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PowerConnect B-Series FCX Configuration Guide 985
53-1002266-01
Overview of BGP4
30
NOTE
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 autonomous systems 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 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 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. 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, refer to “Changing the router ID” on page 809.
• Parameter list – An optional list of additional parameters used in peer negotiation with BGP4
neighbors.