process (Step 6), it usually has no influence on
the BGP selection process. For example, an autonomous system
receiving an MED for a route can change its local preference to
enable the autonomous system to override what the other
autonomous system is advertising with its MED value. When BGP
is comparing MED values for the same destination network in the
path selection process, the lowest MED value is preferred. The
default MED value for each network that an autonomous system
owns and advertises to an EBGP neighbor is set to 0. To change
this value, use the default-metric number
command under the BGP process. Figure displays the command
parameter.
Content 6.6 Manipulating BGP
Path Selection with Route Maps 6.6.8 BGP Using
Route Maps and the MED Example Figure is used in the
following configurations to demonstrate how to manipulate
inbound traffic using route maps to change the BGP MED
attribute. The intention of these route maps is to designate
router A as the preferred path to reach networks
192.168.25.0/24 and 192.168.26.0/24, and to designate router B
as the preferred path to reach network 192.168.24.0/24. The
other networks should still be reachable through each router in
case of a link or router failure.
Content
6.6 Manipulating BGP Path Selection with
Route Maps 6.6.9 BGP Using Route Maps and the
MED Example (continued) The MED is set outbound when a
router is advertising to an EBGP neighbor. In the configuration
example for router A in Figure , a route map named “med_65004”
is linked to neighbor 192.168.28.1 as an outbound route map.
When router A sends an update to neighbor 192.168.28.1 (router
X), it processes the outbound update through route map
med_65004 and uses a set statement to change any values that
are specified, as long as the preceding match statement is met
in that section of the route map. The first line of the route
map is a permit statement with a sequence number of 10 for the
route map med_65004. This defines the first route-map
statement. The match condition for this statement checks all
networks that are permitted by access list 66. The first line
of access list 66 permits any networks that start with the
first three octets of 192.168.25.0, and the second line permits
networks that start with the first three octets of
192.168.26.0. All networks that are permitted by either of
these lines are set to a MED of 100. All other networks are
denied by this access list (there is an implicit deny all at
the end of all access lists), so they are not set to a MED of
100; their MED is not changed. These other networks must
proceed to the next route map statement in the med_65004 route
map. The second statement of the route map is a permit
statement with a sequence number of 100 for the route map
med_65004. The route map does not have any match statements,
just a set metric 200 command. This is a permit all
statement for route maps. Because the network administrator
does not specify a match condition for this portion of the
route map, all networks being processed through this section of
the route map (sequence number 100) are permitted, and they are
set to a MED of 200. If the network administrator did not set
the MED to 200, by default it would have been a MED of 0. Since
0 is less than 100, the routes with a MED of 0 would have been
the preferred paths to the networks in AS 65001. Similarly, in
the configuration example for router B in Figure , a route map
named “med_65004” is linked to neighbor 172.20.50.1 (router Y)
as an outbound route map. Before router B sends an update to
neighbor 172.20.50.1, it processes the outbound update through
route map med_65004 and uses a set statement to change any
values that are specified, as long as the preceding match
statement is met in that section of the route map. The first
line of the route map is a permit statement with a sequence
number of 10 for the route map med_65004, which defines the
first route-map statement. The match condition for that line
checks all networks that are permitted by access list 66.
Access list 66 on router B permits any networks that start with
the first three octets of 192.168.24.0. Any networks that are
permitted by this line are set to a MED of 100. All other
networks are denied by this access list, so they are not set to
a MED of 100. These other networks must proceed to the next
route map statement in the med_65004 route map. The second
statement of the route map is a permit statement with a
sequence number of 100 for the route map med_65004, but it does
not have any match statements, just a set metric 200
command. This is a permit all statement for route maps. Because
the network administrator does not specify a match condition
for this portion of the route map, all networks being
processed through this topic are permitted, but they are set to
a MED of 200. If the network administrator did not set the MED
to 200, by default it would have been set to a MED of 0.
Because 0 is less than 100, the routes with a MED of 0 would
have been the preferred paths to the networks in AS 65001.
Content 6.6 Manipulating BGP Path
Selection with Route Maps 6.6.10 BGP Using
Route Maps and the MED Example (continued) The BGP
forwarding table on router Z in AS 65004 displays the networks
that have been learned from AS 65001. Other networks that do
not affect this example have been omitted. On router Z, there
are multiple paths to reach each network. These paths all have
valid next-hop addresses, have synchronization disabled, and
are loop-free. All networks have a weight of 0 and a local
preference of 100, so Steps 1 and 2 do not determine the best
path. None of the routes were originated by this router or any
router in AS 65004; all networks came from AS 65001, so Step 3
does not apply. All networks have an AS path of one autonomous
system (65001) and were introduced into BGP with network
statements (“i” is the origin code), so Steps 4 and 5 are
equal. Step 6 states that BGP chooses the lowest MED if all
preceding steps are equal or do not apply. For network
192.168.24.0, the next hop of 172.20.50.2 has a lower MED than
the next hop of 192.168.28.2; therefore, for network
192.168.24.0, the path through 172.20.50.2 is the preferred
path. For networks 192.168.25.0 and 192.168.26.0, the next hop
of 192.168.28.2 has a lower MED of 100 compared to the MED of
200 through the next hop of 172.20.50.2; therefore,
192.168.28.2 is the preferred path for those networks.
Content 6.6 Manipulating BGP Path Selection
with Route Maps 6.6.11 Implementing BGP in the
Enterprise Figure depicts a typical enterprise BGP
implementation. The enterprise is multihomed to two different
ISPs to increase the reliability and performance of its
connection to the Internet. The ISPs may pass only default
routes or may also pass other specific routes, or even all
routes, to the enterprise. The enterprise routers connected to
the ISPs run EBGP with the ISP routers and IBGP between
themselves; thus all routers in the transit path within the
enterprise autonomous system run IBGP. These routers pass
default routes to the other routers in the enterprise rather
than redistributing BGP into the interior routing protocol. BGP
attributes may be manipulated, using the methods discussed so
far, by any of the routers running BGP to affect the path of
the traffic to and from the autonomous systems.
Content
6.7 BGP Lab Exercises 6.7.1 Lab 6-1
Configuring BGP with Default Routing Lab
Activity
Lab Exercise: Lab 6-1
Configuring BGP with Default Routing The International
Travel Agency relies extensively on the Internet for sales. The
company has contracted with two ISPs for Internet connectivity
with fault tolerance. You need to configure BGP, which runs
between the San Jose boundary router and the two ISP
routers.
In this lab, you will configure BGP to
exchange routing information with two Internet Service
Providers (ISPs).
Content 6.7 BGP Lab