From: Geatti (geatti@xxxxxxxx)
Date: Sun Jul 30 2000 - 14:08:38 GMT-3
Fred,
The sync rule basically says in order to advertise a route it needs
to be in the RIB (Routing information base) and the routing table.
Turning sync off via the "no sync" command will allow you to advertise
a given route without it being learned via IGP first. Now most people
would say, why on earth would you want to redistribute all the routes
learned via EBGP into IGP, that would be crazy yeah? And you would be
right. Running no sync with IBGP routers is the way to go.
However to run no sync you must meet one of the following criteria....
a. you must be fully meshed IBGP within your AS. OR
b. you are a stub network - not transit.
The reason for the sync rule is to prevent a packet arriving into
your AS destined for another AS (meaning you are transit) getting to a
router within your AS that does not know what to do with it. If you
are running sync with IGP this would not matter as you would have a
IGP route to the external destination. If you are not in sync as soon
as your packet hits the router it won't know how to handle it, doesn't
have an IGP route to the destination nor is it running IBGP, it won't
have a route to that external AS, the packet is dropped.
The sync rule says that IGP must be synchronized with BGP routes, this
is not practical in most cases. Therefore make sure that when you use
no sync on all routers within you AS that you are fully meshed via
IBGP or using something like route reflectors to make appear so. If
you are fully meshed via IBGP there is no need to be sync.
Sync does require you to have an exact match I believe, 10.0.0.0 /8
does not catch 10.10.10.0 /24 and 10.5.0.0 /16. An exact match is
necessary.
Hope this is of some help
Marco
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Fred Nielsen
Sent: Saturday, July 29, 2000 8:51 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.
------
Fred Nielsen [fred_nielsen@hotmail.com]
------
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 08:23:59 GMT-3