Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 747
53-1002269-02
Overview of BGP4
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• AS-path – A list of the other ASs through which a route passes. BGP4 routers can use the
AS-path to detect and eliminate routing loops. For example, if a route received by a BGP4 router
contains the AS that the router is in, the router does not add the route to its own BGP4 table.
(The BGP4 RFCs refer to the AS-path as “AS_PATH”.)
• Additional path attributes – A list of additional parameters that describe the route. The route
origin and next hop are examples of these additional path attributes.
NOTE
The Layer 3 Switch re-advertises a learned best BGP4 route to the Layer 3 Switch neighbors even
when the software does not select that route for installation in the IP route table. The best BGP4
route is the route that the software selects based on comparison of the BGP4 route path attributes.
After a Layer 3 Switch successfully negotiates a BGP4 session with a neighbor (a BGP4 peer), the
Layer 3 Switch exchanges complete BGP4 route tables with the neighbor. After this initial
exchange, the Layer 3 Switch 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 748
for information about BGP4 messages.
How BGP4 selects a path for a route
When multiple paths for the same route 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. (Refer to “Optional configuration tasks”
on page 767.)
1. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the
route.
NOTE
The device does not use the default route to resolve BGP4 next hop. Also refer to “Enabling
next-hop recursion” on page 774.
2. Use the path with the largest weight.
3. If the weights are the same, prefer the route with the largest local preference.
4. If the routes have the same local preference, prefer the route that was originated locally (by
this BGP4 Layer 3 Switch).
5. If the local preferences are the same, prefer the route 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.
6. If the AS-path lengths are the same, prefer the route 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