Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.8.xE and IOS 15.2(4)Ex

Route Maps

The following topics are discussed in the section:

• Understanding Route Maps
• PBR Route-Map Processing Logic
• Load Balancing with Recursive Next Hop
• Packet Matching Criteria
• PBR Route-Map Processing Logic Example

Understanding Route Maps

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 dictate the policy that determines where the packets are forwarded.

Route maps contain statements that can be marked as permit or deny. They are interpreted in the following ways:

• If a statement is marked as deny, the packets meeting the match criteria are sent back using 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.

You can implement PBR 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).

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 denotes 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:

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

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.

A route map statement can have multiple set commands that are applied in the following priority:

set ip next-hop verify-availability

set ip next-hop

set ip next-hop recursive

set interface

set default ip next-hop

set default interface

If both the set ip next-hop and set ip next-hop recursive commands are present in the same route-map statement, the next-hop set command is applied.

If the set ip next-hop command is not available then the set ip next-hop recursive command is applied.

If the set ip recursive-next-hop and the set interface command are not present, then the packet is routed using the default routing table; it is not dropped. If the packet is required to be dropped, use the set next-hop recursive command followed by a set interface null0 configuration command.

Load Balancing with Recursive Next Hop

If multiple equal-cost routes to the subnet have been configured by the set ip next-hop recursive command, load balancing will occur only if all the adjacencies to the routes are resolved. If any of the adjacencies have not been resolved, then load balancing will not happen and only one of the routes whose adjacency is resolved will be used. If none of the adjacencies are resolved, then packets will be processed in software, resulting in at least one of the adjacencies to be resolved and programmed in hardware. PBR relies on routing protocols or other means to resolve all adjacencies and make load balancing happen.

Packet Matching Criteria

Access Control Lists (ACLs) define the allowed match criteria for packets. Each ACL is applied to incoming packets in a certain order, stopping only when the packet characteristics match the ACL being applied. Unlike policy maps, route maps do not support the "match any" match semantics.

IPv6 packets are matched via a match ipv6 address statement in the associated PBR route-map. IPv6 PBR requires IPv6 ACL, although the statement may specify either an IPv6 ACL or an IPv6 Prefixlist,

Packets are matched using the following criteria:

• Input interface
• Source IPv4/IPv6 Address (Prefixlist/Standard/Extended ACL)
• Destination IPv4/IPv6 Address (Standard/Extended ACL)
• Protocol (Extended ACL)
• Source Port and Destination Port (Extended ACL)
• DSCP (Extended ACL)
• Flow-label (Extended ACL)
• Fragment (Extended ACL)

PBR Route-Map Processing Logic Example

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

!

• 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 command takes effect, and the packet is routed using the default IP routing table.

The Catalyst 4500 series switch supports 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 the software. Some packets may be dropped. 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 the software.

Note The scale of hardware-based PBR is determined by the TCAM size and the time required for the CPU to flatten the ACL before programming into the hardware. The time take to flatten the ACL increases when a PBR policy requires a considerable number of route-maps. For example, a PBR policy of 1,200 route-maps (each containing ACLs with permit ACEs only) may require 6-7 minutes of flatten time before programming into hardware. This process may repeat if an adjacency change requires PBR reprogramming.

Policy-Based Routing with Object Tracking

Beginning in Cisco IOS XE Release 3.8.0E and Cisco IOS Release 15.2(4)E, you can configure Policy-Based Routing (PBR) to use object tracking, to verify the most viable next-hop IP address to which to forward packets, using an Internet Control Message Protocol (ICMP) ping as the verification method. PBR used with object tracking is most suitable for devices that have multiple Ethernet connections as the next hop. Normally, Ethernet interfaces connect to digital subscriber line (DSL) modems or cable modems, and do not detect a failure upstream in the ISP broadband network. The Ethernet interface remains up, and any form of static routing points to that interface. Using PBR with object tracking allows you to back-up two Ethernet interfaces, determine the interface that is available by sending ICMP pings to verify if the IP address can be reached, and then route traffic to that interface.

To verify the next-hop IP address for the device, PBR informs the object tracking process that it is interested in tracking a certain object. The tracking process, in turn, informs PBR when the state of the object changes.

Restrictions for Policy-Based Routing with Object Tracking

The set next-hop verify-availability command is not supported with the following:

• VRF instances
• Virtual switching system (VSS)
• IPv6 traffic

IPv4 and IPv6 Policy-Based Routing for VRF Instances

Virtual routing and forwarding (VRF) allows multiple routing instances in Cisco software. Beginning in Cisco IOS Release XE 3.7 0E and IOS 15.2(3)E, the Policy-Based Routing (PBR) feature is VRF-aware and works on multiple routing instances, beyond the default or global routing table.

Incoming packets are filtered through the match criteria that are defined in the route map. After a successful match occurs, the set command configuration determines the VRF through which outbound packets are policy routed.

Inherit-VRF, Inter-VRF, Global-to-VRF, and VRF-to-Global Routing

The Policy-Based Routing feature supports inherit-VRF, inter-VRF, and VRF-to-global routing.

With inherit-VRF, packets arriving at a virtual routing and forwarding (VRF) interface are routed, by looking-up the same VRF’s routing table.

With inter-VRF routing, packets arriving at a VRF interface are routed, by looking-up a different VRF’s routing table, as specified by the set command.

With VRF-to-global routing, packets arriving at a VRF interface are routed via the global routing table.

With global-to-VRF routing, packets arriving at the global interface (an interface that is not part of a VRF) are routed via a VRF routing table.

Restrictions for VRF-Aware Policy-Based Routing

• The same route-map cannot be used to configure PBR:

– on interfaces that belong to different VRFs

– on one VRF interface and another global interface (an interface that is not part of a VRF).

• The set vrf and set ip global next-hop commands can be configured with the set default interface, set interface, set ip default next-hop, and set ip next-hop commands. But the set vrf and set ip global next-hop commands take precedence over the set default interface, set interface, set ip default next-hop, and set ip next-hop commands. No error message is displayed if you attempt to configure the set vrf command with any of these three set commands.
• The set global and set vrf commands cannot be simultaneously applied to a route map.
• When you use the set vrf command you specify the VRF table to be looked-up; this overrides the default or global routing table. If a route is not specified in the VRF routing table, then packets are dropped (even if a route exists in the global routing table).
• The set next-hop verify-availability and the set ip next hop recursive commands are not supported within VRF instances.

Enabling IPv4 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 IPv4 PBR on an interface, perform this task:

Use the set commands 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.

Refer to the section Policy-Based Routing Configuration Examples for examples of IPv4 PBR.

Use the show route-map map-tag command to display the existing route map.

Note Packet and byte counters in the output of the show route-map map-tag command are not updated.

Enabling IPv6 PBR

Note With IOS XE 3.6.0E and IOS 15.2(2)E, IPv6 PBR is not supported on Supervisor Engine 8-E.

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.

For IPv6 PBR to work on the Catalyst 4900M, Catalyst 4948E and Catalyst 4948E-F Series Switches, IPv4 and IPv6 routing must be enabled the device.

To enable IPv6 PBR on an interface, perform this task:

Note The recursive option is supported for IPv4, but not for IPv6. An interface can have either an ipv4 route map or an ipv6 route map. An interface can be bound to only one route map.

Use the set commands 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.

Refer to the following document for IPv6 PBR configuration examples.

http://www.cisco.com/c/en/us/support/docs/ip/ip-version-6-ipv6/112218-policy-based-routing-ipv6-configex.html

Note Packet and byte counters in the output of the show route-map map-tag command are updated only for software switched packets. Counters for hardware switched packets are not updated.

Enabling Local IPv4 and Local IPv6 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 entering this command:

IPv4

IPv6

All packets originating on the switch are then subject to local PBR.

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

Configuring IPv4 and IPv6 PBR for VRF Instances

To enable PBR for multiple routing instances, configure your device in the following way:

Verifying the PBR Configuration for VRF Instances

To verify the PBR configuration for VRF instances, enter the following steps in any order:

The following example shows you how to configure PBR for VRF instances:

Verifying Next-Hop IP using Object Tracking

To verify the next-hop IP address using PBR with Object Tracking, perform the following steps:

Note The set ip next-hop verify-availability command is not supported on VRF instances, on a virtual switching system (VSS), and with IPv6 traffic.

The following example shows you how to verify the next-hop IP address in a route map:

Unsupported Commands

The following PBR commands in config-route-map mode are in the CLI but not supported in Cisco IOS for the Catalyst 4500 series switches. If you attempt to use these commands, an error message displays:

• match-length
• set ip qos and set ipv6 qos
• set ip tos and set ipv6 tos
• set ip precedence and set ipv6 precedence
• set ip df and set ipv6 df
• set ipv6 next-hop recursive
• set ipv6 next-hop verify-availability

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 10.1.1.1 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.

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 10.1.1.1 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.

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 10.1.1.1 skip sequence 10 and jump to sequence 20. All other packets from subnet 10.1.1.0 follow the set statement in sequence 10.

Examples of the show Command

The following show command illustrates that route map pbrv6-test has only one permit sequence.

In the example policy, IPv6 packets with an address matching the criteria defined by the access control list v6_acl are forwarded to the next hop 2006::2. If next-hop 2006::2 is unreachable, the matching packets are forwarded to 2005::2. If both next-hops are unreachable, the packets are forwarded using the routing table lookup. For packets that do not match the filter criteria, a standard routing table lookup is performed for packet forwarding.