7.57 BGP Soft Reconfiguration
• Configure R4 to accept all prefixes from BB3 irrespective of the configured inbound filters and store them all locally.
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Until RFC2918 introduced the Route Refresh capability to BGP it was impossible
to signal a remote BGP peer to re-advertise the prefixes (Adj-RIB-Out) to the
local peer. 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.
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R4:
router bgp 100
neighbor 204.12.1.254 soft-reconfiguration inbound
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Rack1R4#show ip bgp neighbors 204.12.1.254 received-routes
BGP table version is 65, 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#
Rack1R4#show ip bgp neighbors 204.12.1.254 routes
BGP table version is 65, 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#
Rack1R4#show ip bgp neighbors 204.12.1.254 ?
advertised-routes Display the routes advertised to a BGP neighbor
dampened-routes Display the dampened routes received from neighbor (eBGP peers only)
flap-statistics Display flap statistics of the routes learned from neighbor (eBGP peers only)
paths Display AS paths learned from neighbor
policy Display neighbor polices per address-family
received Display information received from a BGP neighbor
received-routes Display the received routes from neighbor
routes Display routes learned from neighbor
| Output modifiers
<cr>
Now, apply a prefix filter to the peering session with BB3, filtering all possible
prefixes. Apply soft reset to the peering session and check the Adj-RIB-In (routes
received from BB3 after filtering) with the total number of routes received from
BB3.
Rack1R4(config)#ip prefix-list TEST permit 119.0.0.0/8
Rack1R4(config)#ip prefix-list TEST deny 0.0.0.0/0 le 32
Rack1R4(config)#router bgp 100
Rack1R4(config-router)#neighbor 204.12.1.254 prefix-list TEST in
Rack1R4(config-router)#^Z
Rack1R4#
Rack1R4#clear ip bgp * soft in
Rack1R4#show ip bgp neighbors 204.12.1.254 routes (這是結果...)
BGP table version is 83, 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
*> 119.0.0.0 204.12.1.254 0 54 i
Total number of prefixes 1
Rack1R4#
Rack1R4#show ip bgp neighbors 204.12.1.254 received-routes (這是store in a separate memory buffer...)
BGP table version is 83, 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#
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