From: Derek Buelna (dameon@xxxxxxxxxxx)
Date: Sun Jul 30 2000 - 13:25:27 GMT-3
Here is an example of the BGP synchronization rule. It is important
for the transit AS. Let's say that you have an AS with two EBGP
routers, one in San Jose and one in Portland and that they are running
also running IBGP between themselves. Let's also assume that within
the AS's internal network, only OSPF knows about all the various paths
from Portland to San Jose.
The Portland router learns how to get to AS 55 and tells the San Jose
router about it. The San Jose router tells it's peers about that and
an AS starts forwarding traffic to the San Jose router in order to get
to AS 55. If the IGP does not know about the routes, the packets will
be dropped along the path from San Jose to Portland.
Given this situation, without some tricks or the use of
synchronization, the network would be broken.
The IGP must exactly match the BGP route.
Hope this helps,
-Derek
Sent: Saturday, July 29, 2000 5:51 PM
To: ccielab@groupstudy.com
Subject: BGP Synchronization rule
I would like to hear the opinions of the group on the synchronization
rule, which states *something* like: a BGP router will not forward an
externally learned route to another external peer until the route is
also present in that router's IGP as well.
The Halabi book touches on this, but didn't spend enough time for me
to really understand the intent behind the rule, other than to prevent
routing loops inside an AS. Because many typical configurations out
there do not redist BGP routes into IGP's, you see the "no
synchronization" command employed fairly often. Why is sync turned on
by default in the IOS? Is it part of the specification perhaps?
Also, referring to "external peer" above, this really means separate
router entities running IBGP within an AS, right? Not between EBGP
peers, where the rule doesn't apply..
And one more question, does the IGP route have to match precisely, or
can a less specific route do the trick? In other words, can the
presence of 10.0.0.0/8 in the IGP allow BGP to forward a 10.1.0.0/16
route?
Hoping all this makes sense.
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Fred Nielsen [fred_nielsen@hotmail.com]
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