In accordance with RFC 4271, a BGP router should not send updated NLRI reachability information to a BGP peer until a specific period of time (the minimum route advertisement interval (MRAI)) has elapsed since the last update. The RFC suggests that the MRAI should be configurable per peer, but does not propose a specific algorithm; consequently, MRAI implementation details vary from one router operating system to another.
On the 7210 SAS, the MRAI is configurable on a per-session basis using the min-route-advertisement command. This CLI command can be configured with any value between 1 and 255 seconds, and the configuration applies to all address families. The default value is 30 seconds, regardless of the session type (eBGP or iBGP). The MRAI timer is started at the configured value when the session is established and counts down continuously. When the timer reaches zero, it resets to the configured value and all pending RIB-OUT routes are sent to the peer.
To send Update messages that advertise new NLRI reachability information more frequently for some address families than others, use the rapid-update command to overrides the remaining time on a peer MRAI timer and immediately send routes belonging to specified address families (and all other pending updates) to the peers receiving these routes. The following address families support rapid-update:
EVPN
L2-VPN
MVPN-IPv4
In many cases, the default MRAI is appropriate for all address families (or at least those not included in the preceding list) when it applies to Update messages that advertise reachable NLRI, but it is not the best option for Update messages that advertise unreachable NLRI (route withdrawals). Fast reconvergence after some types of failures requires route withdrawals to propagate to other routers as quickly as possible so that they can calculate and start using new best paths, which would be impeded by the effect of the MRAI timer at each router hop. This is facilitated by the rapid-withdrawal configuration command.
When rapid-withdrawal is configured, Update messages containing withdrawn NLRI are sent immediately to a peer without waiting for the MRAI timer to expire. Update messages containing reachable NLRI continue to wait for the MRAI timer to expire, or for a rapid-update trigger, if it applies. When rapid-withdrawal is enabled, it applies to all address families.