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986 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Overview of BGP4
30
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 its
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 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.
• 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 BGP4 table but are
no longer feasible. The UPDATE message lists unreachable routes in the same format as new
routes.
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 Layer 3 Switches is 60 seconds.
A parameter related to the Keep Alive Time is the Hold Time. A BGP4 router 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 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.