Configuring Policy-Based Routing

This chapter describes the tasks for configuring policy-based routing (PBR) on a router.

For complete conceptual information, see the section "Policy-Based Routing" in the chapter "Classification Overview" in this book.

For a complete description of the PBR commands in this chapter, refer to the Cisco IOS Quality of Service Solutions Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index, or search online.

Policy-Based Routing Configuration Task List

To configure PBR, perform the tasks in the following sections. The first section is required; the remaining sections are optional.

See the end of this chapter for the section "Policy-Based Routing Configuration Examples."

Enabling PBR

To enable PBR, you must create a route map that specifies the match criteria and the resulting action if all of the match clauses are met. Then, you must enable PBR for that route map on a particular interface. All packets arriving on the specified interface matching the match clauses will be subject to PBR.

To enable PBR on an interface, use the following commands beginning in global configuration mode:

Command Purpose

Step 1

route-map map-tag [permit | deny] [sequence-number]
Defines a route map to control where packets are output. This command puts the router into route-map configuration mode.

Step 2

match length min max
and/or

match ip address {access-list-number | name} [...access-list-number | name]
Specifies the match criteria. You can specify one or both of the following:

Matches the Level 3 length of the packet.

and/or

Matches the source and destination IP address that is permitted by one or more standard or extended access lists.

If you do not specify a match command, the route map applies to all packets.

Step 3

set ip precedence [number | name]
set ip next-hop ip-address [... ip-address]
set interface interface-type interface-number
[... type number] set ip default next-hop ip-address [...ip-address]
set default interface interface-type interface-number [... type ...number]
Specifies the action or actions to take on the packets that match the criteria. You can specify any or all of the following:

Sets precedence value in the IP header. You can specify either the precedence number or name.

Sets next hop to which to route the packet (it need not be adjacent).

Sets output interface for the packet.

Sets next hop to which to route the packet, if there is no explicit route for this destination.

Sets output interface for the packet, if there is no explicit route for this destination.

Step 4

interface interface-type interface-number
Specifies the interface. This command puts the router into interface configuration mode.

Step 5

ip policy route-map map-tag
Identifies the route map to use for PBR. One interface can only have one route-map tag, but you can have multiple route map entries with different sequence numbers. These entries are evaluated in sequence number order until the first match. If there is no match, packets will be routed as usual.

	Command	Purpose
Step 1	route-map map-tag [permit \| deny] [sequence-number]	Defines a route map to control where packets are output. This command puts the router into route-map configuration mode.
Step 2	match length min max and/or match ip address {access-list-number \| name} [...access-list-number \| name]	Specifies the match criteria. You can specify one or both of the following: Matches the Level 3 length of the packet. and/or Matches the source and destination IP address that is permitted by one or more standard or extended access lists. If you do not specify a match command, the route map applies to all packets.
Step 3	set ip precedence [number \| name] set ip next-hop ip-address [... ip-address] set interface interface-type interface-number [... type number] set ip default next-hop ip-address [...ip-address] set default interface interface-type interface-number [... type ...number]	Specifies the action or actions to take on the packets that match the criteria. You can specify any or all of the following: Sets precedence value in the IP header. You can specify either the precedence number or name. Sets next hop to which to route the packet (it need not be adjacent). Sets output interface for the packet. Sets next hop to which to route the packet, if there is no explicit route for this destination. Sets output interface for the packet, if there is no explicit route for this destination.
Step 4	interface interface-type interface-number	Specifies the interface. This command puts the router into interface configuration mode.
Step 5	ip policy route-map map-tag	Identifies the route map to use for PBR. One interface can only have one route-map tag, but you can have multiple route map entries with different sequence numbers. These entries are evaluated in sequence number order until the first match. If there is no match, packets will be routed as usual.

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

Note Enabling PBR disables fast switching of all packets arriving on this interface.

If you want PBR to be fast-switched, see the section "Enabling Fast-Switched PBR," which follows.

Enabling Fast-Switched PBR

IP PBR can now be fast-switched. Prior to Cisco IOS Release 12.0, PBR could only be process-switched, which meant that on most platforms the switching rate was approximately 1000 to 10,000 packets per second. This speed was not fast enough for many applications. Users that need PBR to occur at faster speeds can now implement PBR without slowing down the router.

Fast-switched PBR supports all of the match commands and most of the set commands, with the following restrictions:

The set ip default next-hop and set default interface commands are not supported.
The set interface command is supported only over point-to-point links, unless a route-cache entry exists using the same interface specified in the set interface command in the route map. Also, at the process level, the routing table is consulted to determine if the interface is on a reasonable path to the destination. During fast switching, the software does not make this check. Instead, if the packet matches, the software blindly forwards the packet to the specified interface.

PBR must be configured before you configure fast-switched PBR. Fast switching of PBR is disabled by default. To enable fast-switched PBR, use the following command in interface configuration mode:

Command
Purpose

ip route-cache policy
Enables fast switching of PBR.

Command	Purpose
ip route-cache policy	Enables fast switching of PBR.

To display the cache entries in the policy route cache, use the show ip cache policy command. To display which route map is associated with which interface, use the show ip policy command.

Enabling Local PBR

Packets that are generated by the router are not normally policy-routed. To enable local PBR for such packets, indicate which route map the router should use by using the following command in global configuration mode:

Command
Purpose

ip local policy route-map map-tag
Identifies the route map to use for local PBR.

Command	Purpose
ip local policy route-map map-tag	Identifies the route map to use for local PBR.

All packets originating on the router will then be subject to local PBR.

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

Policy-Based Routing Configuration Examples

The following sections provide PBR configuration examples:

For information on how to configure policy-based routing, see the section "Policy-Based Routing Configuration Task List" in this chapter.

Equal Access Example

The following example provides two sources with equal access to two different service providers. Packets arriving on asynchronous interface 1 from the source 1.1.1.1 are sent to the router at 6.6.6.6 if the router has no explicit route for the packet's destination. Packets arriving from the source 2.2.2.2 are sent to the router at 7.7.7.7 if the router has no explicit route for the packet's destination. All other packets for which the router has no explicit route to the destination are discarded.

access-list 1 permit ip 1.1.1.1 
access-list 2 permit ip 2.2.2.2 
!
interface async 1
 ip policy route-map equal-access
!
route-map equal-access permit 10
 match ip address 1
 set ip default next-hop 6.6.6.6
route-map equal-access permit 20
 match ip address 2
 set ip default next-hop 7.7.7.7
route-map equal-access permit 30
 set default interface null0

Differing Next Hops Example

The following example illustrates how to route traffic from different sources to different places (next hops), and how to set the precedence bit in the IP header. Packets arriving from source 1.1.1.1 are sent to the next hop at 3.3.3.3 with the precedence bit set to priority; packets arriving from source 2.2.2.2 are sent to the next hop at 3.3.3.5 with the precedence bit set to critical.

access-list 1 permit ip 1.1.1.1
access-list 2 permit ip 2.2.2.2
!
interface ethernet 1
 ip policy route-map Texas
!
route-map Texas permit 10
 match ip address 1
 set ip precedence priority
 set ip next-hop 3.3.3.3
!
route-map Texas permit 20
 match ip address 2
 set ip precedence critical
 set ip next-hop 3.3.3.5

Table of Contents

Configuring Policy-Based Routing