Foundry Configuration Guide for the FESX, FSX, and FWSX
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© Foundry Networks, Inc.
December 2005
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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 192.215.129.0/18 indicates a route to
IP network 192.215.129.0 with network mask 255.255.192.0. The binary equivalent of this mask is 18
consecutive one bits, thus “18” in the NLRI entry.
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Path attributes – Parameters that indicate route-specific information such as path information, route
preference, next hop values, and aggregation information. BGP4 uses the path attributes to make filtering
and routing decisions.
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Unreachable routes – A list of routes that have been in the sending router’s BGP4 table but are no longer
feasible. The UPDATE message lists unreachable routes in the same format as new routes:
<IP address>/<CIDR prefix>.
KEEPALIVE Message
BGP4 routers do not regularly exchange UPDATE messages to maintain the BGP4 sessions. For example, if a
Layer 3 Switch configured to perform BGP4 routing has already sent the latest route information to its peers in
UPDATE messages, the router does not send more UPDATE messages. Instead, BGP4 routers send KEEPALIVE
messages to maintain the BGP4 sessions. KEEPALIVE messages are 19 bytes long and consist only of a
message header; they contain no routing data.
BGP4 routers send KEEPALIVE messages at a regular interval, the Keep Alive Time. The default Keep Alive Time
on Foundry Layer 3 Switches is 60 seconds.
A parameter related to the Keep Alive Time is the Hold Time. A BGP4 router’s Hold Time determines how many
seconds the router will wait for a KEEPALIVE or UPDATE message from a BGP4 neighbor before deciding that the
neighbor is dead. The Hold Time is negotiated when BGP4 routers exchange OPEN messages; the lower Hold
Time is then used by both neighbors. For example, if BGP4 Router A sends a Hold Time of 5 seconds and BGP4
Router B sends a Hold Time of 4 seconds, both routers 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 router assumes that its neighbor is alive regardless of how many seconds pass
between receipt of UPDATE or KEEPALIVE messages.
NOTIFICATION Message
When you close the router’s BGP4 session with a neighbor, or the router detects an error in a message received
from the neighbor, or an error occurs on the router, the router sends a NOTIFICATION message to the neighbor.
No further communication takes place between the BGP4 router that sent the NOTIFICATION and the neighbor(s)
that received the NOTIFICATION.
Basic Configuration and Activation for BGP4
BGP4 is disabled by default. To enable BGP4 and place your Foundry Layer 3 Switch into service as a BGP4
router, you must perform at least the following steps:
1.
Enable the BGP4 protocol.
2.
Set the local AS number.
NOTE:
You must specify the local AS number. BGP4 is not functional until you specify the local AS number.
3.
Add each BGP4 neighbor (peer BGP4 router) and identify the AS the neighbor is in.
4.
Save the BGP4 configuration information to the system configuration file.
Содержание FastIron Edge Switch X424
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