The flow-tag propagation feature enables you to establish a co-relation between route-policies and user-policies.
Flow-tag propagation using BGP allows user-side traffic-steering based on routing attributes such as, AS
number, prefix lists, community strings and extended communities. Flow-tag is a logical numeric identifier
that is distributed through RIB as one of the routing attribute of FIB entry in the FIB lookup table. A flow-tag
is instantiated using the 'set' operation from RPL and is referenced in the C3PL PBR policy, where it is
associated with actions (policy-rules) against the flow-tag value.
You can use flow-tag propagation to:
• Classify traffic based on destination IP addresses (using the Community number) or based on prefixes
• Select a TE-group that matches the cost of the path to reach a service-edge based on customer site service
• Apply traffic policy (TE-group selection) for specific customers based on SLA with its clients.
• Divert traffic to application or cache server.
Restrictions for flow-tag propagation
Some restrictions are placed with regard to using Quality-of-service Policy Propagation Using Border Gateway
Protocol (QPPB) and flow-tag feature together in a ASR9K platform. These include:
• A route-policy can have either 'set qos-group' or 'set flow-tag,' but not both for a prefix-set.
• Route policy for qos-group and route policy flow-tag cannot have overlapping routes. The QPPB and
• Mixing usage of qos-group and flow-tag in route-policy and policy-map is not recommended.
Source and destination-based flow tag
The source-based flow tag feature allows you to match packets based on the flow-tag assigned to the source
address of the incoming packets. Once matched, you can then apply any supported PBR action on this policy.
Configure Source and Destination-based Flow Tag
This task applies flow-tag to a specified interface. The packets are matched based on the flow-tag assigned
to the source address of the incoming packets.
You will not be able to enable both QPPB and flow tag feature simultaneously on an interface.
(using Community number or AS number).
level agreements (SLA).
flow tag features can coexist (on same as well as on different interfaces) as long as the route policy used
by them do not have any overlapping route.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x