Multiple Exit Discriminator; Scaling Bgp For Large Networks - Juniper J2300 User Manual

J-series™ Services Router User Guide

Multiple Exit Discriminator

Because the AS path rather than the number of hops between hosts is the primary
criterion for BGP route selection, an AS with multiple connections to a neighbor AS
can have multiple equivalent AS paths. When the routing table contains two routes
to the same host in a neighboring AS, a multiple exit discriminator (MED) metric
assigned to each route can determine which to include in the forwarding table. The
MED metric you assign can force traffic through a particular exit point in an AS.
Figure 70 illustrates how to use an MED metric to determine route selection.
Figure 70: MED Example
Figure 70 shows AS 1 and AS 2 connected by two separate BGP links to routers
C and D. Host E in AS 1 is located nearer router C. Host F also in AS 1, and is
located nearer router D. Because the AS paths are equivalent, two routes exist for
each host, one through router C and one through router D. To force all traffic
destined for host E through router C, network administrator for AS 2 assigns
an MED metric for each router to host E at its exit point. An MED metric of 10
is assigned to the route to host E through router C, and an MED metric of 20
is assigned to the route to host E through router D. BGP routers in AS 2 then
select the route with the lower MED metric for the forwarding table.

Scaling BGP for Large Networks

BGP is not a flooding protocol like RIP or OSPF. Instead, it is a peering protocol that
exchanges routes with fully meshed peers only. However, in large networks, the
full mesh requirement causes scaling problems. BGP combats scaling problems
with the following methods:
280
BGP Overview
AS 1 E
C
E MED = 10
F MED = 20
A
AS 2
Route Reflectors—for Added Hierarchy on page 281
F
D
F MED = 10
E MED = 20
B