How Bgp4 Selects A Path For A Route - Dell PowerConnect B-RX Configuration Manual

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Overview of BGP4
NOTE
The BigIron RX re-advertises a learned best BGP4 route to the BigIron RX'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 BigIron RX 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 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
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 BigIron RX).
5. If the local preferences are the same, prefer the path with the shortest AS-path. An AS-SET
6. If the AS-path lengths are the same, prefer the path with the lowest origin type. From low to
7.
740
path.
NOTE
By default, the device does not use the default route to resolve BGP4 next hop. Also refer to
"Enabling next-hop recursion"
for a BGP4 route" on page 781
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.
high, route origin types are valued as follows:
•
IGP is lowest
•
EGP is higher than IGP but lower than INCOMPLETE
•
INCOMPLETE is highest
If the paths have the same origin type, prefer the path with the lowest MED.
"BGP4 message types"
on page 781 and "Using the IP default route as a valid next hop
on page 741 for information about
BigIron RX Series Configuration Guide
53-1001810-01