BGP Reachability

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R1
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R2—FR CLOUD—R6
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R5

Here in the above scenario R1 and R2 both connected to R6 via Frame-relay cloud and OSPF area 0 is running between all of them, and R6 is advertising default route into OSPF domain.R2 is in BGP AS 100,R6 is in BGP AS 200 and R5 is in AS 54.Here BGP is not running on R1.

The above task describes a case where reachabilty is lost to certain BGP networks when the primary Frame Realy connection of R2 is down.When the Frame Relay connection is down,all of R2’s traffic destined to R6 must transit R1.The problem , however is that R1 does not participate in BGP routing.Therefore,although BGP NLRI(Network Layer Reachability Information) is successfully transmitted throughout the network,traffic may be black holed when it reaches R1.

In order to resolve this issue, BGP has been redistributed into OSPF(IGP).R2 has been configured to redistribute all BGP information learned from AS 54 into OSPF.For traffic in the opposite direction, it doesnt matter,since R6 is originating a default route. The syntax of the commands will be:- on R2

router ospf 1 

redistribute bgp 100 subnets route-map BGP2OSPF

 ip as-path access-list 1 permit ^54_

route-map BGP2OSPF permit 10

match as-path 1

>>>Here _ matching everything in between that is starting ^ from 54.