Hello Rojer,
BGP is designed with scalability as first objective.
the BGP advertise-interval is one of these measures: instead of using flash updates triggered by a change BGP waits for the expiration of the BGP advertise-interval before sending out the BGP update.
In this way if there are other changes that should be advertised the BGP process can prepare a more efficient update.
The price to be payed is the convergence time.
This can be seen in an iBGP including BGP route reflector servers.
For address-family ipv4 unicast (the standard global routing table) route reflector servers propagate only the best path for a destination to other clients and to non clients ( RRS of other clusters).
if the best path fails and it is withdrawn by originator router:first the withdrawn is propagated to the first RRS, that device will propagate after its own BGP advertise interval the update and so on.
The same RRS will pick up a secondary path for the same destination and will propagate in the iBGP domain.
To be noted, also the detection of failed BGP routes that is performed by BGP scanner process is scheduled executed every 60 seconds (default value).
For MPLS L3 VPN, a workaround for multihomed VRF sites (connected to two PE nodes) is to have the two PE nodes to use two slightly different route distinguishers. Being RDs part of VPNv4 prefix the RRS will treat the two prefixes as two different advertisements and both are propagated every where.
Some advanced features to improve BGP convergence have been introduced:
BGP next-hop tracking
BFD
see
http://www.cisco.com/en/US/docs/ios/12_4t/ip_route/configuration/guide/brbadv.html
Edit:
you can modify the advertisement interval for a specific neighbor
neighbor {ip-address | peer-group-name} advertisement-interval seconds
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp3.html#wp1012782
Hope to help
Giuseppe
