AS Path Loops in practice ?

Tue Dec 9 09:42:42 UTC 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
>
>
>
>