Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 216)

202 Brocade Virtual ADX Switch and Router Guide
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Overview of BGP4
8
• OPEN
• UPDATE
• KEEPALIVE
• NOTIFICATION
• ROUTE REFRESH
OPEN message
After a BGP4 device establishes a TCP connection with a neighboring BGP4 device, the devices
exchange OPEN messages. An open message indicates the following:
• BGP4 version – Indicates the version of the protocol that is in use on the device. BGP4 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 the device.
• AS number – An autonomous system number (ASN) identifies the AS to which the BGP4 device
belongs. The number can be up to four bytes.
Hold Time – The number of seconds a BGP4 device will wait for an UPDATE or KEEPALIVE
message (described below) from a BGP4 neighbor before assuming that the neighbor is not
operational. BGP4 devices 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 device
closes the TCP connection to the neighbor and clears any information it has learned and
cached from the neighbor.
You can configure the Hold Time to be 0, in which case a BGP4 device will consider neighbors
to always be up. For directly-attached neighbors, you can configure the device 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 fail
over feature, which is disabled by default.
• BGP4 Identifier – The device ID. The BGP4 Identifier (device ID) identifies the BGP4 device to
other BGP4 devices. The device use the same device ID for OSPF and BGP4. If you do not set a
device ID, the software uses the IP address on the lowest numbered loopback interface
configured on the device. If the device does not have a loopback interface, the default device
ID is the lowest numbered IP address configured on the device.
• 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 a
neighbor when it has a new route to advertise or routes have changed or become unfeasible. An
UPDATE message can contain the following information:
• Network Layer Reachability Information (NLRI) – The mechanism by which BGP4 supports
Classless Interdomain Routing (CIDR). An NLRI entry consists of an IP prefix that indicates a
network being advertised by the UPDATE message. The prefix consists of an IP network
number and the length of the network portion of the number. For example, an UPDATE
message with the NLRI entry 10.215.129.0/18 indicates a route to IP network 10.215.129.0
with network mask 255.255.192.0. The binary equivalent of this mask is 18 consecutive one
bits, thus “18” in the NLRI entry.