Catalyst 4500 Series Switch SW Configuration Guide, Release IOS-XE 3.2.xSG

Configuring Policy-Based Routing

This chapter describes the tasks for configuring policy-based routing (PBR) on a Catalyst 4500 series switch and includes these major sections:

•Overview of Policy-Based Routing

•Policy-Based Routing Configuration Task List

•Policy-Based Routing Configuration Examples

Note For complete syntax and usage information for the switch commands used in this chapter, look at the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html

If the command is not found in the Catalyst 4500 Command Reference, it is located in the larger Cisco IOS library. Refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/ps6350/index.html

Note To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release.

Overview of Policy-Based Routing

This section contains the following subsections:

•About PBR

•Using Policy-Based Routing

PBR gives you a flexible method of routing packets by allowing you to define policies for traffic flows, lessening reliance on routes derived from routing protocols. To this end, PBR gives you more control over routing by extending and complementing the existing mechanisms provided by routing protocols. PBR allows you to specify paths for certain traffic, such as priority traffic over a high-cost link.

You can set up PBR as a way to route packets based on configured policies. For example, you can implement routing policies to allow or deny paths based on the identity of a particular end system, or an application protocol.

PBR allows you to perform the following tasks:

•Classify traffic based on extended access list criteria.

•Route packets to specific traffic-engineered paths.

Policies can be based on IP address, port numbers, or protocols. For a simple policy, use any one of these descriptors; for a complicated policy, use all of them.

About PBR

All packets received on an interface with PBR enabled (except those sent directly to the switch IP) are handled by 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:

•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.

This is explained in more detail in the section Understanding Route-Maps.

You specify PBR on the incoming interface (the interface on which packets are received), not outgoing interface.

Understanding Route-Maps

PBR is implemented by applying a route-map on an incoming interface. A given interface can have only one route-map configured.

A route-map is configured at the global configuration parser mode. You can then apply this route-map on one or more interfaces (in the interface configuration parser sub-mode).

A route-map is comprised of one or more route-map statements. Each statement has a sequence number, as well as a permit or deny clause.

Each route-map statement contains match and set commands. The match command denotes the match criteria to be applied on the packet data. The set command denote the PBR action to be taken on the packet.

The following example shows a single route-map called rm-test and six route-map statements:

    route-map rm-test permit 21

     match ip address 101

     set ip next-hop 21.1.1.1

    !

    route-map rm-test permit 22

     match ip address 102

     set ip next-hop 22.2.2.1

    !

    route-map rm-test permit 23

     match ip address 101 2102

     set interface vlan23

!

    route-map rm-test deny 24

     match ip address 104

     set ip next-hop 24.4.4.1

    !

    route-map rm-test deny 25

     match ip address 105

     set ip next-hop 25.5.5.1

    !

    route-map rm-test permit 26

     match ip address 2104

     set ip next-hop 26.6.6.1

The numbers 21, 22, ... 26 are the sequence numbers of the route-map statements.

The following topics are discussed:

•PBR Route-Map Processing Logic

•PBR Route-Map Processing Logic Example

PBR Route-Map Processing Logic

When a packet is received on an interface configured with a route-map, the forwarding logic processes each route-map statement according to the sequence number.

If the route-map statement encountered is a route-map...permit statement:

•The packet is matched against the criteria in the match command. This command may refer to an ACL that may itself have one or more permit and/or deny expressions. The packet is matched against the expressions in the ACL, and a permit/deny decision is reached.

•If the decision reached is permit, then the PBR logic executes the action specified by the set command on the packet.

•If the decision reached is deny, then the PBR action (specified in the set command) is not applied. Instead the processing logic moves forward to look at the next route-map statement in the sequence (the statement with the next higher sequence number). If no next statement exists, PBR processing terminates, and the packet is routed using the default IP routing table.

If the route-map statement encountered is a route-map... deny statement:

•The packet is matched against the criteria given in the match command. This command may refer to an ACL that may itself have one or more permit and/or deny expressions. The packet is matched against the expressions in the ACL, and a permit/deny decision is reached.

•If the criteria decision is permit, then the PBR processing terminates, and the packet is routed using the default IP routing table.

•If the criteria decision is deny, then the PBR processing logic moves forward to look at the next route-map statement in the sequence (the statement with the next higher sequence number). If no next statement exists, PBR processing terminates, and the packet is routed using the default IP routing table.

Note The set command has no effect inside a route-map... deny statement.

PBR Route-Map Processing Logic Example

Consider a route-map called rm-test defined as follows:

    access-list 101  permit tcp host 61.1.1.1 host 133.3.3.1 eq 101

    access-list 102  deny   tcp host 61.1.1.1 host 133.3.3.1 eq 102

    access-list 2102 permit tcp host 61.1.1.1 host 133.3.3.1 eq 102

    access-list 104  deny   tcp host 61.1.1.1 host 133.3.3.1 eq 104

    access-list 2104 permit tcp host 61.1.1.1 host 133.3.3.1 eq 104

    access-list 105  permit tcp host 61.1.1.1 host 133.3.3.1 eq 105

    route-map rm-test permit 21

     match ip address 101

     set ip next-hop 21.1.1.1

    !

    route-map rm-test permit 22

     match ip address 102

     set ip next-hop 22.2.2.1

    !

    route-map rm-test permit 23

     match ip address 101 2102

     set interface vlan23

    !

    route-map rm-test deny 24

     match ip address 104

     set ip next-hop 24.4.4.1

!

    route-map rm-test deny 25

     match ip address 105

     set ip next-hop 25.5.5.1

    !

    route-map rm-test permit 26

     match ip address 2104

     set ip next-hop 26.6.6.1

•TCP packet from 61.1.1.1 to 133.3.3.1 with destination port 101

–Matches ACL 101 in sequence #21.

–PBR is switched through next-hop 21.1.1.1.

Note ACL 101 is also matched in sequence #23, but the processing doesn't reach that point

•TCP packet from 61.1.1.1 to 133.3.3.1 with destination port 102

–In sequence #21, the ACL 101 action denies this packet (because all ACLs have an implicit deny). Processing advances to sequence #22.

–In sequence #22, ACL 102 matches TCP port 102, but the ACL action is deny. Processing advances to sequence #23.

–In sequence #23, ACL 2102 matches TCP port 102, and the ACL action is permit.

–Packet is switched to output interface VLAN 23.

•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.

Understanding PBR

Supervisor Engine 7-E and Supervisor Engine 7L-E 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.

Supervisor Engine 7-E and Supervisor Engine 7L-E support hardware PBR switching only for set ip next-hop... and set ip default next-hop actions.

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.

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, PBR can be used to provide the following functionality:

•equal access

•protocol-sensitive routing

•source-sensitive routing

•routing based on interactive versus batch traffic

•routing based on dedicated links

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.

Note PBR configuration is only allowed on interfaces belonging to the global routing table. PBR is not supported on interfaces that belong to VRFs.

Policy-Based Routing Configuration Task List

To configure PBR, perform the tasks described in the following sections. The task in the first section is required; the tasks in the remaining sections are optional. See the end of this chapter for the section "Policy-Based Routing Configuration Examples."

•Enabling PBR (Required)

•Enabling Local PBR (Optional)

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 apply that route-map on a particular interface. All packets arriving on the specified interface matching the match clauses are subject to PBR.

To enable PBR on an interface, perform this task:

The set commands can be used in conjunction with each other. These commands are evaluated in the order shown in Step 3 in the previous task table. A usable next hop implies an interface. Once the local switch finds a next hop and a usable interface, it routes the packet.

Enabling Local PBR

Packets that are generated by the switch are not normally policy-routed. To enable local PBR for such packets, indicate which route map the switch should use by performing this task:

All packets originating on the switch are 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.

Unsupported Commands

The following PBR commands in config-route-map mode are in the CLI but not supported in Cisco IOS for the switches. If you attempt to use these commands, an error message displays.

•match-length

•set ip qos

•set ip tos

•set ip precedence

Policy-Based Routing Configuration Examples

The following sections provide PBR configuration examples:

•Equal Access

•Differing Next Hops

•Deny ACE

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

Equal Access

The following example provides two sources with equal access to two different service providers. Packets arriving on interface fastethernet 3/1 from the source 1.1.1.1 are sent to the switch at 6.6.6.6 if the switch has no explicit route for the destination of the packet. Packets arriving from the source 2.2.2.2 are sent to the switch at 7.7.7.7 if the switch has no explicit route for the destination of the packet. All other packets for which the switch has no explicit route to the destination are discarded.

Switch (config)# access-list 1 permit ip 1.1.1.1

access-list 1 permit ip 1.1.1.1

!

interface fastethernet 3/1

 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

Note If the packets you want to drop do not match either of the first two route-map clauses, then change |
set default interface null0 to set interface null0.

Differing Next Hops

The following example illustrates how to route traffic from different sources to different places (next hops). Packets arriving from source 1.1.1.1 are sent to the next hop at 3.3.3.3; packets arriving from source 2.2.2.2 are sent to the next hop at 3.3.3.5.

access-list 1 permit ip 1.1.1.1

access-list 2 permit ip 2.2.2.2

!

interface fastethernet 3/1

 ip policy route-map Texas

!route-map Texas permit 10

 match ip address 1

 set ip next-hop 3.3.3.3

!route-map Texas permit 20

 match ip address 2

 set ip next-hop 3.3.3.5

Deny ACE

The following example illustrates how to stop processing a given route map sequence, and to jump to the next sequence. Packets arriving from source 1.1.1.1 skip sequence 10 and jump to sequence 20. All other packets from subnet 1.1.1.0 follow the set statement in sequence 10.

access-list 1 deny ip 1.1.1.1

access-list 1 permit ip 1.1.1.0 0.0.0.255

access-list 2 permit ip 1.1.1.1

access-list 2 permit ip 2.2.2.2

!

interface fastethernet 3/1

ip policy route-map Texas

!

route-map Texas permit 10

match ip address 1

set ip next-hop 3.3.3.3

!

route-map Texas permit 20

match ip address 2

set ip next-hop 3.3.3.5