Information Technology Reference
In-Depth Information
Example 6-11
Inbound Policy Configuration for R3
R3#show running-config | begin bgp
router bgp 300
no synchronization
bgp log-neighbor-changes
network 172.160.0.0
neighbor 172.160.1.1 remote-as 300
neighbor 172.160.1.1 update-source loopback0
neighbor 172.160.1.2 remote-as 300
neighbor 172.160.1.2 update-source loopback0
neighbor 200.200.200.1 remote-as 200
neighbor 200.200.200.1 send-community
neighbor 200.200.200.1 route-map AS200_OUT_POLICY out
no auto-summary
!
route-map AS200_OUT_POLICY permit 10
set community 200:120
set as-path prepend 300 300
!
Outbound Traffic Policy
The default configuration does not apply any preference to the outbound policy. In the
preceding section about inbound policy, you discovered that AS 200 was better connected.
If full Internet tables were accepted, it is quite likely that the majority of traffic would be
sent outbound over the DS3 link to AS 200. This is undesirable, because it would leave the
OC-3 to AS 100 underutilized, and the DS3 to AS 200 would be overutilized.
The desired outbound policy is for the majority of traffic to flow over the OC-3 link. If the
traffic is destined for AS 200, it should be sent directly over the DS3 on R3 to AS 200.
The first cut at applying this policy is to request partial routes plus the default over all three
BGP sessions. The prefixes received on R1 and R2 from AS 100 should be pretty much the
same. After requesting this from the upstream providers, the traffic balances over all three
links; however, too much traffic follows the default routes over the DS3 to AS 100 and the
DS3 to AS 200.
The solution to this outbound policy issue is to set LOCAL_PREF inbound on the OC-3
to 120. This ensures that the only default route used is the one from the OC-3. The more
specific prefixes received from AS 200 on R3 result in traffic destined for AS 200 being sent
directly there.
The primary failure scenario is that of the OC-3. The failure of the OC-3 results in two
default prefixes. Traffic sent to R3 is sent to AS 200, and traffic sent to R2 is sent to AS 100.
The partial routes ensure that traffic destined for AS 100 is not sent to AS 200 and vice
versa.
