How bgp4 selects a path for a route – Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) User Manual
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Brocade Virtual ADX Switch and Router Guide
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Overview of BGP4
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AS-path – A list of the other ASs through which a route passes. BGP4 devices can use the
AS-path to detect and eliminate routing loops. For example, if a route received by a BGP4
device contains the AS that the device is in, the device does not add the route to its own BGP4
table. (The BGP4 RFCs refer to the AS-path as “AS_PATH”, and RFC 4893 uses “AS4_PATH” in
relation to AS4s.)
•
Additional path attributes – A list of additional parameters that describe the route. The route
MED and next hop are examples of these additional path attributes.
NOTE
The Virtual ADX re-advertises a learned best BGP4 route to its 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 BGP4
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 devices send UPDATE messages to inform neighbors of
new, changed, or no longer feasible routes. BGP4 devices do not send regular updates. However, if
configured to do so, a BGP4 device does regularly send KEEPALIVE messages to its peers to
maintain BGP4 sessions with them if the device does not have any route information to send in an
UPDATE message. Refer to
on page 201 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 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. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the
route.
NOTE
By default, the device does not use the default route to resolve BGP4 next hop. Refer to
“Using the IP default route as a valid next-hop
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 device).
5. If the local preferences are the same, prefer the path with the shortest AS-path. An AS-SET
counts as 1.
NOTE
This step can be skipped if BGP4-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.