Overview Of Bgp4 - Foundry Networks Switch and Router Installation And Configuration Manual

Foundry Switch and Router Installation and Configuration Guide

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.
Routers within an AS can use different Interior Gateway Protocols (IGPs) such as RIP and OSPF to communicate
with one another. However, for routers in different ASs to communicate, they need to use an EGP. BGP4 is the
standard EGP used by Internet routers and therefore is the EGP implemented on Foundry Layer 3 Switches.
Figure 19.1 on page 19-2 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. Foundry Layer 3 Switches can be configured to
redistribute routes among BGP4, RIP, and OSPF. They also can redistribute static routes.
AS 1
IBGP
BigIron
IBGP
OSPF
Figure 19.1 Example BGP4 ASs
Relationship Between the BGP4 Route Table and the IP Route Table
The Foundry Layer 3 Switch's BGP4 route table can have multiple routes 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 Foundry Layer 3 Switch for BGP4, one of the configuration tasks you perform is to identify the Layer
3 Switch'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 BGP4
chooses and sends to the IP route table is the preferred route and will be used by the Foundry Layer 3 Switch. 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 Layer 3 Switch advertises a route to one of its neighbors, the route is
expressed in this format.
19 - 2
BigIron
OSPF
IBGP
BigIron
OSPF
AS 2
EBGP
RIP
IBGP
BigIron
RIP
BigIron
IBGP
BigIron
IBGP
RIP
December 2000