route aggregation and summarization

Discussion in 'Cisco' started by aaabbb16, Mar 24, 2008.

  1. aaabbb16

    aaabbb16 Guest

    Two more questions here:
    1. Anyone can explain route aggregation and summarization. my question
    they are same.
    Also how they are related to CIDR and VLSM.

    2. Right now does CIDR for route summarization
    only ? Anyone use it to assisgn a classless ip addr. such as

    aaabbb16, Mar 24, 2008
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  2. aaabbb16

    Trendkill Guest

    Route aggregation is the grouping of smaller routes together into
    advertisements that are smaller in number (count) then all of the
    individual routes. Generally speaking, you would use as
    an aggregate to,,, etc, which you may have
    configured as vlans in a datacenter. Summarization can refer to many
    things, but can be configured on specific routing protocols to
    automatically summarize by the Class of the network (based on first
    octet). Summarization can also be configured manually, either by
    commands like 'ip summary-route eigrp', or in BGP by creating a route
    to null0 for the summary and putting in the network statement for the
    aggregate network or supernet.

    To answer your second question, with a subnet
    mask would mean that router is advertising -
    via its routing protocol,. Presuming you are asking about a situation
    where is a interface address, and you want
    to summarize a larger range than this, you would put summaries towards
    the WAN or Internet (usually summaries configured in the core and then
    passed into the WAN routers, although some folks prefer to put the
    summaries on the WAN routers themselves). Using a mask of
    would most likely be a very bad idea, unless of course you really have
    a datacenter with 16,000,000 hosts.

    At their core, summaries are used to keep the routing table manageable
    and clean. If you have 10 datacenters, and you were being clean, you
    could have about 10 or so routes in your table, before WAN links or
    default routes. If you did not summarize, you would have 10 x each
    vlan or subnetwork in each datacenter. Particularly when you are
    dealing with retail companies with hundreds or thousands of stores
    connected via point to point links, it was easy in the old days to
    degrade performance on a router or link with all of the updates from
    the various routes. This is why it is best to use stub,
    summarization, and default information originate to keep your tables
    short and succinct.
    Trendkill, Mar 24, 2008
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  3. To add to Trendkill's very valid answer:

    Classes are dead. It's only out of habit that people still frequently
    use a /24 mask with addresses in the 192.168 range. All networks are
    classless nowadays in the sense that you cannot reliably deduce the
    netmask from the first few bits of the address - you need to be told
    what it is. In particular, networks using addresses from the 10.* and
    172.16 to 172.31 private ranges rarely use the /8 or /16 mask
    respectively that would have been associated with them in the old
    classful world.

    Tilman Schmidt, Mar 24, 2008
  4. aaabbb16

    News Reader Guest

    Summarization also has benefits in terms of convergence.

    Using Trendkill's example, if you had a summary route of in
    your table, you would not be aware of the status of individual
    "summarized" routes such as,,, which means
    you would not have to reconverge every time there was a change in status
    of one of these routes.

    A flapping interface on a "summarized" network would not result in you
    having to recalculate an entire OSPF topology (for example).

    It provides stability, and security.

    Best regards,
    News Reader
    News Reader, Mar 24, 2008
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