LAB: BGP Soft ReConfiguration

- Configure R4 to accept all prefixes from BB3 irrespective of the configured inbound filters and store them all locally.

This feature is very helpful in situations when the local peer changes inbound filtering policy and needs the peer to re-advertise the routing information.  The only way to accomplish the policy refresh was to tear down and re-establish the peering session, which could be very resource consuming and causes connectivity disruption. Before the Route Refresh capability became standardized, one workaround was to use the so-called soft-reconfiguration approach.
When a local BGP speaker is configured to apply soft-reconfiguration to a peer using the command 

neighbor <IP> soft-reconfiguration inbound 

the speaker will accept ALL prefixes from the remote peer and store them in a separate memory buffer (of course, a session reset is required for this operation to initialize). The prefixes are then processed via the inbound filters and the resulting information imported into Adj-RIB-In and finally to the BGP table. Every time the local policy changes, there is no need to re-establish the peering session but simply apply the filters to the stored information. The penalty is the extra memory needed to store the routing information from the peer. Of course, this feature became deprecated with the introduction of RR capability.

configuration:

R4:
router bgp 100
neighbor 204.12.1.254 soft-reconfiguration inbound

verification:

Rack1R4#show ip bgp neighbors 204.12.1.254 received-routes
BGP table version is 485, local router ID is 150.1.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 28.119.16.0/24   204.12.1.254             0             0 54 i
*> 28.119.17.0/24   204.12.1.254             0             0 54 i
*> 112.0.0.0        204.12.1.254                           0 54 50 60 i
*> 113.0.0.0        204.12.1.254                           0 54 50 60 i
*> 114.0.0.0        204.12.1.254                           0 54 i
*> 115.0.0.0        204.12.1.254                           0 54 i
*> 116.0.0.0        204.12.1.254                           0 54 i
*> 117.0.0.0        204.12.1.254                           0 54 i
*> 118.0.0.0        204.12.1.254                           0 54 i
*> 119.0.0.0        204.12.1.254                           0 54 i

Total number of prefixes 10 

Rack1R4#clear ip bgp 204.12.1.254 soft in
Rack1R4#show ip bgp neighbors 204.12.1.254 routes
BGP table version is 485, local router ID is 150.1.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 28.119.16.0/24   204.12.1.254             0             0 54 i
*> 28.119.17.0/24   204.12.1.254             0             0 54 i
*> 112.0.0.0        204.12.1.254                           0 54 50 60 i
*> 113.0.0.0        204.12.1.254                           0 54 50 60 i
*> 114.0.0.0        204.12.1.254                           0 54 i
*> 115.0.0.0        204.12.1.254                           0 54 i
*> 116.0.0.0        204.12.1.254                           0 54 i
*> 117.0.0.0        204.12.1.254                           0 54 i
*> 118.0.0.0        204.12.1.254                           0 54 i
*> 119.0.0.0        204.12.1.254                           0 54 i
Total number of prefixes 10