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Re: AS Path Loops in practice ?

  • From: Ben Crocker
  • Date: Tue Dec 09 04:46:32 2003

The "J" vendor expects to see these this when receiving BGP routes, there is a switch in their software to allow this to put the routes into the forwarding table. Their explanation was that the RFC actually said that it was the receiving hosts job to decide what to do with this not that it was forbidden in the RFC to advertise them to anybody (And as Mr Rekhter worked/works for them I kinda thought they would be right).

In our case a customer using BGP to our network from the same AS in two locations but not running an IGP between them (our's is not to reason why and all that) so they wanted us to advertise their own routes to them which we accomplished by putting them into a peer group on the "C" router they where connected to (which circumvents the as-path checks (as well as those checking if it is advertising routes back to the originator) ,or at least did at the time).

Couple o links showing what J are talking about.

http://www.juniper.net/techpubs/software/junos/junos60/swconfig60- routing/html/bgp-summary4.html

http://www.juniper.net/techpubs/software/junos/junos60/swconfig60- routing/html/bgp-config48.html

On Dec 8, 2003, at 7:31 PM, Jaideep Chandrashekar wrote:

Hi.
  Apologies if this posting is off topic.

  I'd observed some loops in the AS Paths as seen by the Route-Views
  routeserver.
  In one particular snapshot -- about 2% of the paths involved such
  loops.

  Here are some examples.....(taken from route-views).

11608 2914 1239 12064 22773 12064 11836
1221 4637 1239 12064 22773 12064 11836
1224 38 7228 1239 12064 22773 12064 11836
1239 12064 22773 12064
1239 12064 22773 12064 11836
19092 3356 1299 766 288 766
3356 1299 766 288 766
4181 3356 1299 766 288 766
6079 3356 1299 766 288 766
8220 1 3356 701 668 5058 48 3356

  RFC 1771  has the following to say:

 -----------------------------
9.3 Route Selection Criteria

    Generally speaking, additional rules for comparing routes among
    several alternatives are outside the scope of this document. There
    are two exceptions:
      - If the local AS appears in the AS path of the new route being
      considered, then that new route cannot be viewed as better than
      any other route.  If such a route were ever used, a routing loop
      would result.
 ------------------------------

 So it seems to me that these paths violate the BGP spec.


 Can anybody comment about whether these paths are in fact valid. Are
 these used in specific settings by ISP's. Most of these loops are of
 length < 3, but there are also some that are really long (5-6).

 Looking closely at the shorter paths, I get the feeling that these
 loops are intentional, perhaps to acheive some traffic engineering
 goal. Note that we tried to traceroute along some of these loops, and
 in almost all the cases, there was no forwarding loop (so probably
 not a flaky route).

 What sort of situations would warrant these AS Path loops.

 From my understanding of the BGP decision process - routes that
 contain the current AS in the path are automatically excluded from
 the path selection process. Is this correct ?

 Any comments would be helpful.

thanks.

-jaideep