Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 36)

8 Multi-Service IronWare Routing Configuration Guide
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Overview of BGP4
• BGP4 Identifier – The device ID. The BGP4 Identifier (device ID) identifies the BGP4 device to
other BGP4 devices. The device use the same device ID for OSPF and BGP4. If you do not set a
device ID, the software uses the IP address on the lowest 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 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.