BGP Update Groups

Update-groups were developed to help BGP processing be more efficient. BGP processing creates routing updates for each BGP peer separately and independently. If a router has three neighbors, it goes through the process of creating routing updates three times, once for each neighbor, even if all three neighbors get exactly the same advertisements.

With the use of update-groups, you can send the same updates to multiple neighbors without the need to recompute them. The router generates a single update and sends it out three times.

The update-group appears in the output of the show ip bgp command:

R1#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1
  3 4
    192.168.13.3 from 192.168.13.3 (192.168.34.3)
      Origin IGP, localpref 100, valid, external
  2 4
    192.168.12.2 from 192.168.12.2 (192.168.24.2)
      Origin IGP, localpref 100, valid, external, best

That value indicates how many times this update was advertised to how many neighbors. In the above case, it has been advertised to only one peer.

https://community.cisco.com/t5/switching/bgp-update-group/td-p/2098204