bgp route selection

Discussion in 'Cisco' started by Bill F, Dec 14, 2003.

  1. Bill F

    Bill F Guest

    BGP routing table entry for 209.167.0.0/16, version 1605584
    Paths: (2 available, best #2, table Default-IP-Routing-Table)
    Advertised to non peer-group peers:
    12.20.134.2
    7018 701
    12.20.134.2 from 12.20.134.2 (12.126.65.194)
    Origin IGP, localpref 100, valid, internal
    6395 701
    216.140.217.117 from 216.140.217.117 (216.140.8.57)
    Origin IGP, metric 23, localpref 100, valid, external, best


    First I'm wondering why the second route is listed with origin igp as
    it's actually learned from it's peer at the isp. I also see "external"
    in the entry. So what's the distinction between igp and "external", and
    what exactly does origin mean in this case?

    I've read the rules on decision making.

    "If multiple routes have same local preference prefer the route that
    was originated by the local router"

    " If routes have the same MED, prefer external paths (ebgp) over
    internal (ibgp)

    Obviously the selection has something to do with one or both of these
    rules, but, which one?

    Thanks
     
    Bill F, Dec 14, 2003
    #1
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  2. In article <>,
    Bill F <> wrote:

    > BGP routing table entry for 209.167.0.0/16, version 1605584
    > Paths: (2 available, best #2, table Default-IP-Routing-Table)
    > Advertised to non peer-group peers:
    > 12.20.134.2
    > 7018 701
    > 12.20.134.2 from 12.20.134.2 (12.126.65.194)
    > Origin IGP, localpref 100, valid, internal
    > 6395 701
    > 216.140.217.117 from 216.140.217.117 (216.140.8.57)
    > Origin IGP, metric 23, localpref 100, valid, external, best
    >
    >
    > First I'm wondering why the second route is listed with origin igp as
    > it's actually learned from it's peer at the isp. I also see "external"
    > in the entry. So what's the distinction between igp and "external", and
    > what exactly does origin mean in this case?


    The origin attribute is something that's set by the AS that originally
    advertised the route. It indicates how that AS learned the route, so
    this means that AS 701 learned the route from its IGP (this may not be
    true -- you can use "set origin" in a route-map to change the origin
    attribute).

    > I've read the rules on decision making.
    >
    > "If multiple routes have same local preference prefer the route that
    > was originated by the local router"
    >
    > " If routes have the same MED, prefer external paths (ebgp) over
    > internal (ibgp)
    >
    > Obviously the selection has something to do with one or both of these
    > rules, but, which one?


    The rules are listed in order of precedence.

    --
    Barry Margolin,
    Woburn, MA
     
    Barry Margolin, Dec 14, 2003
    #2
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  3. Bill F

    Bill F Guest

    Barry Margolin wrote:
    > In article <>,
    > Bill F <> wrote:
    >
    >
    >>BGP routing table entry for 209.167.0.0/16, version 1605584
    >>Paths: (2 available, best #2, table Default-IP-Routing-Table)
    >> Advertised to non peer-group peers:
    >> 12.20.134.2
    >> 7018 701
    >> 12.20.134.2 from 12.20.134.2 (12.126.65.194)
    >> Origin IGP, localpref 100, valid, internal
    >> 6395 701
    >> 216.140.217.117 from 216.140.217.117 (216.140.8.57)
    >> Origin IGP, metric 23, localpref 100, valid, external, best


    I understand your explanation of the origin attribute, but, if the
    second route was selected because of the first rule I mentioned, and
    obviously both routes were learned, the first by the ibgp peer, the
    second from the directly connected isp, I'm still unclear what is meant
    by "originated by".
     
    Bill F, Dec 14, 2003
    #3
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