Policy-Based Routing

Policy-based routing is a process whereby the device puts packets through a route map before routing them. The route map determines which packets are routed to which device next. You might enable policy-based routing if you want certain packets to be routed some way other than the obvious shortest path. Possible applications for policy-based routing are to provide equal access, protocol-sensitive routing, source-sensitive routing, routing based on interactive versus batch traffic, and routing based on dedicated links. Policy-based routing is a more flexible mechanism for routing packets than destination routing.

To enable policy-based routing, you must identify which route map to use for policy-based routing and create the route map. The route map itself specifies the match criteria and the resulting action if all of the match clauses are met.

To enable policy-based routing on an interface, indicate which route map the device should use by using the ip policy route-map map-tag command in interface configuration mode. A packet arriving on the specified interface is subject to policy-based routing except when its destination IP address is the same as the IP address of the device's interface. This ip policy route-map command disables fast switching of all packets arriving on this interface.

To define the route map to be used for policy-based routing, use the route-map map-tag [permit | deny] [sequence-number] global configuration command.

To define the criteria by which packets are examined to learn if they will be policy-based routed, use either the match length minimum-length maximum-length command or the match ip address {access-list-number | access-list-name} [access-list-number | access-list-name] command or both in route map configuration mode. No match clause in the route map indicates all packets.

To display the cache entries in the policy route cache, use the show ip cache policy command.

The precedence setting in the IP header determines whether, during times of high traffic, the packets are treated with more or less precedence than other packets. By default, the Cisco software leaves this value untouched; the header remains with the precedence value that it had.

The precedence bits in the IP header can be set in the device when policy-based routing is enabled. When the packets containing those headers arrive at another device, the packets are ordered for transmission according to the precedence set, if the queueing feature is enabled. The device does not honor the precedence bits if queueing is not enabled; the packets are sent in FIFO order.

You can change the precedence setting, using either a number or name (the names came from RFC 791). You can enable other features that use the values in the set ip precedence route map configuration command to determine precedence. The table below lists the possible numbers and their corresponding name, from lowest to highest precedence.

The set commands can be used with each other. They are evaluated in the order shown in the previous table. A usable next hop implies an interface. Once the local device finds a next hop and a usable interface, it routes the packet.

Packets that are generated by the device are not normally policy-routed. To enable local policy routing for such packets, indicate which route map the device should use by using the ip local policy route-map map-tag global configuration command. All packets originating on the device will then be subject to local policy routing.

Use the show ip local policy command to display the route map used for local policy routing, if one exists.

1.    enable

2.    configure terminal

3.    interface type number

4.    ip policy route-map map-tag

5.    exit

6.    route-map map-tag [permit | deny] [sequence-number]

7.   Enter one or both of the following commands:

8.   set ip next-hop {ip-address[...ip-address] | dynamic dhcp | encapsulate l3vpn profile-name | peer-address | recursive [global | vrf vrf-name] ip-address | verify-availability [ip-address sequence track track-object-number]}

9.   end

Device> enable

Device# configure terminal

Device(config)# interface gigabitethernet 1/0/0

Device(config-if)# ip policy route-map equal-access

Device(config-if)# exit

Device(config)# route-map equal-access permit 10

Device(config-route-map)# match ip address 1

Device(config-route-map)# set ip next-hop 172.16.6.6

Device(config-route-map)# end