Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 217)

Brocade Virtual ADX Switch and Router Guide 203
53-1003246-01
Overview of BGP4
8
• Path attributes – Parameters that indicate route-specific information such as AS 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/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.