From: pkm@xxxxxxxxxx
Date: Tue Aug 01 2000 - 19:48:58 GMT-3
Yes correct. Inside your AS. Check the lab and the book/documentation
I recommended. Obviously, BGP was developed to allow routing among
automous system but in the case of IBGP, you have either the choice to
redistribute BGP into IGP (not recommended) or use the no
synchronization command. BGP always check the IP routing table for
validity of a route. BGP and IGP need to be synchronized, which is on
by default. To bypass this, use the no synch. command inside your IBGP
routers.
Sincerely,
Phillip Moulay
Fred Nielsen wrote:
Thanks for everyone's input, good stuff all. Phillip mentions a
point below that I am was a little stumped on for a while also, not
only will a router not forward a route but the route won't even
make it to the router's local routing table without "no sync" or an
IGP route in place. ------
Fred Nielsen [fred_nielsen@hotmail.com]
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----- Original Message -----
From: pkm@calweb.com
To: GeattiCc: ccielab@groupstudy.com ; asafayan@msdinc.comSent:
Sunday, July 30, 2000 2:55 PMSubject: Re: BGP Synchronization rule
Based on my lab experiences with BGP, I do not think that fully
IBGP speakers is a requirement. Let's say that al your router
inside your transit AS are only BGP speakers and there is no IGP
runnning. You will still have to use the NO SYNCH command on the
IBGP speakers, regardless of having full IBGP mesh topology. If no
synch is used, the network will not appear in the route table even
if it in the BGP route table. There is a good example on the Cisco
web site at : http://www.cisco.com/warp/public/459/14.html (BGP
Case study Section 2):
http://www.cisco.com/warp/public/459/14.html#A15.0. Also, the book
All in oneCCIE lab study guide has good examples of the no synch
rule.
Also, for the CCIE lab, it is most likely that you will have to
apply the next-hop-self statement to the transit AS. The
combination of these two commands (next-hop-self and no synch)
allows the BGP route table to show up in the IP route table of the
IBGP speakers correctly. See all in one CCI Elab study guide.
http://www.mentortlabs.com has a good lab demonstrated the issues
with no synch and next-hop-self in a NBMA network.
Sincerely,
Phillip
Geatti wrote:
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.
ORb. 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
helpMarco-----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]
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