What does auto summary do for rip v2?

I'm doing Cisco Packet Tracer lab 7.6.1 on RIPv2. It asks me to disable auto summary on routers. But with auto summary, all pings works fine. So why should I disable auto summary?

Delete '.doc' from the attached file name to run it in Packet Tracer. Thanks.

pka-83 lab 7.6.1 temp.pka.doc

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Comments

NetworkVeteran

You should disable it because the lab asked you to and you are trying to learn the commands. In the real-world, you would utilize your understanding of what auto-discovery does (as you may gain from reading about it plus doing this lab) to determine whether to enable or disable it on any particular network you're responsible for administering.

leonlimsg

With auto-summary enabled, all your routes will be automatically summarized into classful network.

For example, your router(R1) is connected to two LANS, 10.1.1.0/24 and 10.2.1.0/24, and R1 has to advertise these two networks to another router(R2). Because auto-summary is enabled on R1, R1 will auto-summarized these two networks into 10.0.0.0/8 and pass this routing information to R2.

R2 will receive a summarized route of 10.0.0.0/8 from R1. It will not enable to identify which network on R1 side is 10.1.1.0/24 or 10.2.1.0/24. Hence, ping will not be successful.

Hence, auto-summary is disabled by keying "no auto-summary". With no auto-summary, R1 will not summarize and advertise 10.1.1.0/24 and 10.2.1.0/24 to R2.

dsgm

leonlimsg wrote: »

With auto-summary enabled, all your routes will be automatically summarized into classful network.

For example, your router(R1) is connected to two LANS, 10.1.1.0/24 and 10.2.1.0/24, and R1 has to advertise these two networks to another router(R2). Because auto-summary is enabled on R1, R1 will auto-summarized these two networks into 10.0.0.0/8 and pass this routing information to R2.

R2 will receive a summarized route of 10.0.0.0/8 from R1. It will not enable to identify which network on R1 side is 10.1.1.0/24 or 10.2.1.0/24. Hence, ping will not be successful.

Hence, auto-summary is disabled by keying "no auto-summary". With no auto-summary, R1 will not summarize and advertise 10.1.1.0/24 and 10.2.1.0/24 to R2.

Can i tell you im glad you explained that, was just doing his topic myself, thanks a bunch, great explanation.

workfrom925

leonlimsg wrote: »

With auto-summary enabled, all your routes will be automatically summarized into classful network.

For example, your router(R1) is connected to two LANS, 10.1.1.0/24 and 10.2.1.0/24, and R1 has to advertise these two networks to another router(R2). Because auto-summary is enabled on R1, R1 will auto-summarized these two networks into 10.0.0.0/8 and pass this routing information to R2.

R2 will receive a summarized route of 10.0.0.0/8 from R1. It will not enable to identify which network on R1 side is 10.1.1.0/24 or 10.2.1.0/24. Hence, ping will not be successful.

Hence, auto-summary is disabled by keying "no auto-summary". With no auto-summary, R1 will not summarize and advertise 10.1.1.0/24 and 10.2.1.0/24 to R2.

Thanks for explaining. But I think with the summarized route 10.0.0.0/8, R2 would　send a ping to R1 as long as the ping is intened for an IP address inside 10.0.0.0/8. If the ping is intended for an IP inside either 10.1.1.0/24 or 10.2.1.0/24, R1 would forward along. And if the ping is not for either subnet, R2 would discard the ping. Am I correct?

JoshyJ

That is correct. But let's say that you had another router plugged into R1. Which also is connected to its own networks 10.4.4.0/24 and 10.5.5.0/24. It also has auto-summarized enabled. What will happen if R1 receives two routes for 10.0.0.0/8? Which way would R1 forward packets when trying to get to any of the networks?

workfrom925

JoshyJ wrote: »

That is correct. But let's say that you had another router plugged into R1. Which also is connected to its own networks 10.4.4.0/24 and 10.5.5.0/24. It also has auto-summarized enabled. What will happen if R1 receives two routes for 10.0.0.0/8? Which way would R1 forward packets when trying to get to any of the networks?

Then auto-summary needs to be disabled. Got it.

Merlinmag

JoshyJ wrote: »

That is correct. But let's say that you had another router plugged into R1. Which also is connected to its own networks 10.4.4.0/24 and 10.5.5.0/24. It also has auto-summarized enabled. What will happen if R1 receives two routes for 10.0.0.0/8? Which way would R1 forward packets when trying to get to any of the networks?

Thanks for the explanation .

Llukman1

Quick question guys:

When you said this " That is correct. But let's say that you had another router plugged into R1. Which also is connected to its own networks 10.4.4.0/24 and 10.5.5.0/24. It also has auto-summarized enabled. What will happen if R1 receives two routes for 10.0.0.0/8? Which way would R1 forward packets when trying to get to any of the networks?"

You are talking about discontiguous networks correct? and R1 is the discontiguous network?