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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical AS Path Loops in practice ?
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
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