Information Technology Reference
In-Depth Information
Identifying and Tagging BGP Prefixes That Require Preferential
Treatment
Figure 4-8 shows how this process works. Assume that AS 100 wants to create a special
forwarding policy for traffic between AS 200 and AS 300 for prefix 172.16.0.0/16. When
the prefix is first received from R1 via BGP, R2 tags the prefix with special BGP attributes,
such as a specific community value.
Figure 4-8
How QoS Policy Propagation via BGP Works
Using BGP
Attributes to Set
FIB Policy Entries
172.16.0.0/1
6
AS 100
AS 200
AS 300
R1
R2
R3
R4
R5
BGP Propagates Attributes
Enforce Policy
Set BGP
Attributes
Enable Policy
FIB Lookup
Setting FIB Policy Entries Based on BGP Tagging
As the prefix is propagated via BGP inside AS 100 to R4, the attributes are propagated as
well. When R4 receives the prefix with the matching attributes, it can set various FIB policy
entries using the
table-map
command in BGP. For QPPB, either or both Precedence and
QoS-group ID (a parameter internal to the router) can be set. The Precedence can have eight
values, 0 to 7, and the QoS-group ID can have 99 values, 1 to 99. Each value or a combina-
tion of both values can represent one class of traffic. Note that these settings have no impact
on traffic forwarding until they are used to classify and police the traffic (as discussed next).
Changes to the FIB/RIB tables are made when the IP RIB is cleared using
clear ip route *
,
the BGP session is reset, or a router is reloaded. All of these actions can be disruptive to the
traffic.
NOTE
Within the FIB entry for the prefix 172.16.0.0/16, the following mappings are possible,
depending on the table map configuration:
•
172.16.0.0 Precedence
•
172.16.0.0 QoS-group ID
•
172.16.0.0 Precedence and QoS-group ID
