Brocade BigIron RX Series Configuration Guide - Manual (Page 893)

BigIron RX Series Configuration Guide 815
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Overview of BGP4
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Figure 121 on page 815 shows a simple example of two BGP4 ASs. Each AS contains three BGP4
routers. All of the BGP4 routers within an AS communicate using IBGP. BGP4 routers communicate
with other ASs using EBGP. Notice that each of the routers also is running an Interior Gateway
Protocol (IGP). The routers in AS1 are running OSPF and the routers in AS2 are running RIP. The
device can be configured to redistribute routes among BGP4, ISIS, RIP, and OSPF. They also can
redistribute static routes.
FIGURE 121 Example BGP4 ASs
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:
• 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.
AS 1 AS 2
EBGP
IBGP
OSPF
OSPF
IBGP
IBGP
OSPF
RIP
RIP RIP
IBGP
IBGP
IBGP