Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 621)

• KEEPALIVE
• NOTIFICATION
• ROUTE REFRESH
BGP peering can be internal or external, depending on whether the two BGP peers belong to the same
AS or different ASs. A BGP session between peers within a single AS is referred to as an Interior BGP
(IBGP) session; a session between peers belonging to different ASs is referred to as an Exterior BGP
(EBGP) session.
In order to establish a TCP connection between two IBGP peers, the IP reachability should be
established either by means of the underlying IGP protocol (OSPF) or by means of static routes. When
routes are advertised within IBGP peers, the following primary actions are taken in contrast to EBGP
peering:
• Routes learned from an IBGP peer are not usually advertised to other IBGP peers, in order to
prevent loops within an AS.
• Path attributes are not usually changed, in order to maintain the best path selection at other nodes
within an AS.
• The AS path and next hop are not normally changed.
BGP message types
All BGP messages use a common packet header, with the following byte lengths:
Marker Length Type Data
16 2 1 variable
NOTE
All values in the following tables are in bytes.
Type can be OPEN, UPDATE, NOTIFICATION, or KEEPALIVE, as described below.
OPEN message
After establishing TCP connection, BGP peers exchange OPEN message to identify each other.
Version Autonomous System Hold-Time BGP Identifier Optional Parameter Len Optional Parameters
1 2 or 4 2 4 1 4
Version
Only BGP4 version 4 is supported.
Autonomous System
Both 2-byte and 4-byte AS numbers are supported.
KEEPALIVE and HOLDTIME messages
A BGP timer command specifies both keep-alive and hold-time operands that manage the intervals
for BGP KEEPALIVE and HOLDTIME messages. The first operand sets the number of seconds the
device waits for UPDATE/KEEPALIVE message before closing the TCP connection. The second
operand sets the number of seconds that BGP maintains a session with a neighbor without receiving
BGP message types
Network OS Administrator’s Guide 621
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