Brocade Communications Systems FastIron SX 1600 Configuration Manual Download

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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.

•

Path attributes - Parameters that indicate route-specific information such as Autonomous System path information, route
preference, next hop values, and aggregation information. BGP4 uses path attributes to make filtering and routing decisions.

•

Unreachable routes - A list of routes that have been in the sending device BGP4 table but are no longer feasible. The UPDATE
message lists unreachable routes in the same format as new routes:

IP address

and

CIDR prefix

KEEPALIVE message

BGP4 devices do not regularly exchange UPDATE messages to maintain BGP4 sessions. For example, if a device configured to
perform BGP4 routing has already sent the latest route information to peers in UPDATE messages, the device does not send more
UPDATE messages. Instead, BGP4 devices send KEEPALIVE messages to maintain BGP4 sessions. KEEPALIVE messages are 19
bytes long and consist only of a message header. They do not contain routing data.

BGP4 devices send KEEPALIVE messages at a regular interval, called the Keep Alive Time. The default Keep Alive Time is 60 seconds.

A parameter related to the Keep Alive Time is the Hold Time. The Hold Time for a BGP4 device determines how many seconds the
device waits for a KEEPALIVE or UPDATE message from a BGP4 neighbor before deciding that the neighbor is not operational. The
Hold Time is negotiated when BGP4 devices exchange OPEN messages, the lower Hold Time is then used by both neighbors. For
example, if BGP4 device A sends a Hold Time of 5 seconds and BGP4 device B sends a Hold Time of 4 seconds, both devices use 4
seconds as the Hold Time for their BGP4 session. The default Hold Time is 180 seconds. Generally, the Hold Time is configured to
three times the value of the Keep Alive Time.

If the Hold Time is 0, a BGP4 device assumes that a neighbor is alive regardless of how many seconds pass between receipt of
UPDATE or KEEPALIVE messages.

NOTIFICATION message

When you close the BGP4 session with a neighbor, the device detects an error in a message received from the neighbor, or an error
occurs on the device, the device sends a NOTIFICATION message to the neighbor. No further communication takes place between the
BGP4 device that sent the NOTIFICATION and the neighbors that received the NOTIFICATION.

REFRESH message

BGP4 sends a REFRESH message to a neighbor to request that the neighbor resend route updates. This type of message can be useful
if an inbound route filtering policy has been changed.

Brocade Communications Systems FastIron SX 1600, Configuration Manual

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