Soft Reconfiguration

Two routes and one inbound policy

R1 advertises two host routes to R2. Begin with no inbound policy; both appear in R2’s accepted BGP table. This lab changes MED locally to make policy reprocessing visible.

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface Loopback10
 ip address 198.51.100.1 255.255.255.255
exit
interface Loopback11
 ip address 198.51.100.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router bgp 65001
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65002
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
  network 198.51.100.1 mask 255.255.255.255
  network 198.51.100.2 mask 255.255.255.255
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
router bgp 65002
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65001
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
 exit-address-family
end

R2# show ip bgp
     Network          Next Hop        Metric LocPrf Weight Path
*>   198.51.100.1/32  192.0.2.1            0             0 65001 i
*>   198.51.100.2/32  192.0.2.1            0             0 65001 i

Prepare the replay mechanism

router bgp 65002
 address-family ipv4 unicast
  neighbor 192.0.2.1 soft-reconfiguration inbound
 exit-address-family

Enable storage before the advertisements that must be retained. If configured after the session is up, populate the copy through a supported refresh or a planned session restart; the command cannot recreate information already discarded.

Change inbound MED

route-map IN-MED permit 10
 set metric 222
exit

router bgp 65002
 address-family ipv4 unicast
  neighbor 192.0.2.1 route-map IN-MED in
 exit-address-family

R2# clear ip bgp 192.0.2.1 soft in

R2# show ip bgp 198.51.100.1/32
BGP routing table entry for 198.51.100.1/32
  65001
    192.0.2.1 from 192.0.2.1
      Origin IGP, metric 222, localpref 100, valid, external, best

The changed attribute is visible after replay. TCP need not restart, but the resulting routing-policy change can still alter or remove forwarding paths; soft does not promise zero traffic impact.

Compare before-policy and after-policy information

R2# show ip bgp neighbors 192.0.2.1 received-routes
     Network             Next Hop       Metric Path
*>   198.51.100.1/32      192.0.2.1       0     65001 i
*>   198.51.100.2/32      192.0.2.1       0     65001 i

The retained received copy has MED 0; the accepted entry has MED 222. received-routes is not universally available without storage. Retaining unmodified paths costs memory per neighbor; the protocol’s conceptual Adj-RIB-In does not mandate this exact implementation.

Remove the experiment and re-evaluate

router bgp 65002
 address-family ipv4 unicast
  no neighbor 192.0.2.1 route-map IN-MED in
 exit-address-family

R2# clear ip bgp 192.0.2.1 soft in

R2# show ip bgp 198.51.100.1/32
BGP routing table entry for 198.51.100.1/32
  65001
    192.0.2.1 from 192.0.2.1
      Origin IGP, metric 0, localpref 100, valid, external, best

If storage was enabled only for this experiment, remove neighbor soft-reconfiguration inbound after verifying a suitable refresh method remains available.