AS prepending

Fri Apr 8 16:05:59 UTC 2005

Update 2:

More info. When I have tested the failover by pulling
the plug on the preferred ISP, I do not see my network
in looking glass. Secondly, the backup provider has
told me the the route is not in the (rib).

Philip

--- Mark Kasten <mark.kasten at savvis.net> wrote:
> <offlist>
> 
> fwiw, it's in the routing table (rib), not their
> forwarding table (fib). 
>   if they look on their side of the session, they
> will have the prefix 
> in "show ip bgp" or "show route", but it will not
> propogate beyond that 
> router because their network prefers the other path
> with the short AS. 
> a router doesn't forward all rib entries, only fib
> entries.
> 
> 
> for example:
> 
> dcr4.nyr> show route 141.77.0.0/16
> 141.77.0.0/16      *[BGP/170] 3w4d 01:53:30, MED 98,
> localpref 100, from 
> 206.24.194.105
>                        AS path: 1273 ?
>                      > via so-0/0/0.1510
>                        via so-1/1/0.10
>                      [BGP/170] 9w3d 12:26:09, MED
> 128, localpref 80
>                        AS path: 3356 1273 I
>                      > to 4.68.127.205 via
> so-6/1/0.0
>                      [BGP/170] 21:32:50, MED 128,
> localpref 80
>                        AS path: 1239 1273 ?
>                      > to 144.232.9.117 via
> so-3/1/0.0
> 
> 
> on a router, one hop away:
> 
> 
> kar1.nyr> show route 141.77.0.0/16
> 
> inet.0: 173067 destinations, 345433 routes (172951
> active, 0 holddown, 
> 547 hidden)
> Restart Complete
> + = Active Route, - = Last Active, * = Both
> 
> 141.77.0.0/16      *[BGP/170] 3w4d 01:53:59, MED 98,
> localpref 100, from 
> 206.24.194.105
>                        AS path: 1273 ?
>                      > to 208.174.228.1 via ae0.0
> 
> 
> 
> no evidence of the 3356_1273 or the 1239_1273 path. 
> if i lose the 
> direct 1273 path, then one of those paths would then
> be propogated as 
> the preferred path.
> 
> 
> hth's.
> 
> mark
> 
> 
> Philip Lavine wrote:
> 
> > Update:
> > 
> > I am prepending my AS 3 times to the un-preferred
> ISP.
> > Both ISP's are my peers. The un-preferred ISP
> claims
> > the see my advertisement yet they do not add it to
> > their routing table (suggests filtering??). They
> claim
> > all the filtering they are doing is based on the
> > networks I told them over the phone that I was
> using
> > with that AS.
> > 
> > Philip
> > 
> > --- Jon Lewis <jlewis at lewis.org> wrote:
> > 
> >>On Fri, 8 Apr 2005, Philip Lavine wrote:
> >>
> >>
> >>>I am using AS prepending to favor one ISP over
> >>>another, in a BGP multihomed/multiISP scenario.
> >>
> >>Why
> >>
> >>>does the ISP receiving the prepends fail to add
> my
> >>>network into their routing table? Is this a
> >>
> >>"feature"
> >>
> >>>of BGP, or have I gone too far with 3 prepend
> >>>statements.
> >>
> >>Who's ASN are you prepending on your advertised
> >>routes?
> >>
> >>
> > 
> >
>
----------------------------------------------------------------------
> > 
> >> Jon Lewis                   |  I route
> >> Senior Network Engineer     |  therefore you are
> >> Atlantic Net                |
> >>_________ http://www.lewis.org/~jlewis/pgp for PGP
> >>public key_________
> >>
> > 
> > 
> > __________________________________________________
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> 

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