What does auto summary do for rip v2?

workfrom925workfrom925 Member Posts: 196
in CCNA & CCENT
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.
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    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.
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  • leonlimsgleonlimsg Member Posts: 10 ■□□□□□□□□□
    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.
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  • dsgmdsgm Member Posts: 228 ■■■□□□□□□□
    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.
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  • workfrom925workfrom925 Member Posts: 196
    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?
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  • JoshyJJoshyJ Member Posts: 32 ■■□□□□□□□□
    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?
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  • workfrom925workfrom925 Member Posts: 196
    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.
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  • MerlinmagMerlinmag Registered Users Posts: 1 ■□□□□□□□□□
    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 .

    :)
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  • Llukman1Llukman1 Member Posts: 21 ■□□□□□□□□□
    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?
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