This BGP synchronization!

[jaighobahi]

Greetings!

Here is an except from the Cisco Press I am using to study:

"The BGP synchronization rule, which states that a BGP router should not use, or advertise to an external neighbor, a route learned by IBGP, unless that route is local or is learned from the IGP." 

I do not understand it.  Please, break it down for me if you can.

1.  What does the word local mean - local with respect to the autonomous system or local with respect to the router?

2.  How does BGP learn routes from IGP?

3.  When IGP routes are redistributed into BGP, is it correct to say that the routes were learned from IGP?

Thank you.

[Lei Tian]

Hi,

Local refers to the router itself, connected or static. The rules says same prefix learned from iBGP neighbor must be learned from IGP as well. Here is the reason of the rule. Consider you have R1-R2-R3, R1 and R3 are iBGP neighbor, all three routers are running OSPF as IGP. R3 learns route 10/8 from R1, but not via OSPF from R2 because R1 is not redistributing BGP into OSPF. In this case, if R3 forwards traffic in 10/8 to R2, it will be discarded on R2, because IGP doesn't have the route.

HTH,
Lei Tian

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[Peter Paluch]

Hello,

Perhaps you would be interested in reading this thread:

https://supportforums.cisco.com/message/3454824#3454824

The BGP Synchronization is an often discussed topic - and admittedly, it can be quite confusing at first.

Best regards,

Peter

[jaighobahi]

Thank you all.  I think I understand it now.  This statement makes the concept clearer:

"So the state of seeing an iBGP-learned route as IGP-learned in your routing table means that the route itself is synchronized."

In otherwords, if I see a route in the routing table and I also see the exact same route in the BGP forwarding table, it means that route is synchronized.  Correct?

[Peter Paluch]

Hello,

if I see a route in the routing table and I also see the exact same  route in the BGP forwarding table, it means that route is synchronized.   Correct? 

More precisely: If I see a route learned via an IGP protocol (RIP, EIGRP, OSPF, ISIS) in my routing table, and I also see the exact same route in the BGP database learned from an internal BGP neighbor, the route is synchronized.

Best regards,

Peter

[jaighobahi]

Thank you Sir.