BGP route refresh and soft-reconfiguration

R1 ---------------------R2



R2(config-router)#do sh ip bgp nei 100.100.12.1
BGP neighbor is 100.100.12.1,  remote AS 100, external link
  BGP version 4, remote router ID 10.10.30.1
  BGP state = Established, up for 00:44:20
  Last read 00:00:20, last write 00:00:20, hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
    Route refresh: advertised and received(old & new)
    Address family IPv4 Unicast: advertised and received
    ......

Without soft-reconfiguration, clear ip bgp * soft command sends Refresh Request, as long as the peer router has Route Refresh capability, it sends back updates with entire BGP table. this command does the same work with command clear ip bgp * in.

R2#debug ip bgp
BGP debugging is on for address family: IPv4 Unicast
R2#debug ip bgp up
BGP updates debugging is on for address family: IPv4 Unicast
R2#
R2#
R2#clear ip bgp * soft
R2#
*Mar  1 00:06:01.987: BGP: 100.100.12.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar  1 00:06:01.987: BGP: 100.100.12.1 send message type 5, length (incl. header) 23
*Mar  1 00:06:02.083: BGP(0): 100.100.12.1 rcvd UPDATE w/ attr: nexthop 100.100.12.1, origin i, metric 0, path 100
*Mar  1 00:06:02.087: BGP(0): 100.100.12.1 rcvd 10.10.3.0/24...duplicate ignored
*Mar  1 00:06:02.087: BGP(0): 100.100.12.1 rcvd 10.10.2.0/24...duplicate ignored
*Mar  1 00:06:02.091: BGP(0): 100.100.12.1 rcvd 10.10.1.0/24...duplicate ignored
*Mar  1 00:06:02.091: BGP(0): 100.100.12.1 rcvd 10.10.0.0/24...duplicate ignored

R2#clear ip bgp 100.100.12.1  in
R2#
*Mar  1 00:53:33.883: BGP: 100.100.12.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar  1 00:53:33.883: BGP: 100.100.12.1 send message type 5, length (incl. header) 23

R2(config)#ip prefix-list INBOUND permit 10.10.0.0/24
R2(config)#ip prefix-list INBOUND permit 10.10.20.0/24

R2(config)#router bgp 200
R2(config-router)#nei 100.100.12.1 prefix-list INBOUND in

R2#sh ip bgp
BGP table version is 5, local router ID is 100.100.12.2
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
*> 10.10.0.0/24     100.100.12.1             0             0 100 i
*> 10.10.10.0/24    100.100.12.1             0             0 100 i
*> 10.10.20.0/24    100.100.12.1             0             0 100 i
*> 10.10.30.0/24    100.100.12.1             0             0 100 I
clear ip bgp * soft  and clear ip bgp * soft request the peer router to re-send BGP table so the new filter can apply on it.

R2#clear ip bgp 100.100.12.1 in
R2#
*Mar  1 00:56:04.899: BGP: 100.100.12.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar  1 00:56:04.899: BGP: 100.100.12.1 send message type 5, length (incl. header) 23
R2#
R2#
R2#sh ip bgp                  
BGP table version is 7, local router ID is 100.100.12.2
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
*> 10.10.0.0/24     100.100.12.1             0             0 100 i
*> 10.10.20.0/24    100.100.12.1             0             0 100 i
R2#
R2#

Without soft-reconfiguration, following command doesn't work.

R2#sh ip bgp nei 100.100.12.1 received-routes
% Inbound soft reconfiguration not enabled on 100.100.12.1
When enable soft-reconfiguration, the local router immediately sends out a Refresh Request to get a copy of the peer BGP table and stores it locally.

R2#config t
R2(config)#router bgp 200
R2(config-router)#nei 100.100.12.1 soft-reconfiguration in
R2(config-router)#
R2(config-router)#
R2(config-router)#
*Mar  1 00:57:43.879: BGP: 100.100.12.1 sending REFRESH_REQ(5) for afi/safi: 1/1
*Mar  1 00:57:43.879: BGP: 100.100.12.1 send message type 5, length (incl. header) 23
R2(config-router)#^Z
R2#

With soft-reconfiguration, the following command can display received route from local cached copy.
R2#sh ip bgp nei 100.100.12.1 received-routes
BGP table version is 11, local router ID is 100.100.12.2
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
*> 10.10.0.0/24     100.100.12.1             0             0 100 i
*  10.10.10.0/24    100.100.12.1             0             0 100 i
*> 10.10.20.0/24    100.100.12.1             0             0 100 i
*  10.10.30.0/24    100.100.12.1             0             0 100 i
Total number of prefixes 4
R2#
R2#
R2#sh ip bgp 10.10.0.0/24                        
BGP routing table entry for 10.10.0.0/24, version 11
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  100, (received & used)
    100.100.12.1 from 100.100.12.1 (10.10.30.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
The routes didn't pass the filter will be marked as received-only in the BGP table.

R2#sh ip bgp 10.10.10.0/24
BGP routing table entry for 10.10.10.0/24, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  100, (received-only)
    100.100.12.1 from 100.100.12.1 (10.10.30.1)
      Origin IGP, metric 0, localpref 100, valid, external


With soft-reconfiguration enabled, both commands clear ip bgp x.x.x.x soft and clear ip bgp x.x.x.x in no longer send out Refresh Request, instead, they re-apply policy on the local cached copy of the peer BGP table.

R2#
R2#clear ip bgp 100.100.12.1 soft
R2#
*Mar  1 01:54:39.887: BGP(0): start inbound soft reconfiguration for 100.100.12.1
*Mar  1 01:54:39.887: BGP(0): process 10.10.0.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 01:54:39.887: BGP(0): No inbound policy. Prefix 10.10.0.0/24 accepted unconditionally
*Mar  1 01:54:39.891: BGP(0): process 10.10.10.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 01:54:39.891: BGP(0): Prefix 10.10.10.0/24 rejected by inbound distribute/prefix-list.
*Mar  1 01:54:39.891: BGP(0): update denied
*Mar  1 01:54:39.895: BGP(0): process 10.10.20.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
R2#
*Mar  1 01:54:39.895: BGP(0): No inbound policy. Prefix 10.10.20.0/24 accepted unconditionally
*Mar  1 01:54:39.895: BGP(0): process 10.10.30.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 01:54:39.899: BGP(0): Prefix 10.10.30.0/24 rejected by inbound distribute/prefix-list.
*Mar  1 01:54:39.899: BGP(0): update denied
*Mar  1 01:54:39.899: BGP(0): complete inbound soft reconfiguration, ran for 12ms
R2#
R2#
R2#clear ip bgp 100.100.12.1 in
R2#
*Mar  1 02:02:35.891: BGP(0): start inbound soft reconfiguration for 100.100.12.1
*Mar  1 02:02:35.891: BGP(0): process 10.10.0.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 02:02:35.891: BGP(0): No inbound policy. Prefix 10.10.0.0/24 accepted unconditionally
*Mar  1 02:02:35.895: BGP(0): process 10.10.10.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 02:02:35.895: BGP(0): Prefix 10.10.10.0/24 rejected by inbound distribute/prefix-list.
*Mar  1 02:02:35.895: BGP(0): update denied
*Mar  1 02:02:35.899: BGP(0): process 10.10.20.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 02:02:35.899: BGP(0): No inbound policy. Prefix 10.10.20.0/24 accepted unconditionally
*Mar  1 02:02:35.899: BGP(0): process 10.10.30.0/24, next hop 100.100.12.1, metric 0 from 100.100.12.1
*Mar  1 02:02:35.903: BGP(0): Prefix 10.10.30.0/24 rejected by inbound distribute/prefix-list.
*Mar  1 02:02:35.903: BGP(0): update denied
*Mar  1 02:02:35.903: BGP(0): complete inbound soft reconfiguration, ran for 12ms
R2#

So, when the router has Route Refresh capability, the only benefit to enable soft-reconfiguration is to use following command:
show ip bgp nei x.x.x.x received-routes

However, without soft-reconfiguration, issue the following command also display the received routes:

R2#sh ip bgp nei 100.100.12.1 received-routes
% Inbound soft reconfiguration not enabled on 100.100.12.1
R2#

R2#sh ip bgp nei 100.100.12.1 route
BGP table version is 5, local router ID is 100.100.12.2
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
*> 10.10.0.0/24     100.100.12.1             0             0 100 i
*> 10.10.1.0/24     100.100.12.1             0             0 100 i
*> 10.10.2.0/24     100.100.12.1             0             0 100 i
*> 10.10.3.0/24     100.100.12.1             0             0 100 i
Total number of prefixes 4
R2#