[79477] in North American Network Operators' Group
Re: AS prepending
daemon@ATHENA.MIT.EDU (Philip Lavine)
Fri Apr 8 12:06:30 2005
Date: Fri, 8 Apr 2005 09:05:59 -0700 (PDT)
From: Philip Lavine <source_route@yahoo.com>
To: Mark Kasten <mark.kasten@savvis.net>, nanog <nanog@merit.edu>
In-Reply-To: 6667
Errors-To: owner-nanog@merit.edu
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@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@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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