Internetwork Expert WB VOL 1 - BGP Lab 4

4)iBGP Synchronization

Objective: Configure BGP per the diagram to obtain connectivity from AS 1 to R4 and R5's loopback interfaces. R4 and R5 should have static default routes pointing towards R1 and R2 respectively. BGP synchronization should be enabled on R1 and R2.

Wow, this was a big step ..the topology was much better here. Let me get moving and then blog more on this one. Well to understand this lab better here is the bgp rule of synchronization, as quoted by Brian of IE:

‘Paths marked as "not synchronized" in the show ip bgp longer-prefixes output. If BGP synchronization is enabled, which it is by default in Cisco IOS® Software, there must be a match for the prefix in the IP routing table in order for an internal (iBGP) path to be considered a valid path. If the matching route is learned from an OSPF neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization using the no synchronization BGP subcommand. See the Synchronization section in Using the Border Gateway Protocol for Interdomain Routing for more information.
• Note: Synchronization is disabled by default in Cisco IOS Software version 12.2(8)T and later.’

Actually it’s a legacy rule, which was designed for a specific purpose, which is “Designed to prevent traffic black holes, in an transit AS”. As CCIE candidates it’s important to understand its purpose and how it works, irrespective of whether it is on or off and what are the possible solutions to prevent black-holes in our transit AS!!

And yes, btw what are black-holes?! In an transit AS, whenever you see a non-BGP speaker chances are that the packets will be dropped.

To prevent this black-hole problem, I am going to go ahead and list some of the possible solutions,

1) Run full mesh iBGP within the transit AS. (synchronization is off)
2) Redistribute BGP into the IGP, thereby making the two to be in sync with each other. (synchronization is on)
3) Impelement tunnels, (synchronization is off)

• Gre
• IPIP
• MPLS ..et all

I went ahead and using the same topology, I did make all the three above mentioned solutions work!! Towards, the end of the day I will again work with solutions, as I really want to have it wrapped around my head.

Let’s proceed to our next lab.