• LLGR is not supported for multicast address-families.

BGP paths received from neighbor are removed immediately when it detects that the session is down. This behavior is to keep BGP table and forwarding table updated with current network state. Eventually this avoids traffic blackhole and routing loops. But in some scenarios keeping the routes for longer time during control plane failure helps the services that are less IP sensitive to continue uninterrupted for longer duration. The traffic flow does not get affected in the following scenarios, even if the BGP routes are stored for longer time during the BGP neighbor failures:

• When route advertisement path is different than the forwarding path that is, through MPLS tunnels. For example, VPN routes.

• When the purpose of route advertisement is to push configuration that is, filter programming on the router. For example, flow-spec, route-targets.

• When route advertisement is used for auto-discovery. For example, VPLS.

BGP persistence enables the local router to retain routes that it has learnt from the configured neighbor even when the neighbor session is down. BGP persistence is also referred as long lived graceful restart (LLGR). LLGR comes into effect after graceful restart (GR) ends. LLGR ends either when the LLGR stale timer expires or when the neighbor sends the end-of-RIB marker after it has sent its routes. When LLGR for a neighbor ends, all routes from that neighbor that are still LLGR stale gets deleted. The LLGR capability is signaled to a neighbor in the BGP OPEN message, if configured. With BGP persistence the paths are held for very long time (days), and unlike graceful-restart behavior the paths are de-preferenced so that a non-stale path is chosen over a stale path.

BGP speaker advertises LLGR capability including all address-families configured with LLGR. LLGR stale time is per address-family. The BGP persistence feature is supported on the following address family indicators (AFIs):

• VPNv4 and VPNv6

• Flow spec (IPv4, IPv6, VPNv4 and VPNv6)

Persistence is executed by the helper router only after the graceful-restart is completed. Persistence ends when neighbor sends end-of-row (EoR) or persistence timer expires.

1. To verify LLGR capability advertise and receive status, use the command show ip bgp vpnv4 unicast neighbors neighbor-id .

   
   show ip bgp vpn4 unicast neighbors 1.1.1.1
   ………
   BGP neighbor is 1.1.1.1,  remote AS 1, internal link
    Description: test1
    …….
   Neighbor capabilities:
       Route refresh: advertised and received(new)
       Four-octets ASN Capability: advertised and received
       Address family IPv4 Unicast: advertised and received
       Address family VPNv4 Unicast: advertised and received
       Address family VPNv6 Unicast: advertised and received
       Graceful Restart Capability: advertised and received
         Remote Restart timer is 10 seconds
         Address families advertised by peer:
           none
         Address families advertised by peer before restart:
           none
       Long-lived Graceful Restart Capability:
           VPNv4 Unicast: advertised and received(was preserved)
           VPNv6 Unicast: received(was preserved)
       Enhanced Refresh Capability: advertised and received
       Multisession Capability: 
       Stateful switchover support enabled: NO for session 1
   
   For address family: VPNv4 Unicast
     Session: 1.1.1.1
   ………
   ………
   Long-lived Graceful-Restart(was preserved)
       Stalepath-time: sent 2000s, received 50s, accepted 2000s, used 50s
   

2. To verify restarting of the peer, use the command show ip bgp vpnv4 unicast neighbors neighbor-id .

   
   show ip bgp vpn4 unicast neighbors 1.1.1.1
   ..............
    BGP neighbor is 1.1.1.1,  remote AS 1, internal link
    Description: test1
   ..............
     BGP version 4, remote router ID 0.0.0.0
     BGP state = Active, down for 00:00:23
     Configured hold time is 15, keepalive interval is 5 seconds
     Minimum holdtime from neighbor is 0 seconds
     Neighbor sessions:
       0 active, is not multisession capable (disabled)
       Stateful switchover support enabled: NO for session 0
     Message statistics:
       InQ depth is 0
   …….
   …….
   For address family: VPNv4 Unicast
     Session: 1.1.1.1
   ………
   ………
   Long-lived Graceful-Restart
       Stalepath-time: sent 2000s, accepted 2000s, used 2000s
       Stalepath-timer running 37s remaining
   
   

3. To verify routes marked with long-lived stale routes, use the command show ip bgp vpnv4 all .

   
   Device# show ip bgp vpnv4 all 
   BGP table version is 33, local router ID is 19.19.19.19
   Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
                 r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
                 x best-external, a additional-path, c RIB-compressed, 
   	       L long-lived-stale-path
   Origin codes: i - IGP, e - EGP, ? - incomplete
   RPKI validation codes: V valid, I invalid, N Not found
    
        Network          Next Hop            Metric LocPrf Weight Path
   Route Distinguisher: 2:2 (default for vrf example)
    *L>i 20.0.0.0/16      1.1.88.1                 0    100      0 81 ?
    *Li  38.1.1.0/24      1.1.88.1                 0    100      0 81 ?
    *L>i 180.180.180.180/32
                          1.1.88.1                 0    100      0 81 ?
   
   Router#show ip bgp vpnv4 all 20.0.0.0/16
   BGP routing table entry for 2:2:20.0.0.0/16, version 9
   Paths: (1 available, best #1, table example)
     Advertised to update-groups:
        1         
     Refresh Epoch 1
     81 (long-lived-stale), imported path from 5:5:20.0.0.0/16 (global)
       1.1.88.1 (metric 40) (via default) from 1.1.1.188 (1.1.1.188)
         Origin incomplete, metric 0, localpref 100, valid, internal, best
         Community: 100:100
         Extended Community: RT:1:1
         Originator: 1.1.88.1, Cluster list: 1.1.1.188
         mpls labels in/out nolabel/44
         rx pathid: 0, tx pathid: 0x0