1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Auto-Summary

Discussion in 'General Cisco Certifications' started by Headache, Mar 13, 2007.

  1. Headache

    Headache Gigabyte Poster

    1,092
    9
    85
    I'm paraphasing here, but from what I've read, auto-summary on EIGRP does not advertise the subnets of connected networks. In order to be able to view these components in the topology table, auto-summary must first of all be disabled with the "no auto-summary" command. Furthermore, it appears that unless it is disabled, auto-summary can also impede connectivity between these networks.

    Basically, I'm having difficulty understanding why connectivity should be impeded. Is it because all the connected subnets must be advertised in order for connectivity to take place, or is there another reason ?

    (EDIT)
     
    Certifications: CCNA
    WIP: CCNP
  2. laup

    laup Byte Poster

    121
    1
    24
    think its down to the fact that summarising uses common high order bits of the network, therefore will advertise the multiple networks as one, rather than individual/seperate networks in there own right


    If u get what i mean :P
     
    Certifications: IC3,A+, N+, 70-270, CCNA
    WIP: CCDA/Job :>
  3. Headache

    Headache Gigabyte Poster

    1,092
    9
    85
    Yeah, I think so. Still not hundred percent though.
     
    Certifications: CCNA
    WIP: CCNP
  4. Spice_Weasel

    Spice_Weasel Kilobyte Poster

    254
    45
    45
    Auto-summary will make eigrp advertise networks on classful boundaries. As well, eigrp will create classful routes to null0 for connected networks. These routes will be eigrp summary routes and thus will have an admin distance of 5, making the route to null0 preferred over routes received from routing protocols. You still will be able to see received routes in the topology table, but the they will not be installed in the routing table as the eigrp summary route has a better admin distance.

    For example, suppose there are 3 routers in series:

    R1 --- R2 --- R3

    Suppose R1 has an interface with an ip address of 172.16.1.1 255.255.255.0, R2 has an interface with an ip address of 172.16.2.1 255.255.255.0 and R3 has an interface with an ip address of 172.16.3.1 255.255.255.0. Suppose the networks connecting the routers are 10.10.30.0/30 for R1 to R2 and 192.168.45.64/30 for R2 to R3.

    If auto-summary is enabled the routers will be able to reach each other via the networks that connect them but will not be able to ping any other router's 172.16.x.y network. R1 will not be able to ping 172.16.2.1 or 172.16.3.1, for example. This is because each router will be advertising 172.16.0.0/16 networks, and will have an entry in their topology table pointing the 172.16.0.0/16 network to null0. Because of that, the routing table on R1 would look something like:

    172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
    C 172.16.1.0/24 is directly connected, Vlan10
    D 172.16.0.0/16 is a summary, 00:01:22, Null0

    So, pinging 172.16.2.1, which should go to R2, will instead go to null0. Removing auto-summary will correct this problem. Note that if you remove auto-summary from R2, R1 (which still has auto-summary enabled) will be able to ping 172.16.2.1. R2 will be able to ping 172.16.1.1, but check in the routing table and you will see the summary route advertised by R1, which will blackhole classless networks in the 172.16.0.0/16 network if you're not careful.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  5. Headache

    Headache Gigabyte Poster

    1,092
    9
    85
    Thanks Spice_Weasel, that explains a lot.

    As soon as I have some time, I'm gonna have to crank up the routers and check this out for real.
     
    Certifications: CCNA
    WIP: CCNP
  6. laup

    laup Byte Poster

    121
    1
    24
    There explained much better :>
     
    Certifications: IC3,A+, N+, 70-270, CCNA
    WIP: CCDA/Job :>

Share This Page

Loading...