Bgp4 Message Types - Foundry Networks FESX Manual

10. If the internal paths also are the same and BGP4 load sharing is enabled, load share among the paths.
Otherwise, 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. For EBGP routes, load sharing applies only when the paths are from neighbors
within the same remote AS. EBGP paths from neighbors in different ASs are not compared.

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.
• BGP Identifier – The router ID. The BGP Identifier (router ID) identifies the BGP4 router to other BGP4
routers. Foundry Layer 3 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
Layer 3 Switch does not have a loopback interface, the default router ID is the lowest numbered IP address
configured on the device. For more information or to change the router ID, see "Changing the Router ID" on
page 16-23.
• Parameter list – An optional list of additional parameters used in peer negotiation with BGP4 neighbors.
UPDATE Message
After BGP4 neighbors establish a BGP4 connection over TCP and exchange their BGP4 routing tables, they do
not send periodic routing updates. Instead, a BGP4 neighbor sends an update to its neighbor when it has a new
route to advertise or routes have changed or become unfeasible. An UPDATE message can contain the following
information:
December 2005 © Foundry Networks, Inc.
Configuring BGP4
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