Advanced Configuration and Management Guide

•INCOMPLETE is highest

7.If the routes have the same origin type, prefer the route with the lowest MED.

NOTE: If the path does not have the MED attribute, HP’s BGP4 uses zero as the MED value for the comparison.

8.If the routes have the same MED, prefer routes in the following order:

•Routes received through EBGP from a BGP neighbor outside of the confederation

•Routes received through EBGP from a BGP router within the confederation

•Routes received through IBGP

9.If all the comparisons above are equal, prefer the route that can be reached using the closest IGP neighbor. This is the closest internal path inside the AS to reach the destination.

10.If the internal paths also are the same, prefer the route that comes from the BGP4 router with the lowest router ID.

NOTE: HP routing switches support BGP4 load sharing among multiple equal-cost paths. BGP4 load sharing enables the routing switch to balance the traffic across the multiple paths instead of choosing just one path based on router ID. See “Changing the Maximum Number of Paths for BGP4 Load Sharing” on

page 10-25for more information.

BGP4 Message Types

BGP4 routers communicate with their neighbors (other BGP4 routers) using the following types of messages:

•OPEN

•UPDATE

•KEEPALIVE

•NOTIFICATION

OPEN Message

After a BGP4 router establishes a TCP connection with a neighboring BGP4 router, the routers exchange OPEN messages. An OPEN message indicates the following:

•BGP version – Indicates the version of the protocol that is in use on the router. BGP version 4 supports Classless Interdomain Routing (CIDR) and is the version most widely used in the Internet. Version 4 also is the only version supported on HP routing switches.

•AS number – A two-byte number that identifies the AS to which the BGP4 router belongs.

•Hold Time – The number of seconds a BGP4 router will wait for an UPDATE or KEEPALIVE message (described below) from a BGP4 neighbor before assuming that the neighbor is dead. BGP4 routers exchange UPDATE and KEEPALIVE messages to update route information and maintain communication. If BGP4 neighbors are using different Hold Times, the lowest Hold Time is used by the neighbors. If the Hold Time expires, the BGP4 router closes its TCP connection to the neighbor and clears any information it has learned from the neighbor and cached.

You can configure the Hold Time to be 0, in which case a BGP4 router will consider its neighbors to always be up. For directly-attached neighbors, you can configure the HP routing switch to immediately close the TCP connection to the neighbor and clear entries learned from an EBGP neighbor if the interface to that neighbor goes down. This capability is provided by the fast external fallover feature, which is disabled by default.

•BGP Identifier – The router ID. The BGP Identifier (router ID) identifies the BGP4 router to other BGP4 routers. HP routing switches use the same router ID for OSPF and BGP4. If you do not set a router ID, the software uses the IP address on the lowest numbered loopback interface configured on the router. If the

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