I know you may know about Auto Summary, but there are very few documents about Auto Summary. I decided to describe it in simple words. I saw that many CCNA and CCNP students face difficulties in the same way. Let's check the automatic summary in simple words:
RIP Version 2 supports automatic route summarization by default. The software summarizes subprefixes to the classful network boundary when classful network boundaries are crossed.
If you have disconnected subnets, disable automatic route summarization to advertise the subnets. When route summarization is disabled, the software sends subnet and host routing information across classful network boundaries. To disable automatic summarization, use the noauto-summary command in router configuration mode.
Let's Understand with an example:
There are a few loopback interfaces on the R1 as:
R1#sho ip route | in C Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP C 10.10.1.0/24 is directly connected, Loopback0 C 10.10.2.0/24 is directly connected, Loopback1 C 10.10.3.0/24 is directly connected, Loopback2 C 10.10.4.0/24 is directly connected, Loopback3 C 10.10.254.0/24 is directly connected, FastEthernet0/0 C 22.214.171.124/24 is directly connected, Loopback10 C 126.96.36.199/24 is directly connected, Loopback11 C 188.8.131.52/24 is directly connected, Loopback12 C 184.108.40.206/24 is directly connected, Loopback13 R1#
As you can see there are few loopback interfaces on subnet 10.10.x.0 / 24 and some loopback interfaces are on subnet 11.10.x.0 / 24
Let check Routing protocol configuration details:
R1#sho ip protocols *** IP Routing is NSF aware ***
Routing Protocol is "rip" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Sending updates every 30 seconds, next due in 26 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Redistributing: rip Default version control: send version 2, receive version 2 Interface Send Recv Triggered RIP Key-chain FastEthernet0/0 2 2 Loopback0 2 2 Loopback1 2 2 Loopback2 2 2 Loopback3 2 2 Loopback10 2 2 Loopback11 2 2 Loopback12 2 2 Loopback13 2 2 Automatic network summarization is in effect Maximum path: 4 Routing for Networks: 10.0.0.0 220.127.116.11 Routing Information Sources: Gateway Distance Last Update 10.10.254.2 120 00:00:09 Distance: (default is 120)
As you can see, the automatic summary is in effect. But can you imagine the R2 routing table? Will both 10.10.x.0 / 24 and 11.10.x.0 / 24 be summarized on route 10.0.0./8 and 18.104.22.168/8? let's check:
R2#sho ip route rip
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks R 10.10.1.0/24 [120/1] via 10.10.254.1, 00:00:01, FastEthernet0/0 R 10.10.2.0/24 [120/1] via 10.10.254.1, 00:00:01, FastEthernet0/0 R 10.10.3.0/24 [120/1] via 10.10.254.1, 00:00:01, FastEthernet0/0 R 10.10.4.0/24 [120/1] via 10.10.254.1, 00:00:01, FastEthernet0/0 R 22.214.171.124/8 [120/1] via 10.10.254.1, 00:00:01, FastEthernet0/0 R2#
Than? Why only 126.96.36.199/24 summary route in the table? All other 10.10.x.0 / 24 routes are classless that is advertised from R1. Before we get the answer, let's check the route on R3:
R3#sho ip route rip
Gateway of last resort is not set
R 10.0.0.0/8 [120/1] via 172.16.19.1, 00:00:03, FastEthernet1/0 R 188.8.131.52/8 [120/2] via 172.16.19.1, 00:00:03, FastEthernet1/0 R3#
ohhh.. Both routes are summarized on the R3? Why those are not summarized in R2?
As per definition "The software summarizes subprefixes to the classful network boundary when classful network boundaries are crossed." So a link between R1 and R2 (10.10.254.0/24) is still falling in classful network boundary of 10.0.0./8 network as it did not cross the boundary yet. It is summarized on the R3 because it crossed the classful boundary network.
Let add one more loopback in the R2:
R2#sho ip route | in Loopback0 C 184.108.40.206/24 is directly connected, Loopback0 L 220.127.116.11/32 is directly connected, Loopback0 R2#
Now what will be an impact on R1, R2, and R3 routing table:
Let's check on the RIP debug on R2:
*May 24 19:35:39.705: RIP: received v2 update from 10.10.254.1 on FastEthernet0/0 *May 24 19:35:39.705: 10.10.1.0/24 via 0.0.0.0 in 1 hops *May 24 19:35:39.709: 10.10.2.0/24 via 0.0.0.0 in 1 hops *May 24 19:35:39.709: 10.10.3.0/24 via 0.0.0.0 in 1 hops *May 24 19:35:39.709: 10.10.4.0/24 via 0.0.0.0 in 1 hops *May 24 19:35:39.709: 18.104.22.168/8 via 0.0.0.0 in 1 hops
R1 is sending a route 11.0.0./8 to R2, so R2 will install the route in the routing table because there is no other better route for 11.0.0 / 8 in the routing table.
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