Bgp4 Message Types - Foundry Networks Switch and Router Installation And Configuration Manual

Foundry Switch and Router Installation and Configuration Guide
• IGP is lowest
• EGP is higher than IGP but lower than INCOMPLETE
• 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, Foundry'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: Foundry Layer 3 Switches support BGP4 load sharing among multiple equal-cost paths. BGP4 load
sharing enables the Layer 3 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 19-28 for 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 Foundry Layer 3 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 Foundry Layer 3 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.
19 - 4 December 2000