Routing Table masks

Discussion in 'Cisco' started by ciscortp@hotmail.com, Jun 2, 2006.

  1. Guest

    I'm trying to figure out how these routing protocols figure out these
    masks. The reason being is that I need to advertise BGP routes without
    using a Null0 static.

    So for example. Sometimes if I do a "sh ip route 192.168.100.0" The
    routing table may show that there it is a 192.168.100.0 /24 with
    multiple /32 subnets. I assume this is because there are many /32 all
    over the network and the router is doing some type of summarization

    But on other occasions if I have a few interfaces configured with
    10.100.10.0 /26 10.100.10.64/26 and 10.100.10.128/26, 10.100.11.0/26
    When I do a "sh ip route 10.100.10.0" it will show up as 10.100.10.0
    /26 with no other subnets.. Why doesn't it show up as a 10.100.10.0
    /23 and list the various subnets?

    Please clarify

    Thanks in advance
     
    , Jun 2, 2006
    #1
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  2. In article <>,
    wrote:

    > I'm trying to figure out how these routing protocols figure out these
    > masks. The reason being is that I need to advertise BGP routes without
    > using a Null0 static.
    >
    > So for example. Sometimes if I do a "sh ip route 192.168.100.0" The
    > routing table may show that there it is a 192.168.100.0 /24 with
    > multiple /32 subnets. I assume this is because there are many /32 all
    > over the network and the router is doing some type of summarization
    >
    > But on other occasions if I have a few interfaces configured with
    > 10.100.10.0 /26 10.100.10.64/26 and 10.100.10.128/26, 10.100.11.0/26
    > When I do a "sh ip route 10.100.10.0" it will show up as 10.100.10.0
    > /26 with no other subnets.. Why doesn't it show up as a 10.100.10.0
    > /23 and list the various subnets?
    >
    > Please clarify


    It sounds like it's doing "auto-summary" based on address classes. So
    10.100.10.128/26 is a subnet of the Class A address 10.0.0.0/8.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Jun 3, 2006
    #2
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