Quick Reference Guide

746 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Overview of BGP4
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Figure 115 on page 746 shows a simple example of two BGP4 ASs. Each AS contains three BGP4
switches. All of the BGP4 switches within an AS communicate using IBGP. BGP4 switches
communicate with other ASs using EBGP. Notice that each of the switches also is running an
Interior Gateway Protocol (IGP). The switches in AS1 are running OSPF and the switches in AS2 are
running RIP. Layer 3 Switches can be configured to redistribute routes among BGP4, RIP, and OSPF.
They also can redistribute static routes.
FIGURE 115 Example BGP4 ASs
Relationship between the BGP4 route table and the IP route table
The Layer 3 Switch BGP4 route table can have multiple routes to the same destination, which are
learned from different BGP4 neighbors. A BGP4 neighbor is another switch that also is running
BGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGP
communication. When you configure the Layer 3 Switch for BGP4, one of the configuration tasks
you perform is to identify the Layer 3 Switch BGP4 neighbors.
Although a Layer 3 Switch 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 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.
AS 1
AS 2
EBGP
IBGP
OSPF
OSPF
IBGP
IBGP
OSPF
RIP
RIP
RIP
IBGP
IBGP
IBGP
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch