default, RIPv2 and EIGRP perform automatic network summarization at classful boundaries, just like a classful protocol does.
  • The diffrence between these two protocols and their predecessors (RIPv1 and IGRP) is that you can turn off automatic summarization.
  • To turn off, use the router configuration command:

no auto-summary

• OSPF and IS-IS RIP and EIGRP perform automatic network summarization by default.

Example:

• The diagram above shows a RIPv2 network where autosummarization occurs.

Dynamips .net Configuration:

autostart = False
ghostios = true
sparsemem = true

[localhost]

    [[7200]]
        image = \Program Files\Dynamips\images\c7200-js-mz.124-3\C7200-JS.BIN
        # On Linux / Unix use forward slashes:
        # image = /opt/7200-images/c7200-jk9o3s-mz.124-7a.image
        npe = npe-400
        ram = 160

    [[ROUTER A]]
        S1/0 = B s1/0
        model = 7200
        console = 2001
        idlepc = 0x6082d7a0

    [[router B]]
        s1/1 = C s1/0
        model = 7200
        console = 2002
        idlepc = 0x607016a0

    [[router C]]
        model = 7200
        console = 2003
        idlepc = 0x607016a0

Router A Config:

!
interface FastEthernet0/0
 ip address 172.16.2.1 255.255.255.0
 duplex half
 no keepalive
!
interface Serial1/0
 ip address 172.16.1.1 255.255.255.0
 serial restart-delay 0
!
router rip
 version 2
 network 172.16.0.0

Router B Config:

!
interface Serial1/0
 ip address 172.16.1.2 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 192.168.5.2 255.255.255.0
 serial restart-delay 0
!
router rip
 version 2
 network 172.16.0.0
 network 192.168.5.0
!

Router C Config:

!
interface Serial1/0
ip address 192.168.5.1 255.255.255.0
serial restart-delay 0
!
router rip
version 2
network 192.168.5.0
!

• In the RIPv2 network above, Router B performs a defualt behavior of automatically summarizing the 172.16.1.0/24 and 172.16.2.0/24 networks learned from B’s connected subnet and A’s advertised subnet.

C# sh ip route
Gateway of last resort is not set

R    172.16.0.0/16 [120/1] via 192.168.5.2, 00:00:05, Serial1/0
C    192.168.5.0/24 is directly connected, Serial1/0

• In Router C’s routing table, notice that it, indeed, learns of a summarized route from it’s neighbor 192.168.5.2, which is Router B.
• A simple no auto-summary command on Router B, changes the routing table on Router C.

B(config)#router rip
B(config-router)#no auto-summary

• Now looking at Router C’s IP routing table, we see:

C# sh ip route
Gateway of last resort is not set

     172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
R       172.16.0.0/16 [120/1] via 192.168.5.2, 00:00:29, Serial1/0
R       172.16.1.0/24 [120/1] via 192.168.5.2, 00:00:00, Serial1/0
R       172.16.2.0/24 [120/2] via 192.168.5.2, 00:00:00, Serial1/0
C    192.168.5.0/24 is directly connected, Serial1/0

• Notice now that both 172.16.1.0/24 and 172.16.2.0/24 networks are advertised with both prefix and subnet mask.