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

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Overview of BGP4
Relationship between the BGP4 route table and the IP route

table

The device's BGP4 route table can have multiple routes or paths to the same destination, which are
learned from different BGP4 neighbors. A BGP4 neighbor is another router that also is running
BGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGP
communication. When you configure the device for BGP4, one of the configuration tasks you
perform is to identify the device's BGP4 neighbors.
Although a router's BGP4 route table can have multiple routes to the same destination, the BGP4
protocol evaluates the routes and chooses only one of the routes to send to the IP route table. The
route that BGP chooses and sends to the IP route table is the preferred route. This route is what the
device advertises to other BGP neighbors. If the preferred route goes down, BGP4 updates the
route information in the IP route table with a new BGP4 preferred route.
NOTE
If IP load sharing is enabled and you enable multiple equal-cost paths for BGP4, BGP4 can select
more than one equal-cost path to a destination.
A BGP4 route consists of the following information:
•
•
•
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
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.
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Network number (prefix) – A value comprised of the network mask bits and an IP address (<IP
address>/ <mask bits>); for example, 192.215.129.0/18 indicates a network mask of 18 bits
applied to the IP address 192.215.129.0. When a BGP4 device advertises a route to one of its
neighbors, the route is expressed in this format.
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
MED and next hop are examples of these additional path attributes.
"BGP4 message types" on page 734 for information about BGP4
BigIron RX Series Configuration Guide
53-1001986-01