From: Padhu@xxxxxxxxxxxx
Date: Sun Jul 30 2000 - 20:27:27 GMT-3
Does the next-hop-self need to be on all IBGP neighbor commands ?
I have a scneario in test where routers A ,B & C are in AS 100.
Routers D & E are in AS 200 and 300 respectively.
A,B& C - FULL IBGP MESH .
EBGP
A<------------->D ( 128.10.0.0)
IBGP
B C-------->E ( 150.150.0.0)
EBGP
On rtr-a the neighbor to c has next hop self included
I have the next hop self on rtr c also...
however A brings in the ip of D as the next hop and that is what is visible
in C also for the 128.10 network instead of bringing in the IP of A itself
( Inspite of having the next hop self )..
I did clear ip bgp * and all that ...I am missing something
here...Appreciate you help on this. Thanks .
Cheers,Padhu
EBGP
-----Original Message-----
From: pkm@calweb.com
To: Geatti
Cc: ccielab@groupstudy.com; asafayan@msdinc.com
Sent: 7/30/00 4:55 PM
Subject: 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
<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
<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
<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
<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
<mailto:fred_nielsen@hotmail.com> ]
------
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