6.1 Multiple Networks Per Message

The BGP protocol allows for multiple address prefixes with the same AS path and next-hop gateway to be specified in one message. Making use of this capability is highly recommended. With one address prefix per message there is a substantial increase in overhead in the receiver. Not only does the system overhead increase due to the reception of multiple messages, but the overhead of scanning the routing table for updates to BGP peers and other routing protocols (and sending the associated messages) is incurred multiple times as well. One method of building messages containing many address prefixes per AS path and gateway from a routing table that is not organized per AS path is to build many messages as the routing table is scanned. As each address prefix is processed, a message for the associated AS path and gateway is allocated, if it does not exist, and the new address prefix is added to it. If such a message exists, the new address prefix is just appended to it. If the message lacks the space to hold the new address prefix, it is transmitted, a new message is allocated, and the new address prefix is inserted into the new message. When the entire routing table has been scanned, all allocated messages are sent and their resources released. Maximum compression is achieved when all the destinations covered by the address prefixes share a gateway and common path attributes, making it possible to send many address prefixes in one 4096-byte message.

When peering with a BGP implementation that does not compress multiple address prefixes into one message, it may be necessary to take steps to reduce the overhead from the flood of data received when a peer is acquired or a significant network topology change occurs. One method of doing this is to limit the rate of updates. This will eliminate the redundant scanning of the routing table to provide flash updates for BGP peers and other routing protocols. A disadvantage of this approach is that it increases the propagation latency of routing information. By choosing a minimum flash update interval that is not much greater than the time it takes to process the multiple messages this latency should be minimized. A better method would be to read all received messages before sending updates.