Understanding Pbr; Understanding Pbr Flow Switching; Using Policy-Based Routing - Cisco 4500M Software Manual

Overview of Policy-Based Routing
PBR allows you to perform the following tasks:
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Policies can be based on IP address, port numbers, or protocols. For a simple policy, you can use any
one of these descriptors; for a complicated policy, you can use all of them.

Understanding PBR

All packets received on an interface with PBR enabled are passed through enhanced packet filters known
as route maps. The route maps used by PBR dictate the policy, determining to where the packets are
forwarded.
Route maps are composed of statements. The route map statements can be marked as permit or deny, and
they are interpreted in the following ways:
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You specify PBR on the incoming interface (the interface on which packets are received), not outgoing
interface.

Understanding PBR 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 will be 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 obvious shortest path. For
example, PBR can be used to provide the following functionality:
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Some applications or traffic can benefit from source-specific routing; for example, you can transfer stock
records to a corporate office on a higher-bandwidth, higher-cost link for a short time while sending
routine application data, such as e-mail, over a lower-bandwidth, lower-cost link.
Software Configuration Guide—Release 12.2(25)EW
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Classify traffic based on extended access list criteria. Access lists, then establish the match criteria.
Route packets to specific traffic-engineered paths.
If a statement is marked as deny, the packets meeting the match criteria are sent back through the
normal forwarding channels and destination-based routing is performed.
If the statement is marked as permit and a packet matches the access-lists, then the first valid set
clause is applied to that packet.
equal access
protocol-sensitive routing
source-sensitive routing
routing based on interactive versus batch traffic
routing based on dedicated links
Chapter 26 Configuring Policy-Based Routing
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