Information Technology Reference
In-Depth Information
Shifting the traffic pattern inbound to make use of the OC-3 exclusively for traffic from
AS 100 can be achieved by AS_PATH prepending on the DS3 link to AS 100. R2 prepends
twice. This causes AS 100 to prefer the path from R1 and to make use of the OC-3 link
exclusively.
To prevent the DS3 link in AS 200 from becoming overloaded, the majority of traffic is
shifted to AS 100. The AS_PATH prepending mechanism is used to prepend outbound to
AS 200 twice.
After adding the AS_PATH prepends, the traffic flow is given time to settle down to the new
policy. For some reason, all inbound traffic enters through the OC-3, and none enters through
either of the DS3s. This is not quite the desired policy. Traffic originating in AS 200 should
enter through that DS3. After checking with the upstream provider for AS 200, you discover
that they have direct peering with AS 100 and are receiving the prefix 172.160.0.0/16 with
an AS_PATH of 300 100, which is being preferred over the AS_PATH 300 300 300, which
you are advertising to them.
After discussing your intentions for traffic flow, it is suggested that you continue sending
the prefix to AS 200 over the DS3 with the two prepends. In addition, you will send them
the community 200:120, which will cause them to set a higher local preference for the pre-
fix and then prefer it over the prefix received from AS 100. Setting this community is based
on a predefined policy of AS 200, which allows the following communities to be used:
200:80 = Set LOCAL_PREF 80
200:100 = Set LOCAL_PREF 100 (default)
200:120 = Set LOCAL_PREF 120
After you apply the community, traffic begins to flow inbound over the DS3 link from
AS 200. At this point, the inbound traffic flow is consistent with the desired policy when all
circuits are active. The next step is to check the failure scenarios.
Because no traffic is flowing inbound over the DS3 from AS 100, turning down this circuit
does not affect traffic flow. Turning down the DS3 to AS 200 causes the path from AS 100
to be the only remaining path, and the OC-3 will be used for all traffic. So far, everything
looks good.
The last failure scenario is turning down the OC-3 link to AS 100. This makes the prepended
path from R2 take precedence in AS 100. The desired behavior is for traffic to flow over
both DS3 links, to AS 100 and AS 200 in a fairly even manner. Perfect load balancing is
not possible. However, after turning down the OC-3 link, the DS3 to AS 200 runs at about
35 Mbps, and the DS3 to AS 100 runs at about 12 Mbps. This distribution is not in line with
the desired inbound traffic policy.
The LOCAL_PREF attribute that is being set in AS 200 does not have an effect outside
AS 200, so it should not affect traffic balancing. This appears to be a result of AS 200's
being better connected than AS 100, which means that a majority of autonomous systems
on the Internet have a shorter AS_PATH to AS 200 than AS 100. Based on this assumption,
