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  1. Member
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    #1

    Default Auto Summarization

    Hey... confused about auto summary...

    If RIPv2 is a Classless Routing protocol.. that means that in addition to the network number, it also sends the subnet mask.... so why then is the no auto-summary still required. The fact that it sends the subnet mask along with the network number should achieve the same goal as the no auto-summary command... shouldn't it?

    I am looking at the ICND1 Official Guide 2nd Edition by Odom (on page 440), and he does not use the no auto summary command.... however, even though he does not use no auto summary, there is no auto-summarization happening... I don't get how this is happening...

    For those with the book... can you please have a look? This is driving me insane.


    Thank you...
    Last edited by new2net; 09-04-2009 at 03:29 AM.
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  3. Senior Member jovan88's Avatar
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    #2
    i think autosummary was put in RIPv2 and EIGRP to annoy the crap out of everyone.

    With autosummary on, it will send the subnet mask of the classful boundary.. With autosummary off it will send the real subnet mask.
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    #3
    K... I think I get it..

    But what i dont understand is.... In the example from the book, he does not use the no auto-summary command, yet the routing tables are not summarized at the classfull boundaries...

    He has 3 routers (R1, R2, and R3)... R1 connects to R2, R2 connects to R3, and R3 connects back to R1 (the topology looks like a triangle).

    Also, each router has a network of its Fa0/0 interface

    This is what confuses me... The links between all routers, and the networks off each Fa0/0 interface use 10.0.0.0, but subnetted to fit each network

    The routing table does not display a summarized network... instead it lists the subnets (10.1.2.0/24, and 10.1.3.0/24 ect). This was achieved without using no auto-summary...

    I dont get what I am missing here... Ive read through this like 8 times now..

    Since the no auto-summary command was not used, why doesnt R2 tell everyone "hey I know about 10.0.0.0/8" and why doesnt R1 and R3 do the same? It seems as though they are telling each other specifically what subnet they know about, instead of saying "i know about 10.0.0.0/8." This would totally make sense if he used the no auto-summary command... but since he hasn't I am getting so messed up. If he had used a Class C addres throughout the diagram I would undestand how this happened as well... but I dont get it since subnetted Class A was used.

    See what i mean..?

    Here is a picture of the topology...

    Example of where I am confused: Why doesn't Seville Router tell Albuquerque "i know about 10.0.0.0/8?" Instead it tells Albuquerque "hey I know about 10.1.3.0/24" and it does this even though no auto-summary has not been given.
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    Last edited by new2net; 09-04-2009 at 04:16 AM.
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  5. Senior Member trackit's Avatar
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    #4
    this configuration has to use no auto-summary, otherwise it wouldnt work. I guess the book author just didnt mention that he used the command.

    EDIT: You know what? just through 3 routers together in your lab, put the same networks in and enable RIPv2 and see what happens
    Last edited by trackit; 09-04-2009 at 04:49 AM.
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    #5
    Auto-summary occurs at the major network boundary.In your example there is no Major network boundary as all links share the same major network 10.x.x.x.
    If a router had a link with 11.x.x.x, and auto-summary was enabled all the 10x.x.x networks would be advertised out the 11.x.x.x link as 10.0.0.0/8.

    10.1.1.1 is a class A address its major net number is 10.x.x.x
    172.168.1.1 is a class B address with major network 172.168.x.x
    etc..
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  7. Senior Member trackit's Avatar
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    #6
    ahh ok, thanks EdTheLad, never heard about that before but it makes sense.
    Last edited by trackit; 09-04-2009 at 04:55 AM.
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    #7
    Quote Originally Posted by EdTheLad View Post
    Auto-summary occurs at the major network boundary.In your example there is no Major network boundary as all links share the same major network 10.x.x.x.
    If a router had a link with 11.x.x.x, and auto-summary was enabled all the 10x.x.x networks would be advertised out the 11.x.x.x link as 10.0.0.0/8.

    10.1.1.1 is a class A address its major net number is 10.x.x.x
    172.168.1.1 is a class B address with major network 172.168.x.x
    etc..
    172.16.x.x, but yeah.
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    #8
    Quote Originally Posted by EdTheLad View Post
    Auto-summary occurs at the major network boundary.In your example there is no Major network boundary as all links share the same major network 10.x.x.x.
    Thanks EdTheLad...!
    So if all the links are using the same major netwok, doesn't that still mean that there is a boundary? What I am trying to say is... Why doesn't each link advertise 10.0.0.0/8 since we do not have no auto-summary on? It's not clicking yet... Could you please expand on this a little? To me, each link should say "hey i know about 10.0.0.0/8" since auto summarization is in use by default, but it appears (by looking at the routing table in the book based on the example), that each link is saying "i know about 10.1.3.0/24 ect" or "i know about 10.1.128.0/24" ect ect ect, when it should be saying "hey i know about 10.0.0.0/8

    Quote Originally Posted by EdTheLad View Post
    If a router had a link with 11.x.x.x, and auto-summary was enabled all the 10x.x.x networks would be advertised out the 11.x.x.x link as 10.0.0.0/8.
    Ohh I see how that works! That makes sense!

    Thanks so much for the help so far, really appreciate all your responses... this has been driving me so crazy..
    Last edited by new2net; 09-04-2009 at 07:29 AM.
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  10. Senior Member billscott92787's Avatar
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    #9
    Quote Originally Posted by new2net View Post
    Thanks EdTheLad...!
    So if all the links are using the same major netwok, doesn't that still mean that there is a boundary? What I am trying to say is... Why doesn't each link advertise 10.0.0.0/8 since we do not have no auto-summary on? It's not clicking yet... Could you please expand on this a little? To me, each link should say "hey i know about 10.0.0.0/8" since auto summarization is in use by default, but it appears (by looking at the routing table in the book based on the example), that each link is saying "i know about 10.1.3.0/24 ect" or "i know about 10.1.128.0/24" ect ect ect, when it should be saying "hey i know about 10.0.0.0/8



    Ohh I see how that works! That makes sense!

    Thanks so much for the help so far, really appreciate all your responses... this has been driving me so crazy..





    No, there wouldn't be a boundary. Think of it as an example with a boundary router. This is a case where there is one major network, which then connects to another major network. See the example that I created for you.


    Here is the routing table of the boundary Router:


    10.0.0.0/10 is subnetted, 2 subnets
    C 10.64.0.0 is directly connected, Serial1/0
    C 10.128.0.0 is directly connected, Serial1/1
    11.0.0.0/10 is subnetted, 1 subnets
    C 11.64.0.0 is directly connected, Serial1/2

    All routers are connected, there are no RIP routes because they are all directly connected. See how They give you the /10 address ----> 255.192.0.0


    Here is the routing table of R1:



    10.0.0.0/10 is subnetted, 2 subnets
    C 10.64.0.0 is directly connected, Serial0/0
    R 10.128.0.0 [120/1] via 10.64.0.1, 00:00:10, Serial0/0
    R 11.0.0.0/8 [120/1] via 10.64.0.1, 00:00:10, Serial0/0


    Here is the routing table of R2:


    10.0.0.0/10 is subnetted, 2 subnets
    R 10.64.0.0 [120/1] via 10.128.0.1, 00:00:00, Serial0/0
    C 10.128.0.0 is directly connected, Serial0/0
    R 11.0.0.0/8 [120/1] via 10.128.0.1, 00:00:00, Serial0/0



    Here is the routing table of RemoteNetwork router:

    R 10.0.0.0/8 [120/1] via 11.64.0.1, 00:00:03, Serial0/0
    11.0.0.0/10 is subnetted, 1 subnets
    C 11.64.0.0 is directly connected, Serial0/0



    As you can see here the RIPv2 route to the 10.0.0.0 network is summarized down to 10.0.0.0/8. Since RIPv2 defaults to auto summary, you can see the fact that it sends this update:

    RIP: sending v2 update to 224.0.0.9 via Serial1/2 (11.64.0.1)
    RIP: build update entries
    10.0.0.0/8 via 0.0.0.0, metric 1, tag

    See how it summerized the routes by the classful mask? I'm sure it has something to do with the fact of being able to send and receive RIPv1 updates, even though you have turned RIPv2 on, for all routers to only send and receive RIPv2 updates.


    Now if we disable auto summary on all the routers we get the following:




    Look at the RIP update that the Remote Network now receives from the Boundary Router:

    IP: s=11.64.0.1 (Serial0/0), d=224.0.0.9 len 0, rcvd 2RIP: received v2 update from 11.64.0.1 on Serial0/0
    10.64.0.0/10 via 0.0.0.0 in 1 hops
    10.128.0.0/10 via 0.0.0.0 in 1 hop

    Where as before it was just sending the update 10.0.0.0/8. I hope this help you understand what happens with it.

    It is a pain in the crack, but if you visit this cisco page: Cisco IOS IP Command*Reference, Volume 2*of 3: Routing Protocols, Release*12.2 - RIP Commands [Cisco IOS Software Releases 12.2 Mainline] - Cisco Systems you will see that there explanation is basically that auto summarization is used to limit the amount of routes in the routing table. Thereby making the routing table smaller, which in turn, allows the routing process to be performed faster.


    Hope this helps ya.
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    #10
    Quote Originally Posted by billscott92787 View Post
    No, there wouldn't be a boundary. Think of it as an example with a boundary router. This is a case where there is one major network, which then connects to another major network. See the example that I created for you.
    Thanks so much for that. I still have one question regarding your example...

    Let us say that we connected R1 directly to R2 in your example, and gave that link the address 10.192.0.0/10 (so it looks like a triangle similar to the Odom example)

    With auto-smmary turned off, would R1 tell the Boundary Router "hey I know about 10.0.0.0/8" (when referring to the link between R1 and R2) or would R1 tell the Boundary Router "hey i know about 10.192.0.0/10" (again, when referring to the link between R1 and R2)

    If auto-summary is turned off, i think that R1 would tell the boundary router "i know of 10.0.0.0/8 which is between R1 and R2"... but I have a feeling that is not the case... and I really dont understand why this isn't the case since we are not using auto summary. If we had used auto-summary I would totally get it.


    I ran the same lab you made... I attached a picture of the topology.

    Here is the routing table on the Boundary Router:

    Gateway of last resort is not set

    10.0.0.0/10 is subnetted, 3 subnets
    C 10.64.0.0 is directly connected, Serial0/3/0
    C 10.128.0.0 is directly connected, Serial0/1/0
    R 10.192.0.0 [120/1] via 10.64.0.2, 00:00:22, Serial0/3/0
    [120/1] via 10.128.0.2, 00:00:00, Serial0/1/0
    11.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
    R 11.0.0.0/8 [120/3] via 10.128.0.2, 00:00:22, Serial0/1/0
    C 11.64.0.0/10 is directly connected, Serial0/0/0


    The only commands I did on each router was:
    router rip
    version 2
    network [classfull network number]


    I do not understand why Boundary Router has this line:
    R 10.192.0.0 [120/1] via 10.64.0.2, 00:00:22, Serial0/3/0
    [120/1] via 10.128.0.2, 00:00:00, Serial0/1/0

    It seems to me that both R1 and R2 are telling Boundary Router "hey i know of 10.192.0.0/10"... but since auto-summary is in effect, why are they not telling Boundary Router "hey i know of 10.0.0.0/8. I dont understand why they are not summarizing this update.

    Sorry if i repeated myself, I am trying to be detailed as possible.
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  12. CCIE Bound kryolla's Avatar
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    #11
    auto summarization happens on the router that has both networks for example

    10.x/24 fa0/0 -----R1 ---- fa1/0 11.x/24

    R1 will summarize the 10.network into /8 install a null route (EIGRP) and advertise it out fa1/0. It will also do the same thing for 11. network and summarize it to/8 install a null route (EIGRP) and advertise out fa0/0.

    In your example R1 and R2 has all its interfaces in 10. network so it will not auto summarize. The auto summary command has no affect on R1 and R2 so you can turn it on or off and it wont do anything
    Last edited by kryolla; 09-04-2009 at 04:21 PM.
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    #12
    Quote Originally Posted by kryolla View Post
    auto summarization happens on the router that has both networks for example

    10.x/24 fa0/0 -----R1 ---- fa1/0 11.x/24

    R1 will summarize the 10.network into /8 install a null route (EIGRP) and advertise it out fa1/0. It will also do the same thing for 11. network and summarize it to/8 install a null route (EIGRP) and advertise out fa0/0.

    In your example R1 and R2 has all its interfaces in 10. network so it will not auto summarize. The auto summary command has no affect on R1 and R2 so you can turn it on or off and it wont do anything

    So auto-summarization (even though it is on by default) will not occur if a router has all interfaces using the same major network with the same mask? Example: A router has S0/0/0 with 172.16.1.0/24 and S0/1/0 with 172.16.2.0/24 and Fa0/0 with 172.16.3.0/24. Auto summarization will not occur here?
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    #13
    Edit - ignore me, was talking about RIPv1!
    Last edited by Neeko; 09-04-2009 at 09:52 PM.
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  15. CCIE Bound kryolla's Avatar
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    #14
    Quote Originally Posted by Neeko View Post
    Yes, the specifics of this are outlined here: Behavior of RIP and IGRP When Sending and Receiving Updates - Cisco Systems

    I too stumbled across what you have realised and went mad for a while trying to figure it out, this link helped me a lot. The routers do checks; the sending router checks the subnet being advertised against its sending interface and the receiving router checks the subnet being advertised against the interface it was received on. If the subnet is part of the same major network and has the same mask as both interfaces involved in a particular update then both routers deal with the subnet in it's entirety.

    Basically the network you have posted a picture of is contiguous, meaning subnets of the same major network are directly connected and using the same subnet masks. If the masks were different or there was a different classful network involved (causing the classful boundary to be crossed) then you would see auto summarization. As it is though the contiguous setup and same length masks allows for full updates since the routers can use their interfaces to correctly assume the full subnet mask.

    Edit: to be clear, this will only be the case for advertised subnets that fall into the category of same major network and same subnet mask. So each 10 network in the diagram will be advertised as the full subnet and installed in each routing table as the full subnet due to the logic outlined in the link I have copied in. But lets say for example in the diagram from the book Seville had a subnet connected to it on the 50.100.25.0 /24 network. Seville would advertise this subnet out of all links enabled for routing, and for example when sending out of its interface on the 10.1.130.0 /24 network, it will see the 50.100.25.0 /24 subnet is on a different major network to 10.1.130.0 /24 so automatically summarize it to 50.0.0.0 /8.

    Furthermore, if you had a subnet connected to Seville let's say... 10.2.1.0 /29, the router would apply the same logic by checking if it is on the same major network which it is. Then it would check if it has the same length mask, which it does not so it would be summarized to 10.0.0.0 /8. Lab it up to see.

    In writing that I've just realised there is a typo in that book on the network between Seville and Albuquerque, the subnet has an extra 1 making it 10.1.1.130.0 /24. The author has his own IP scheme lol.
    Your link is for RIPv1 and auto summarization is v2 and EIGRP
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  16. CCIE Bound kryolla's Avatar
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    #15
    Quote Originally Posted by new2net View Post
    So auto-summarization (even though it is on by default) will not occur if a router has all interfaces using the same major network with the same mask? Example: A router has S0/0/0 with 172.16.1.0/24 and S0/1/0 with 172.16.2.0/24 and Fa0/0 with 172.16.3.0/24. Auto summarization will not occur here?
    That is correct, it will only happen when you cross a major network boundary regardless of your subnet mask so you have a subnet of 172.16.x.x and when it crosses another boundary that does not have 172.16 it will summarize it to 172.16.0.0/16
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    #16
    Quote Originally Posted by kryolla View Post
    Your link is for RIPv1 and auto summarization is v2 and EIGRP
    Ummm.. good point lol.

    Is the same logic applied though? In that unless the subnet being advertised is on the same major network and has the same mask, it will be summarized?
    Last edited by Neeko; 09-04-2009 at 09:23 PM.
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  18. CCIE Bound kryolla's Avatar
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    #17
    Quote Originally Posted by Neeko View Post
    Ummm.. good point lol.

    Is the same logic applied though?
    No RIPv1 doesnt send a mask with the updates so the link you provided explain how a router running RIPv1 chooses to send and accept rip routes with v2 and auto summarization you can have a subnet mask of varying lengths as long as it is still part of the major network i.e 172.16.15.x /30 then a /28 then a /24 then 172.16.16 /30 etc anything in 172.16.0.0 - 172.16.255.255 it will not summarize until you cross the major network like 172.17 or 172.18 hope this makes sense
    Last edited by kryolla; 09-04-2009 at 09:29 PM.
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    #18
    Yeah that makes sense, basically routers running RIPv2 with auto-summarization on will check the classful network, where as routers running RIPv1 will check the classful network and then the mask due to no mask being sent.
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    #19
    Quote Originally Posted by Neeko View Post
    Edit - ignore me, was talking about RIPv1!

    Oh man... All the stuff I read in your post and the link you gave me made so much sense!!!! I got so happy for a bit there after I understood it all... now I just wanna... die

    Quote Originally Posted by kryolla View Post
    v2 and auto summarization you can have a subnet mask of varying lengths as long as it is still part of the major network i.e 172.16.15.x /30 then a /28 then a /24 then 172.16.16 /30 etc anything in 172.16.0.0 - 172.16.255.255 it will not summarize until you cross the major network like 172.17 or 172.18 hope this makes sense
    So that link doesn't apply to v2?

    Okay...So..... with RIPv2 enabled, and auto-summary turned off... that means summarization only happens when a major network boundary is crossed? Otherwise it will not summarize even if we have not issued no auto-summary?
    Last edited by new2net; 09-04-2009 at 09:55 PM.
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    #20
    Haha sorry man, whats happening is similar but not quite the same so kryolla was right to point out the difference. I think what I was saying was correct for v1, but due to v2 sending the subnet mask with routes it acts slightly different.

    No since v2 sends the mask, if you have auto summary turned off nothing will be summarized. If auto summary is on routes will be summarized if they cross classful boundaries.

    kryolla has this down though so I'll let him confirm.
    Last edited by Neeko; 09-04-2009 at 10:08 PM.
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  22. CCIE Bound kryolla's Avatar
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    #21
    Turning auto summarization off which is on by default ONLY matters if you have a router with mulptiple interfaces running ripv2 or eigrp with different network boundaries.
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    #22
    Quote Originally Posted by Neeko View Post
    Haha sorry man, it is close but not quite there is a slight difference.

    No since v2 sends the mask, if you have auto summary turned off nothing will be summarized. If it is on routes will be summarized if they cross classful boundaries.

    kryolla has this down though so I'll let him confirm.
    Thanks!

    My biggest problem when staring at the Odom example is this...

    The routing table on Albuquerque says:

    10.0.0.0/24 is subnetted, 6 subnets
    R 10.1.3.0 [120/1] via 10.1.130.253 ...
    R 10.1.2.0 [120/1] via 10.1.128.252 ...
    C 10.1.1.0 is directly connected, FastEthernet0/0
    C 10.1.130.0 is directly connected, Serial 0/1/0
    R 10.1.129 [120/1] via 10.1.130.253 ...
    10.1.129 [120/1] via 10.1.128.252 ...
    C 10.1.128.0 is directly connected, Serial 0/0/1

    Okay... based on the first image in the thread... how does Albuquerque know the exact subnets of the 10.0.0.0 network that Seville and Yosemite told it about (namely 10.1.2.0/24 and 10.1.3.0/24 AND 10.1.129.0/24).

    This must mean Seville is saying to Albuquerque, "hey Albuquerque, I know about 10.1.2.0/24." I dont understand this because Seville should be saying "hey Albuquerque, i know about 10.0.0.0/8." since we have not issued the command no auto-summary in the config.

    This also must mean that Yosemite is saying to Albuquerque, "hey Albuquerque , I know about 10.1.3.0/24." I don't get this either. Yosemite should be summarizing and should be saying "hey Albuquerque I know about 10.0.0.0/8"

    Both of Yosemite and Seville are also telling Albuquerque "hey Albuquerque, I know about 10.1.129.0/24"

    So it appears to me that Yosemite and Seville are advertising non-summarized routes even though summarization should be in effect.
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  24. CCIE Bound kryolla's Avatar
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    #23
    Quote Originally Posted by new2net View Post
    Thanks!

    My biggest problem when staring at the Odom example is this...

    The routing table on Albuquerque says:

    10.0.0.0/24 is subnetted, 6 subnets
    R 10.1.3.0 [120/1] via 10.1.130.253 ...
    R 10.1.2.0 [120/1] via 10.1.128.252 ...
    C 10.1.1.0 is directly connected, FastEthernet0/0
    C 10.1.130.0 is directly connected, Serial 0/1/0
    R 10.1.129 [120/1] via 10.1.130.253 ...
    10.1.129 [120/1] via 10.1.128.252 ...
    C 10.1.128.0 is directly connected, Serial 0/0/1

    Okay... based on the first image in the thread... how does Albuquerque know the exact subnets of the 10.0.0.0 network that Seville and Yosemite told it about (namely 10.1.2.0/24 and 10.1.3.0/24 AND 10.1.129.0/24).

    This must mean Seville is saying to Albuquerque, "hey Albuquerque, I know about 10.1.2.0/24." I dont understand this because Seville should be saying "hey Albuquerque, i know about 10.0.0.0/8." since we have not issued the command no auto-summary in the config.

    This also must mean that Yosemite is saying to Albuquerque, "hey Albuquerque , I know about 10.1.3.0/24." I don't get this either. Yosemite should be summarizing and should be saying "hey Albuquerque I know about 10.0.0.0/8"

    Both of Yosemite and Seville are also telling Albuquerque "hey Albuquerque, I know about 10.1.129.0/24"

    So it appears to me that Yosemite and Seville are advertising non-summarized routes even though summarization should be in effect.
    In the first image as Ed has stated ALL interfaces for ALL routers belong to the major network of 10 so it doesnt matter if auto-summarization is on or off you will NOT get a 10./8 route
    Studying for CCIE and drinking Home Brew
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  25. Senior Member
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    #24
    Quote Originally Posted by new2net View Post
    This must mean Seville is saying to Albuquerque, "hey Albuquerque, I know about 10.1.2.0/24." I dont understand this because Seville should be saying "hey Albuquerque, i know about 10.0.0.0/8." since we have not issued the command no auto-summary in the config.

    This also must mean that Yosemite is saying to Albuquerque, "hey Albuquerque , I know about 10.1.3.0/24." I don't get this either. Yosemite should be summarizing and should be saying "hey Albuquerque I know about 10.0.0.0/8"

    Both of Yosemite and Seville are also telling Albuquerque "hey Albuquerque, I know about 10.1.129.0/24"

    So it appears to me that Yosemite and Seville are advertising non-summarized routes even though summarization should be in effect.
    They are saying that yes, but no, summarization will not happen in this example because no network boundaries are being crossed.

    When RIPv2 with auto summarization turned on advertises a subnet, it will only summarize the subnet down to 10.0.0.0 /8 if the interface it is going out of is on a different classful network e.g 11.0.0.0 /8. That is what we mean by crossing classful boundaries. In this case when Seville advertises 10.x.x.x subnets it advertises them out of interfaces on 10.x.x.x subnets too, so no classful network boundaries are crossed.

    The links between the routers in the example are on the same major network as the subnets being advertised, so they are not summarized.

    This is why kryolla said this:

    Quote Originally Posted by kryolla View Post
    Turning auto summarization off which is on by default ONLY matters if you have a router with mulptiple interfaces running ripv2 or eigrp with different network boundaries.
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  26. Member
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    #25
    Quote Originally Posted by Neeko View Post
    They are saying that yes, but no, summarization will not happen in this example because no network boundaries are being crossed.

    When RIPv2 with auto summarization turned on advertises a subnet, it will only summarize the subnet down to 10.0.0.0 /8 if the interface it is going out of is on a different classful network e.g 11.0.0.0 /8. That is what we mean by crossing classful boundaries. In this case when Seville advertises 10.x.x.x subnets it advertises them out of interfaces on 10.x.x.x subnets too, so no classful network boundaries are crossed.

    The links between the routers in the example are on the same major network as the subnets being advertised, so they are not summarized.

    This is why kryolla said this:

    Ohhhhhhhhhhhhh!! It is making sense!!!!





    Thanks so much (all of u) for all your time and patience by the way...


    So.... would I be correct in saying this: even though auto-summary is on by default, auto-summary will only happen when crossing classful boundaries. Otherwise auto-summary will not occur if we are using the same major network address everywhere (even though auto-summary is on by default).

    (this may have been said already.. i just need to understand it in my own words i guess)

    Was I right in my statement..?
    Last edited by new2net; 09-04-2009 at 10:48 PM.
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