BGP synchronization requirement

EDWIN.GONZALEZ · ‎02-12-2009

I was reading a Cisco doc "BGP best path selection algorithm" and I was left a bit confused on a section about synchronization.

I understand that with synchronization enabled there must be a matching IGP route in the routing table in order for an IBGP route to be considered valid.

But then I read this;

"If the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor".

Is due to the BGP Next_Hop reachability requirement?

B/W this is the link to the doc;

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Harold Ritter · ‎02-12-2009

Edwin,

Indeed, there is an additional requirement if the IGP is OSPF, which is as you said that the BGP RID of the iBGP peer needs to match the OSPF RID probably just to check that the IGP route is for the right iBGP peer.

This check will never be satisfied in an environment where a RR is used. Yet another good reason to disable synchronization, which has been disabled by default for quite a while now in IOS.

Regards

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

foxbatreco · ‎02-12-2009

this is a specific case where ospf is igp within bgp.By default,BGP identifier & ospf rid is same on each router as both of them use same rule for getting rid..viz.loopback or highest physical ip address.The ospf rid here basically means rid of ASBR speaker within ospf which wud interact with the BGP speaker for injection of routes.this especially comes to fore in cases where Route reflectors are used with an AS IGP not having direct bgp peering between two speakers.

this link could be more useful.

http://www.faqs.org/rfcs/rfc1745.html

Pls Rate if this helps!