Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 386)

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 device’s BGP4 route table can have multiple routes to the same destination, the BGP4
protocol evaluates the routes and chooses only one to send to the IP route table. The route that BGP4
chooses and sends to the IP route table is the preferred route . This route is what the device
advertises to other BGP4 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:
• Network number (prefix) - A value made up of the network mask bits and an IP address; for
example, 10.215.129.0/18 indicates a network mask of 18 bits applied to the IP address
10.215.129.0. When a BGP4 device advertises a route to one of its neighbors, it uses this format.
• AS-path - A list of the other autonomous systems 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 device re-advertises a learned best BGP4 route to the device’s neighbors even when the software
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.
How BGP4 selects a path for a route (BGP best path selection
algorithm)
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
The device does not use the default route to resolve BGP4 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.
How BGP4 selects a path for a route (BGP best path selection algorithm)
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