According to the BGP standard (RFC 4271), a BGP router should not send updated reachability information for an NLRI to a BGP peer until a specific period of time, Min Route Advertisement Interval (MRAI), has elapsed from the last update. The RFC suggests the MRAI should be configurable per peer but does not propose a specific algorithm, and therefore, MRAI implementation details vary from one router operating system to another.
In SR OS, the MRAI is configurable, on a per-session basis, using the min-route-advertisement command. The min-route-advertisement command can be configured with any value between 1 and 255 seconds and the setting 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, resetting to the configured value whenever it reaches zero. Every time it reaches zero, 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, SR OS offers a rapid-update command that overrides the remaining time on a peer's MRAI timer and immediately sends routes belonging to specified address families (and all other pending updates) to the peers receiving these routes. The address families that can be configured with rapid-update support are:
EVPN
L2-VPN
label-IPv4
label-IPv6
MCAST-VPN-IPv4
MCAST-VPN-IPv6
MDT-SAFI
MVPN-IPv4
MVPN-IPv6
VPN-IPv4
VPN-IPv6
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 re-convergence 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When rapid-withdrawal is configured, UPDATE messages containing withdrawn NLRI are sent immediately to a peer without waiting for the MRAI timer to expire.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.
When there is a change to a labeled-unicast route that requires reprogramming of the label operations in the dataplane, these IOM updates are not made until the changed route is advertised to a peer, which depends on MRAI. Lowering the MRAI value or using rapid-update improves the speed of this operation.