iBGP Routing Question

Discussion in 'Cisco' started by amyl@paxemail.com, Aug 1, 2007.

  1. Guest

    I am hoping someone can shed some light on why this route is not being
    advertised via iBGP between two routers.

    The layout is as follows (hopefully the drawing will format right):
    AS 1 (0.0.0.0)
    | \
    | --------------------
    AS 2 |
    | |
    | |
    AS 3 (WEST) ----AS 3 (EAST)

    Summary:
    AS 1 is advertising 0.0.0.0 to AS 2 and AS 3.
    AS 3 WEST router connected to AS 2 router receives the 0.0.0.0 via AS
    PATH 2, 1
    AS 3 EAST router connected to AS 1 router received the 0.0.0.0 via AS
    PATH 1

    In the BGP table for the AS 3 WEST router shows both of the routes
    0.0.0.0 via AS 1
    0.0.0.0 via AS 2, 1

    In the BGP table for the AS 3 EAST router we only see
    0.0.0.0 via AS 1

    Question:
    Why in the bgp table for AS 3 EAST wouldn't we also see the route
    0.0.0.0 via AS 2, 1.

    When checking the advertised routes from AS 3 WEST to AS 3 EAST its
    not being advertised. There is absolutely no route filtering
    happening between the iBGP routers.

    The only thing I can think of is that AS 3 WEST sees the better path
    from AS 3 EAST and decides not to send its 0.0.0.0 route to AS 3 EAST.

    My expectation would be that both "sh ip bgp" on each router would
    have listed the 0.0.0.0 route, but that clearly is not the case in my
    lab.

    Amy.
    , Aug 1, 2007
    #1
    1. Advertising

  2. John Agosta Guest

    <> wrote in message
    news:...
    >I am hoping someone can shed some light on why this route is not being
    > advertised via iBGP between two routers.
    >
    > The layout is as follows (hopefully the drawing will format right):
    > AS 1 (0.0.0.0)
    > | \
    > | --------------------
    > AS 2 |
    > | |
    > | |
    > AS 3 (WEST) ----AS 3 (EAST)
    >
    > Summary:
    > AS 1 is advertising 0.0.0.0 to AS 2 and AS 3.
    > AS 3 WEST router connected to AS 2 router receives the 0.0.0.0 via AS
    > PATH 2, 1
    > AS 3 EAST router connected to AS 1 router received the 0.0.0.0 via AS
    > PATH 1
    >
    > In the BGP table for the AS 3 WEST router shows both of the routes
    > 0.0.0.0 via AS 1
    > 0.0.0.0 via AS 2, 1
    >
    > In the BGP table for the AS 3 EAST router we only see
    > 0.0.0.0 via AS 1
    >
    > Question:
    > Why in the bgp table for AS 3 EAST wouldn't we also see the route
    > 0.0.0.0 via AS 2, 1.
    >
    > When checking the advertised routes from AS 3 WEST to AS 3 EAST its
    > not being advertised. There is absolutely no route filtering
    > happening between the iBGP routers.
    >
    > The only thing I can think of is that AS 3 WEST sees the better path
    > from AS 3 EAST and decides not to send its 0.0.0.0 route to AS 3 EAST.
    >
    > My expectation would be that both "sh ip bgp" on each router would
    > have listed the 0.0.0.0 route, but that clearly is not the case in my
    > lab.
    >
    > Amy.
    >


    The picture doesn't quite match up to the wording, but here's a shot at
    answering:

    An 'x' router may have multiple pathways that it has learned to consider,
    but that 'x' router will advertise only the route that it is USING to other
    routers.
    It will not advertise the pathways that have not been selected as "best."

    If an 'x' router is using a path through a "y" neighbor,
    then the "x" router will not advertise that path to the "y" neighbor.
    The "x" router will advertise that path to other neighbors, but not the
    neighbor that is providing the pathway itself.

    Make sense ?

    -ja
    John Agosta, Aug 2, 2007
    #2
    1. Advertising

  3. Guest

    On Aug 2, 12:01 am, "John Agosta" <j_agosta@remove_wideopenwest.kom>
    wrote:
    > <> wrote in message
    >
    > news:...
    >
    >
    >
    > >I am hoping someone can shed some light on why this route is not being
    > > advertised via iBGP between two routers.

    >
    > > The layout is as follows (hopefully the drawing will format right):
    > > AS 1 (0.0.0.0)
    > > | \
    > > | --------------------
    > > AS 2 |
    > > | |
    > > | |
    > > AS 3 (WEST) ----AS 3 (EAST)

    >
    > > Summary:
    > > AS 1 is advertising 0.0.0.0 to AS 2 and AS 3.
    > > AS 3 WEST router connected to AS 2 router receives the 0.0.0.0 via AS
    > > PATH 2, 1
    > > AS 3 EAST router connected to AS 1 router received the 0.0.0.0 via AS
    > > PATH 1

    >
    > > In the BGP table for the AS 3 WEST router shows both of the routes
    > > 0.0.0.0 via AS 1
    > > 0.0.0.0 via AS 2, 1

    >
    > > In the BGP table for the AS 3 EAST router we only see
    > > 0.0.0.0 via AS 1

    >
    > > Question:
    > > Why in the bgp table for AS 3 EAST wouldn't we also see the route
    > > 0.0.0.0 via AS 2, 1.

    >
    > > When checking the advertised routes from AS 3 WEST to AS 3 EAST its
    > > not being advertised. There is absolutely no route filtering
    > > happening between the iBGP routers.

    >
    > > The only thing I can think of is that AS 3 WEST sees the better path
    > > from AS 3 EAST and decides not to send its 0.0.0.0 route to AS 3 EAST.

    >
    > > My expectation would be that both "sh ip bgp" on each router would
    > > have listed the 0.0.0.0 route, but that clearly is not the case in my
    > > lab.

    >
    > > Amy.

    >
    > The picture doesn't quite match up to the wording, but here's a shot at
    > answering:
    >
    > An 'x' router may have multiple pathways that it has learned to consider,
    > but that 'x' router will advertise only the route that it is USING to other
    > routers.
    > It will not advertise the pathways that have not been selected as "best."
    >
    > If an 'x' router is using a path through a "y" neighbor,
    > then the "x" router will not advertise that path to the "y" neighbor.
    > The "x" router will advertise that path to other neighbors, but not the
    > neighbor that is providing the pathway itself.
    >
    > Make sense ?
    >
    > -ja


    That makes absolute sense and that was what I was thinking.

    Thanks
    Amy
    , Aug 2, 2007
    #3
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Charles Jennings

    Redundant VPN with IBGP - Newbie Question

    Charles Jennings, Jun 3, 2004, in forum: Cisco
    Replies:
    1
    Views:
    558
    Vincent C Jones
    Jun 4, 2004
  2. Todd Adamson

    ibgp routing tables not filling up

    Todd Adamson, Oct 13, 2004, in forum: Cisco
    Replies:
    2
    Views:
    484
    John Agosta
    Oct 14, 2004
  3. Jarek Jarzebowski

    iBGP question.

    Jarek Jarzebowski, Jul 5, 2008, in forum: Cisco
    Replies:
    3
    Views:
    455
    Bartosz Gagat
    Jul 7, 2008
  4. Jarek Jarzebowski

    Re: iBGP question.

    Jarek Jarzebowski, Jul 7, 2008, in forum: Cisco
    Replies:
    1
    Views:
    531
  5. John Agosta

    Re: iBGP question.

    John Agosta, Jul 7, 2008, in forum: Cisco
    Replies:
    1
    Views:
    424
Loading...

Share This Page