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Re: how are backups implemented?
From: Shiva <zhiva () pacbell net>
Date: Thu, 20 Sep 2001 21:58:27 -0700


This would be seen when an enterprise (with ASY) is multihomed to to ISPs and with smaller blocks out ISPs (Say A and B) blocks and do conditional advertisement... such that when link to A goes down the smaller block from A is advertised out via B. Maybe 4hours is the ckt restore interval... Does this fit what you are seeing?

-shiva


Ratul Mahajan wrote:


let me repeat my question, this time more clearly. we, at uw, are
analyzing bgp tables for possible errors (misconfigurations). one of the
strange things (question 3 below) we are observing is the following.

a prefix 10.10.0.0/16 (for instance) is announced by AS X. sometimes, some
of its more-specifics (like 10.10.1.0/24, 10.10.56.0/24 ....) would appear
for a short time (for example, 4 hours) and then disappear again. furthermore, these more-specifics would have an origin AS Y (Y != X).
i am curious if this behavior can be caused by some sort of backup
arrangements i don't understand, or some router/administrator mess-up. clues?

        thanks,
        -- ratul

On Thu, 20 Sep 2001, Ratul Mahajan wrote:


[posting this message after having looked for answers elsewhere including
the archives, but found no satisfactory answers]

i wanted to ask the operations community about how backups are typically
implemented. i am more interested in backup implementations, in which a
failure would expose a different origin AS (this would exclude prepending
based backups).

1. when a network is multihomed, and one of the links fails, would you
expect a smooth transition (as seen in the bgp tables of a remote AS) from
one origin AS to another (modulo convergence effects)?

2. can a failure (anywhere in the network) ever expose another origin AS
for some AS's while it stays the same for some?  i guess it can, when the
network is being persistently announced from both origins, and under
normal scenario one origin could be hidden from some AS's. would this also
hold for a routing table as rich as routeviews?

3. can a failure ever cause more-specifics with a different (from the
origin of the less-specific) origin AS to appear (again, as seen from a
remote AS)? this might depend on how backups are implemented - so what i
am asking is, is this a common/possible case?


        thanks,
        -- ratul








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