Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 623)

• Finite state-machine error
• Cease (voluntarily)
Error Subcode
Provides specific information about the error reported.
Error Data
Contains data based on error code and subcode.
KEEPALIVE message
Because BGP does not regularly exchanges route updates to maintain a session, KEEPALIVE
messages are sent to keep the session alive. A KEEPALIVE message contains just the BGP header
without data field. Default KEEPALIVE time is 60 seconds and is configurable.
REFRESH message
A REFRESH message is sent to a neighbor requesting that the neighbor resend the route updates. This
is useful when the inbound policy has been changed.
BGP attributes
BGP attributes are passed in UPDATE messages to describe the characteristics of a BGP path by the
advertising router. At a high level, there are only two types of attributes: well-known and optional. All of
the well-known attributes as described in RFC 4271 are supported in Network OS 4.0 and later.
Best-path algorithm
The BGP decision process is applied to the routes contained in the Routing Information Base, Incoming
(RIB-In), which contains routes learned from inbound update messages. The output of the decision
process is the set of routes that will be advertised to BGP speakers in local or remote autonomous
systems and are stored in the Adjacency RIB, Outgoing (RIB-Out).
When multiple paths for the same route prefix are known to a BGP4 device, the device uses the
following algorithm to weigh the paths and determine the optimal path for the route. (The optimal path
depends on various parameters, which can be modified.)
1. Verify that the next hop can be resolved by means of Interior Gateway Protocol (IGP).
2. Use the path with the largest weight.
3. Prefer the path with the higher local preference.
4. Prefer the route that was self-originated locally.
5. Prefer the path with the shortest AS-path. An AS-SET counts as 1. A confederation path length, if
present, is not counted as part of the path length. (An as-path ignore command disables the
comparison of the AS path lengths of otherwise equal paths.)
6. Prefer the path with the lowest origin type. From low to high, route origin types are valued as follows:
• IGP is lowest.
• EGP is higher than IGP, but lower than INCOMPLETE.
• INCOMPLETE is highest.
7. Prefer the path with the lowest MED.
KEEPALIVE message
Network OS Administrator’s Guide 623
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