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Introduction to BGP Synchronization rule

BGP Synchronization rule

BGP Synchronization rule is one of the rule to avoid traffic black hole inside the internal network and says that if the route is learned to the router NDNA_R1 from external router NDNA_R5 will not send to the router NDNA_R2 until unless it is known to the internal network which is connected via OSPF with NDNA_R1 and NDNA_R2.

Below is the topology showing the BGP Synchronization and the configuration on the respective routers as below. Hope you get the idea how configurations looks like on the routers.

Fig 1.1- BGP Synchronization rule

Let's talk how this rule in place in the above topology

How BGP Synchronization rule works
if Router NDNA_R5 sends updates about network 60.0.0.0 and received by Router NDNA_R1. As you see in the diagram that Routers NDNA_R1 and NDNA_R2 are are connected via iBGP and Router NDNA_R2 receives routing updates about network 60.0.0.0 via iBGP.

If Router NDNA_R2 wants to reach network 60.0.0.0, it sends traffic to Router NDNA_R3. If Router NDNA_R1 does not redistribute network 60.0.0.0 into an OSPF, Router NDNA_R3 has no way of knowing that network 60.0.0.0 exists and will drop the packets. 

Commands on Router NDNA_R1
NDNA_R1# Config t
NDNA_R1(config)# router bgp 100
NDNA_R1(config)# network 50.0.0.0
NDNA_R1(config)# neighbor 5.5.5.2 remote-as 100 (iBGP)
NDNA_R1(config)# neighbor 10.10.10.2 remote-as 200 (eBGP)
NDNA_R1(config)# no synchronization

Commands on Router NDNA_R2
NDNA_R2# Config t  
NDNA_R2(config)#router bgp 100
NDNA_R2(config)# network 50.0.0.0
NDNA_R2(config)# neighbor 5.5.5.1 remote-as 100 (iBGP)
NDNA_R2(config)# neighbor 20.20.20.2 remote-as 300 (eBGP)
NDNA_R2(config)# neighbor 30.30.30.2 remote-as 400 (eBGP)
NDNA_R2(config)# no synchronization

Commands on Router NDNA_R5
NDNA_R5# Config t  
NDNA_R5(config)#router bgp 200
NDNA_R5(config)# network 60.0.0.0
NDNA_R5(config)# neighbor 10.10.10.1 remote-as 100 (eBGP)

Commands on Router NDNA_R6
NDNA_R6# Config t  
NDNA_R6(config)#router bgp 300
NDNA_R6(config)# network 70.0.0.0
NDNA_R6(config)# neighbor 20.20.20.1 remote-as 100 (eBGP)

Commands on Router NDNA_R7
NDNA_R6# Config t  
NDNA_R6(config)#router bgp 400
NDNA_R6(config)# network 80.0.0.0
NDNA_R6(config)# neighbor 30.30.30.1 remote-as 100 (eBGP)

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