Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 389)

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.
Grouping of RIB-out peers
To improve efficiency in the calculation of outbound route filters, the device groups BGP4 peers
together based on their outbound policies. To reduce RIB-out memory usage, the device then groups
the peers within an outbound policy group according to their RIB-out routes. All peers sharing a single
RIB-out route (up to 32 peers per group) also share a single physical RIB-out entry, resulting in as much
as a 30-fold memory usage reduction.
KEEPALIVE message
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