Auto-Summary Question

[up2thetime]

Hey everyone,

I was wondering if someone would clarify "no auto-summary" for me.

I have set up a lab in Packet Tracer and trying to see it for myself.

My question is, if RIPv2 sends subnet masks with advertisements, then why do we need auto summary?

Here is the setup:

192.168.1.0/24---RouterA---192.168.2.0/30---RouterB---192.168.3.0/30

When RIP is enabled on both routers, RouterA has this in its Routing Table:

R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:04, Serial0/0/0


So I went over to RouterB and enabled no auto-summary

I went back to RouterA and now the routing table has this:

192.168.3.0/30 is subnetted, 1 subnets
R 192.168.3.0 [120/1] via 192.168.2.2, 00:00:12, Serial0/0/0

To me, that seems like the no auto-summary command caused RouterB to send the /30 subnet mask along with the network it advertised.

My confusion comes from knowing that one of the features of RIPv2 is that it sends the mask along with the advertisement. However, in my test lab it seems like the addition of no auto-summary caused it to send the correct mask along with the advertisement.

Thanks in advance.

[Forsaken_GA]

By default, rip acts as a classful routing protocol, which means it will summarize on classful boundaries unless you tell it not to.

In your first example, with auto-summary on, 192.168.3.0 is a classful network, a class C, hence it's being advertised with a /24 mask.

When you turned it off, it advertised it as a /30, which tells me that the interface is configured with a 255.255.255.252 mask.

If auto-summary is on, you shouldn't be seeing the /30 mask.

As for why it's needed, it's really not needed. But if you're designing your network along class boundaries, it's useful. You do want to summarize when possible, it keeps your routing tables small

[up2thetime]

So RIPv2 still sends the subnet mask, but it by default sends the classful mask unless we say no auto-summary?

Telling the router no auto-summary actually tells the router to send the subnet mask that the network is configured with (ie. the subnet mask we chose to use when designing the network), and not the default subnet mask for this network address based on Class A, B, or C?

Correct?

[Forsaken_GA]

Exactly right!

EIGRP behaves the same way.

OSPF, otoh, does not auto-summarize by default

[up2thetime]

Originally Posted by Forsaken_GA

Exactly right!

EIGRP behaves the same way.

OSPF, otoh, does not auto-summarize by default

Hey thanks for clearing that up! Appreciate it!

[up2thetime]

Originally Posted by up2thetime

192.168.1.0/24---RouterA---192.168.2.0/30---RouterB---192.168.3.0/30

Just as an example from my diagram:


If we have auto summary on, when PC1 on the 192.168.1.0/24 network (left side of RouterA) wants to ping the IP Address 192.168.3.10, will RouterA send it out to RouterB? I believe it will do this because, since we have auto summary on, RouterA believes that 192.168.3.0/24 can be reached via RouterB. It believes RouterB has the route to 192.168.3.1 - 192.168.3.255

However, if we turn no auto-summary on, RouterA will receive the message destined for 192.168.3.10 and realize RouterB only has the route to 192.168.3.1, 192.168.3.2, and 192.168.3.3 (for broadcasts).

Therefore RouterA drops the packet (assume no gateway of last resort on RouterA).

Is that right?

[Forsaken_GA]

Exactly. This is why discontiguous subnets and classful routing protocols don't mix

[up2thetime]

Originally Posted by Forsaken_GA

Exactly. This is why discontiguous subnets and classful routing protocols don't mix

Awesome! I understand. Thank you again!