Implementing IBM System Networking 10Gb Ethernet Switches

66 Implementing IBM System Networking 10Gb Ethernet Switches

2.3.5 Border Gateway Protocol

Border Gateway Protocol (BGP) is an Internet protocol that enables routers on an IPv4

network to share and advertise routing information with each other about the segments of the

IPv4 address space they can access within their network and with routers on external

networks. You can use BGP to decide what is the “best” route for a packet to take from your

network to a destination on another network rather than setting a default route from your

border routers to your upstream providers. BGP is defined in RFC 1771, found at the

following website:

http://www.ietf.org/rfc/rfc1771.txt

External networks (outside your own) that are under the same administrative control are

referred to as

autonomous systems (AS). Sharing of routing information between autonomous

systems is known as external routing.

External BGP (eBGP) is used to exchange routes between different autonomous systems,

and internal BGP (iBGP) is used to exchange routes within the same autonomous system. An

iBGP is a type of internal routing protocol you can use to do active routing inside your

network. It also carries AS path information, which is important when you are an ISP or doing

BGP transit.

The iBGP peers must maintain reciprocal sessions to every other iBGP router in the same AS

(in a full-mesh manner) to propagate route information throughout the AS.

If the iBGP session shown between the two routers in AS 20 is not present (Figure 2-10), the

top router does not learn the route to AS 50, and the bottom router does not learn the route to

AS 11, even though the two AS 20 routers are connected through the IBM System

Networking switch.

Figure 2-10 iBGP and eBGP

Typically, an AS has one or more border routers, which are peer routers that exchange

routes with other ASs, and an internal routing scheme that enables routers in that AS to reach

every other router and destination within that AS. When you advertise routes to border

routers on other autonomous systems, you are committing to carry data to the IPv4 space

represented in the route that is advertised. For example, if you advertise 192.204.4.0/24, you

are declaring that if another router sends you data destined for any address in

192.204.4.0/24, you know how to carry that data to its destination.