Re: bgp convergence problem

[ISP Services <nanog () isp-services nl>]

I suggest you work your way down :-)

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Dennis Hagens

Song Li schreef op 5/6/14 1:42 PM:
> Hi Dennis,
>
> I think there are two possible convergence results:
>
> 1/ AS3 accepted route 16.1/16(2 4 5) from AS1, then it will withdraw
> announce of 16.1/16(5) towards AS1. And AS1 will remain 16.1/16 (2 4 5).
>
> 2/ AS1 accepted route 16.1/16(3 5) from AS3, then it withdraw 16.1/16(2
> 4 5), and AS3 will remain 16.1/16(5).
>
> I simulated this case in GNS3, and only got the first kind of result, i
> don't know why?
>
> Song
>
> 于 2014/5/6 18:13, ISP Services 写道:
> > Hi Song Li,
> >
> > As far as I know there are 2 mechanisms that should prevent this
> > situation you describe from happening:
> >
> > - Not advertising routes that are not in the RIB
> > Once AS1's peering with AS3 comes back up, the route through AS3 is
> > learned and preferred. Therefore the route via AS2 is purged from the
> > RIB. Once it is no longer in the RIB, AS1 cannot announce that path
> > anymore.
> >
> > - AS Path loop prevention
> > If AS1 still leaks the prefix to AS3, it can only announce the active
> > path which points to AS3 itself. Therefore AS3 will see a prefix with
> > its own ASN in the path and (should) drop the prefix. Crisis avoided.
> >
> > My textbook knowledge is a bit rusty though..
> >
> > Dennis Hagens
> >
> > Song Li schreef op 5/6/14 5:58 AM:
> > > Hi everyone,
> > >
> > > I have one bgp convergence problem which confused me. The problem is as
> > > follows:
> > >
> > >                  +--------+
> > >                  |  AS5   |
> > >           +------+16.1/16 |
> > >           |      +-----+--+
> > >           |            |
> > >       +---+--+         |
> > >       | AS4  |         |
> > >       |      |         |
> > >       ++-----+         |
> > >        |               |
> > >        |               |
> > >        |               |
> > > +-----+--+          +-+-----+
> > > |  AS2   |          | AS3   | 16.1/16 (5)
> > > |  ISP   |          | ISP   |
> > > +---^----+          +---^---+
> > >      |                   |
> > >      |     +--------+    |
> > >      +-----+  AS1   +----+
> > >            |customer|
> > >            +--------+
> > >           16.1/16 (2 4 5)
> > >
> > > AS1 multihomed to AS2 and AS3, for some reasons AS1 disconnect from AS3,
> > > and as a resutl the route to 16.1/16 will be 16.1/16 (2 4 5).
> > >
> > > After a while, the BGP seesion between AS1 and AS3 reestablished  but
> > > AS1 leaks the route 16.1/16 (2 4 5) to AS3. At this point,
> > >
> > > 1/ AS1 will have two bgp routes for prefix 16.1/16: 16.1/16(2 4 5)and
> > > 16.1/16(3 5), according to shorter AS_PATH it will select 16.1/16(3 5)
> > > as best route.
> > >
> > > 2/ AS3 also have two bgp routes: 16.1/16(2 4 5) and 16.1/16(5),
> > > according to local_pref it will select 16.1/16(2 4 5).
> > >
> > > in this case, AS1 and AS3 select each other as the best route to AS5, i
> > > wonder which route will be the final best route after bgp convergence in
> > > AS1 and AS3.
> > >
> > > Thanks!