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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 35 Configuring Policy-Based Routing About Policy-Based Routing
TCP packet from 61.1.1.1 to 133.3.3.1 with destination port 105
Processing moves from sequence #21 to #24, because all ACLs in these sequence numbers have
a deny action for port 105.
In sequence #25, ACL 105 has a permit action for TCP port 105.
The route-map deny takes effect, and the packet is routed using the default IP routing table.

PBR on Supervisor Engine 6-E, Supervisor Engine 6L-E, Catalyst 4900M, and Catalyst 4948E

Supervisor Engine 6-E, Supervisor Engine 6L-E, Catalyst 4900M, and Catalyst 4948E support matching
route-map actions with a packet by installing entries in the TCAM that match the set of packets described
by the ACLs in the match criteria of the route map. These TCAM entries point at adjacencies that either
perform the necessary output actions or forward the packet to software if either hardware does not
support the action or its resources are exhausted.
If the route-map specifies a set interface … action, packets that match the match statement are routed
in software. Similarly, if the route-map specifies a set default interface… action and there is no
matching IP route for the packet, the packet is routed in software.
Note The scale of hardware-based PBR is determined by TCAM size and the time required for the CPU to
flatten the ACL before programming into hardware. The latter will noticeably increase if a PBR policy
requires a considerable number of class-maps. For example, a PBR policy of 1,200 class-maps may
require 60-90 minutes of "flatten" time before programming into hardware. This process may repeat if
an adjacency change requires PBR reprogramming.
PBR Flow Switching
Note Supervisor Engine 6-E, Supervisor Engine 6L-E, Catalyst 4900M, and Catalyst 4948E do not implement
PBR using flow switching.
The Catalyst 4500 switching engine supports matching a set next-hop route-map action with a packet on
a permit ACL. All other route-map actions, as well as matches of deny ACLs, are supported by a flow
switching model. In this model, the first packet on a flow that matches a route-map is delivered to the
software for forwarding. Software determines the correct destination for the packet and installs an entry
into the TCAM so that future packets on that flow are switched in hardware. The Catalyst 4500 switching
engine supports a maximum of 4096 flows.
Using Policy-Based Routing
You can enable PBR to change the routing path of certain packets from the default path that would be
chosen by IP routing. For example, you can use PBR to provide the following functionality:
Equal access
Protocol-sensitive routing
Source-sensitive routing
Routing based on interactive versus batch traffic