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7151 Views 12 Replies Latest reply: Sep 18, 2011 12:22 PM by tnewshott RSS

Currently Being Moderated

auto summarization

Sep 17, 2011 8:58 AM

Stephen 488 posts since
Apr 22, 2011

Hi guys,


I've read a lot of info about auto summarization, however, I never ever ever ever get questions right about it.  It's like I'm totally misunderstanding it or something.  I need you guys to help.  I had a question (forwarded from a mate who also doesn't understand the answer) regarding a similar layout below:

 

 

 

EIGRP is configured with the default configuration on all routers.  Autosummarization is enabled on router R2 and R3, but is disabled on router R1.  Which two EIGRP routes will be seen in the routing table of router R3? (choose 2)

autosummarization.jpg

 

R1#show running-config

!

router eigrp 4

network 10.0.0.0

network 172.16.0.0

no auto-summary

!

 

 

A  10.0.0.0/8

B  10.0.0.0/16

C  10.10.10.0/24

D  172.16.0.0/16

E  172.16.0.0/24

F  172.16.10.0/24

 

The correct answers were C and D.

 

In my head, auto-summarization summarises networks at classful boundaries.  So when the routes hit R2, R2 will summarise the

 

10.10.10.0/24 network into  ----->10.0.0.0/8 and

172.16.10.0/24  network into -----> 172.16.0.0/16

 

So how do you arrive at the answer of C and D?

 

Cheers,

Stephen

 

PS: this is something I've been trying to work out for a while, so any explination is really appreciated!  Thanks in advance

  • Paul Stewart  -  CCIE Security, CCSI 6,993 posts since
    Jul 18, 2008
    Currently Being Moderated
    1. Sep 17, 2011 9:14 AM (in response to Stephen)
    Re: auto summarization

    You are correct in how auto summarization would work.  C AND D are correct because auto-summary was disabled with the "no auto-summary" command. 

  • Jared 5,502 posts since
    Jul 27, 2008
    Currently Being Moderated
    Re: auto summarization

    In todays networks, auto-summarization can be dangerous.  I had a jr engineer forget to turn it off and we caused some temporary routing issues there for a while.

  • Paul Stewart  -  CCIE Security, CCSI 6,993 posts since
    Jul 18, 2008
    Currently Being Moderated
    4. Sep 17, 2011 9:26 AM (in response to Stephen)
    Re: auto summarization

    My bad.   I didn't read the question thoroughly.  I think you are right and the question is wrong.

  • Paul Stewart  -  CCIE Security, CCSI 6,993 posts since
    Jul 18, 2008
    Currently Being Moderated
    6. Sep 17, 2011 10:25 AM (in response to Stephen)
    Re: auto summarization

    I labbed it up and got those exact results. 10.10.10.0 is not summarizes because R2 doesn't handle any 10.0.0.0/8 networks.

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    8. Sep 18, 2011 4:07 AM (in response to Stephen)
    Re: auto summarization

    R2 will not "auto-summarize" the routes received from R1, unless it also has directly connected routes for the same major network.  As Paul pointed out, since R2 does not have any directly connected subnets of the 10.0.0.0/8 network, then he does not "auto-summarize" this network to R3.

     

    I did get some very different results between GNS3 and Cisco PT in the above topology.  GNS3 produced the same results as Stephen showed, but Cisco PT included a summary route for the 10.0.0.0/8 network on R2 and advertised both the 10.0.0.0/8 and 10.10.10.0/24 networks to R3.

     

    Brian

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    9. Sep 18, 2011 5:42 AM (in response to Stephen)
    Re: auto summarization

    Here are some sample outputs for EIGRP.  R1 has "disabled" auto-summary using the "no auto-summary" command and R2 and R3 have auto-summary "enabled".

     

    Notice the outputs below.  The first output shows R3's routing table output and how R2 does NOT auto-summarize the 10.10.10.0/24 network to 10.0.0.0/8.  This is because R2 does NOT have any directly connected subnets of this major network.

     

    The second output shows R3's routing table output, but this time R2 summarized the route to 10.0.0.0/8.  This is because I configured a loopback on R2 with the following IP address 10.12.12.2/24 and included this interface in the EIGRP routing process on R2.

     

    R3#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

    D    172.16.0.0/16 [90/2681856] via 192.168.23.2, 00:04:40, Serial0/0

        10.0.0.0/24 is subnetted, 1 subnets

    D       10.10.10.0 [90/2809856] via 192.168.23.2, 00:04:40, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/0

         30.0.0.0/24 is subnetted, 1 subnets

    C       30.30.30.0 is directly connected, Loopback30

    R3#

     

    ====================================================================

     

    R3#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

    D    172.16.0.0/16 [90/2681856] via 192.168.23.2, 00:00:14, Serial0/0

    D    10.0.0.0/8 [90/2297856] via 192.168.23.2, 00:00:14, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/0

         30.0.0.0/24 is subnetted, 1 subnets

    C       30.30.30.0 is directly connected, Loopback30

    R3#

     

    Brian

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    11. Sep 18, 2011 12:12 PM (in response to Stephen)
    Re: auto summarization

    Only in the case of EIGRP.  If you used RIPv2, then RIPv2 would perform the "auto-summarization" regardless if R2 had directly connected routes or not.

     

    RIPv2 and EIGRP do behave differently regarding "auto-summarization" across major network boundaries.  Below are the sample outputs for EIGRP vs RIPv2.  I used the same topology you had in the original post, R1 <---> R2 <---> R3.

     

    =====================================================================

    =====================================================================

    ======= EIGRP CONFIGURATION - AUTO SUMMARY ON R2 AND R3 ONLY ========

     

    R1#sh run

    Building configuration...

     

    Current configuration : 823 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R1

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback10

    ip address 10.10.10.1 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 172.16.12.1 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    router eigrp 1

    network 10.0.0.0

    network 172.16.0.0

    no auto-summary

    !

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R1#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

         172.16.0.0/24 is subnetted, 1 subnets

    C       172.16.12.0 is directly connected, Serial0/0

         10.0.0.0/24 is subnetted, 1 subnets

    C       10.10.10.0 is directly connected, Loopback10

    D    192.168.23.0/24 [90/2681856] via 172.16.12.2, 00:04:16, Serial0/0

    R1#

     

    =====================================================================

     

    R2#sh run

    Building configuration...

     

    Current configuration : 905 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R2

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback20

    ip address 20.20.20.2 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 172.16.12.2 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/1

    ip address 192.168.23.2 255.255.255.0

    clock rate 2000000

    !       

    router eigrp 1

    network 172.16.0.0

    network 192.168.23.0

    auto-summary

    !

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R2#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

         20.0.0.0/24 is subnetted, 1 subnets

    C       20.20.20.0 is directly connected, Loopback20

         172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

    C       172.16.12.0/24 is directly connected, Serial0/0

    D       172.16.0.0/16 is a summary, 00:04:29, Null0

         10.0.0.0/24 is subnetted, 1 subnets

    D       10.10.10.0 [90/2297856] via 172.16.12.1, 00:04:29, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/1

    R2#

     

    =====================================================================

     

    R3#sh run

    Building configuration...

     

    Current configuration : 805 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R3

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback30

    ip address 30.30.30.3 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 192.168.23.3 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    router eigrp 1

    network 192.168.23.0

    auto-summary

    !       

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R3#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

    D    172.16.0.0/16 [90/2681856] via 192.168.23.2, 00:04:40, Serial0/0

         10.0.0.0/24 is subnetted, 1 subnets

    D       10.10.10.0 [90/2809856] via 192.168.23.2, 00:04:40, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/0

         30.0.0.0/24 is subnetted, 1 subnets

    C       30.30.30.0 is directly connected, Loopback30

    R3#

     

    =====================================================================

    =====================================================================

    ======= RIPv2 CONFIGURATION - AUTO SUMMARY ON R2 AND R3 ONLY ========

     

    R1#sh run

    Building configuration...

     

    Current configuration : 830 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R1

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback10

    ip address 10.10.10.1 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 172.16.12.1 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    router rip

    version 2

    network 10.0.0.0

    network 172.16.0.0

    no auto-summary

    !

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R1#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

         172.16.0.0/24 is subnetted, 1 subnets

    C       172.16.12.0 is directly connected, Serial0/0

         10.0.0.0/24 is subnetted, 1 subnets

    C       10.10.10.0 is directly connected, Loopback10

    R    192.168.23.0/24 [120/1] via 172.16.12.2, 00:00:16, Serial0/0

    R1#

     

    =====================================================================

     

    R2#sh run

    Building configuration...

     

    Current configuration : 898 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R2

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback20

    ip address 20.20.20.2 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 172.16.12.2 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/1

    ip address 192.168.23.2 255.255.255.0

    clock rate 2000000

    !       

    router rip

    version 2

    network 172.16.0.0

    network 192.168.23.0

    !

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R2#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

         20.0.0.0/24 is subnetted, 1 subnets

    C       20.20.20.0 is directly connected, Loopback20

         172.16.0.0/24 is subnetted, 1 subnets

    C       172.16.12.0 is directly connected, Serial0/0

         10.0.0.0/24 is subnetted, 1 subnets

    R       10.10.10.0 [120/1] via 172.16.12.1, 00:00:03, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/1

    R2#sh ip rip da

    R2#sh ip rip database

    10.0.0.0/8    auto-summary

    10.10.10.0/24

        [1] via 172.16.12.1, 00:00:18, Serial0/0

    172.16.0.0/16    auto-summary

    172.16.12.0/24    directly connected, Serial0/0

    192.168.23.0/24    auto-summary

    192.168.23.0/24    directly connected, Serial0/1

    R2#

     

    =====================================================================

     

    R3#sh run

    Building configuration...

     

    Current configuration : 798 bytes

    !

    version 12.4

    service timestamps debug datetime msec

    service timestamps log datetime msec

    no service password-encryption

    !

    hostname R3

    !

    boot-start-marker

    boot-end-marker

    !

    no aaa new-model

    !

    resource policy

    !

    memory-size iomem 5

    ip cef

    !

    interface Loopback30

    ip address 30.30.30.3 255.255.255.0

    !

    interface FastEthernet0/0

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    interface Serial0/0

    ip address 192.168.23.3 255.255.255.0

    clock rate 2000000

    !

    interface FastEthernet0/1

    no ip address

    shutdown

    duplex auto

    speed auto

    !

    router rip

    version 2

    network 192.168.23.0

    !       

    ip http server

    no ip http secure-server

    !

    control-plane

    !

    line con 0

    logging synchronous

    line aux 0

    line vty 0 4

    login

    !

    end

     

    R3#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

    R    172.16.0.0/16 [120/1] via 192.168.23.2, 00:00:11, Serial0/0

    R    10.0.0.0/8 [120/2] via 192.168.23.2, 00:00:11, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/0

         30.0.0.0/24 is subnetted, 1 subnets

    C       30.30.30.0 is directly connected, Loopback30

    R3#sh ip rip da

    R3#sh ip rip database

    10.0.0.0/8    auto-summary

    10.0.0.0/8

        [2] via 192.168.23.2, 00:00:16, Serial0/0

    172.16.0.0/16    auto-summary

    172.16.0.0/16

        [1] via 192.168.23.2, 00:00:16, Serial0/0

    192.168.23.0/24    auto-summary

    192.168.23.0/24    directly connected, Serial0/0

    R3#

     

    =====================================================================

     

    R2#sh run

    Building configuration...

     

    interface Loopback10

    ip address 10.12.12.2 255.255.255.0

    !

    router eigrp 1

    network 10.0.0.0

    network 172.16.0.0

    network 192.168.23.0

    auto-summary

    !

     

     

    R3#sh ip rou

    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

           D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

           N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

           E1 - OSPF external type 1, E2 - OSPF external type 2

           i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

           ia - IS-IS inter area, * - candidate default, U - per-user static route

           o - ODR, P - periodic downloaded static route

     

    Gateway of last resort is not set

     

    D    172.16.0.0/16 [90/2681856] via 192.168.23.2, 00:00:14, Serial0/0

    D    10.0.0.0/8 [90/2297856] via 192.168.23.2, 00:00:14, Serial0/0

    C    192.168.23.0/24 is directly connected, Serial0/0

         30.0.0.0/24 is subnetted, 1 subnets

    C       30.30.30.0 is directly connected, Loopback30

    R3#

     

    =====================================================================

     

     

    Brian

  • Currently Being Moderated
    12. Sep 18, 2011 12:22 PM (in response to Stephen)
    Re: auto summarization

    As Brian noted - R2 is receiving the 10.10.10.0/24 route, as opposed to originating it.  The reason this is important is because unless you change something with a summary command, EIGRP is still distance vector at it's heart and will route by rumor - basically passing along what it received.

     

    Auto-summarization happens when the routes are locally originated and installed in the RIB/FIB - received routes are not affected.

     


    FWIW - took me a while to wrap my mind around this concept as well.  What helped me was starting to think about things at the router level - what exact operations are occurring, in what order, and how does the configuration impact it?  At the CCIE level that is imperative to success, no reason not to start thinking that way at the CCNP level as well.  Set yourself up for success.

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