Brocade BigIron RX Series Configuration Guide - Manual (Page 894)

816 BigIron RX Series Configuration Guide
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Overview of BGP4
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NOTE
The device re-advertises a learned best BGP4 route to the device’s neighbors even when the route
table manager does not select that route for installation in the IP route table. This can happen if a
route from another protocol, for example, OSPF, is preferred. The best BGP4 route is the route that
BGP selects based on comparison of the BGP4 route path’s attributes.
After a device successfully negotiates a BGP4 session with a neighbor (a BGP4 peer), the device
exchanges complete BGP4 route tables with the neighbor. After this initial exchange, the device
and all other RFC 1771-compliant BGP4 routers send UPDATE messages to inform neighbors of
new, changed, or no longer feasible routes. BGP4 routers do not send regular updates. However, if
configured to do so, a BGP4 router does regularly send KEEPALIVE messages to its peers to
maintain BGP4 sessions with them if the router does not have any route information to send in an
UPDATE message. Refer to
“BGP4 message types” on page 817 for information about BGP4
messages.
How BGP4 selects a path for a route
When multiple paths for the same route prefix are known to a BGP4 router, the router 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. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the
path.
NOTE
By default, the device does not use the default route to resolve BGP4 next hop. Also refer to
“Enabling next-hop recursion” on page 858 and “Using the IP default route as a valid next hop
for a BGP4 route” on page 858
2. Use the path with the largest weight.
3. If the weights are the same, prefer the path with the largest local preference.
4. Prefer the route that was originated locally (by this BGP4 device).
5. If the local preferences are the same, 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.
NOTE
This step can be skipped if bgp-as-path-ignore is configured.
6. If the AS-path lengths are the same, 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. If the paths have the same origin type, prefer the path with the lowest MED.