Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 213)

Brocade Virtual ADX Switch and Router Guide 199
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Chapter
8
Configuring BGP4 (IPv4)
This chapter provides details on how to configure Border Gateway Protocol version 4 (BGP4).
Overview of BGP4
BGP4 is the standard Exterior Gateway Protocol (EGP) used on the Internet to route traffic between
Autonomous Systems (AS) and to maintain loop-free routing. An autonomous system is a collection
of networks that share the same routing and administration characteristics. For example, a
corporate Intranet consisting of several networks under common administrative control might be
considered an AS. The networks in an AS can but do not need to run the same routing protocol to
be in the same AS, nor do they need to be geographically close.
Devices within an AS can use different Interior Gateway Protocols (IGPs) such as OSPF to
communicate with one another. However, for devices in different ASs to communicate, they need to
use an EGP. BGP4 is the standard EGP used by Internet devices and therefore is the EGP
implemented on the device.
Relationship between the BGP4 route table and
the IP route table
The Virtual ADX 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 device that also is running
BGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGP4
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 BGP4 route table on a Virtual ADX 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 BGP4 chooses and sends to the IP route table is the preferred route. This
route is what the Virtual ADX 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 comprised of the network mask bits and an IP address (IP
address/ mask bits); 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.