Brocade BigIron RX Series Configuration Guide - Manual (Page 897)

BigIron RX Series Configuration Guide 819
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Brocade implementation of BGP4
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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.
• 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 device 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 device 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 neighbors that received the NOTIFICATION.
REFRESH message
BGP sends a REFRESH message to a neighbor to request the neighbor to resend route updates.
This type of message can be useful if an inbound route filtering policy has been changed.
Brocade implementation of BGP4
BGP4 is described in RFC 1771 and the latest BGP drafts. The Brocade implementation fully
complies with RFC 1771 and also supports the following:
• RFC 1745 (OSPF Interactions)
• RFC 1997 (BGP Communities Attributes)
• RFC 2385 (TCP MD5 Signature Option)