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Table Of Contents
Prerequisites for Multi-Topology Routing
Restrictions for Multi-Topology Routing
Information About Multi-Topology Routing
Unicast Topology Configuration for MTR
Multicast Topology Configuration for MTR
Routing Protocol Support for MTR
BGP Routing Protocol Support for MTR
BGP Sessions for Class-Specific Topologies
Topology Translation Using BGP
Network Management Support for MTR
Guidelines for Enabling and Disabling MTR
How to Configure Multi-Topology Routing
Configuring a Unicast Topology for MTR
Configuring a Multicast Topology for MTR
Configuring MTR Traffic Classification
MTR and QoS Traffic Classification in the Same Network
Activating an MTR Topology Using OSPF
Activating an MTR Topology Using EIGRP
Activating an MTR Topology Using IS-IS
Activating an MTR Topology Using BGP
Importing Routes from an MTR Topology Using BGP
Configuring an MTR Topology in Interface Configuration Mode
Activating an MTR Topology in Interface Configuration Mode Using OSPF
OSPF Interface Topology Configuration
Activating an MTR Topology in Interface Configuration Mode Using EIGRP
EIGRP Interface Topology Configuration
Activating an MTR Topology in Interface Configuration Mode Using IS-IS
IS-IS Interface Topology Configuration
Configuring SNMP Support for MTR
Associating an SNMP Context with a VRF for MTR
Associating an SNMP Context with a Data Topology for MTR
Associating an SNMP Context with a Routing Protocol for MTR
Enabling and Monitoring MTR Topology Statistics Accounting
Enabling Topology Statistics Accounting for MTR
Monitoring Interface and Topology IP Traffic Statistics for MTR
Testing Network Connectivity for MTR
Configuration Examples for Multi-Topology Routing
Unicast Topology for MTR: Examples
Global Interface Configuration Example
Incremental Forwarding Configuration Example
Unicast Topology Verification Example
Multicast Topology for MTR: Examples
Route Replication Configuration Example
Using a Unicast RIB for Multicast RPF Configuration Example
Multicast Verification Example
MTR Traffic Classification: Examples
Activating an MTR Topology Using OSPF: Examples
Activating an MTR Topology Using EIGRP: Examples
Activating an MTR Topology Using IS-IS: Examples
Activating an MTR Topology Using BGP: Examples
BGP Topology Translation Configuration
BGP Scope Global and VRF Configuration
Importing Routes from an MTR Topology Using BGP: Example
MTR Topology in Interface Configuration Mode: Examples
MTR OSPF Topology in Interface Configuration Mode: Examples
MTR EIGRP Topology in Interface Configuration Mode: Examples
MTR IS-IS Topology in Interface Configuration Mode: Examples
SNMP Support for MTR: Examples
Monitoring Interface and Topology IP Traffic Statistics: Examples
Testing Network Connectivity for MTR: Examples
Feature Information for Multi-Topology Routing
Multi-Topology Routing
First Published: February 27, 2007Last Updated: June 4, 2007Multi-Topology Routing (MTR) allows the configuration of service differentiation through class-based forwarding. MTR supports multiple unicast topologies and a separate multicast topology. A topology is a subset of the underlying network (or base topology) characterized by an independent set of Network Layer Reachability Information (NLRI). A topology can overlap with another or share any subset of the underlying network. MTR provides separate forwarding capabilities on a per topology basis. A separate forwarding table is maintained for each topology, allowing you to broadly apply independent forwarding configurations or add a level of granularity to independent forwarding configurations. MTR can be used, for example, to define separate topologies for voice, video, and data traffic classes.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Multi-Topology Routing" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for Multi-Topology Routing
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Prerequisites for Multi-Topology Routing
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Restrictions for Multi-Topology Routing
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Information About Multi-Topology Routing
You should understand the following concepts before configuring MTR in a production network:
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MTR Overview
MTR introduces the capability to configure service differentiation through class-based forwarding. There are two primary components to configuring MTR: independent topology configuration and traffic classification configuration.
A topology is defined as a subset of routers and links in a network, for which a separate set of routes is calculated. The entire network itself, for which the usual set of routes is calculated, is known as the base topology. The base topology (or underlying network) is characterized by the NLRI that a router uses to calculate the global routing table to make routing and forwarding decisions. In other words, the base topology is the default routing environment that exists prior to enabling MTR.
Any additional topologies are known as class-specific topologies and are a subset of the base topology. Each class-specific topology carries a class of traffic and is characterized by an independent set of NLRI that is used to maintain a separate Routing Information Base (RIB) and Forwarding Information Base (FIB). This design allows the router to perform independent route calculation and forwarding for each topology.
Within a given router, MTR creates a selection of routes upon which to forward to a given destination. The specific choice of route is based on the class of the packet being forwarded, a class that is an attribute of the packet itself. This design allows packets of different classes to be routed independently from one another. The path that the packet follows is determined by classifiers configured on the routers and interfaces in the network. Figure 1 shows the base topology, which is a superset of the red, blue, and green topologies.
Figure 1 MTR Base Topology
Figure 2 shows an MTR-enabled network that is configured using the service separation model. The base topology (shown in black) uses NLRI from all reachable devices in the network. The blue, red, and purple paths each represent a different class-specific topology. Each class-specific topology calculates a separate set of paths through the network. Routing and forwarding are independently calculated based on individual sets of NLRI that are carried for each topology.
Figure 2 Defining MTR Topologies
Figure 3 shows that the traffic is marked at the network edge. As the traffic traverses the network, the marking is used during classification and forwarding to constrain the traffic to its own colored topology.
Figure 3 Traffic Follows Class-Specific Forwarding Paths
The same topology can have configured backup paths. In Figure 4, the preferential path for the voice topology is represented by the solid blue line. In case this path becomes unavailable, MTR can be configured to choose the voice backup path represented by the dotted blue line. Both of these paths represent the same topology and none overlap.
Figure 4 MTR Backup Contingencies Within a Topology
Figure 5 shows the MTR forwarding model at the system level. When a packet arrives at the incoming interface, the marking is examined. If the packet marking matches a topology, the associated topology is consulted, the next hop for that topology is determined, and the packet is forwarded. If there is no forwarding entry within a topology, the packet is dropped. If the packet does not match any classifier, it is forwarded to the base topology. The outgoing interface is a function of the colored route table in which the lookup is done.
Figure 5 MTR Forwarding at the System Level
MTR is implemented in Cisco IOS software on a per address family and subaddress family basis. Only the IPv4 (unicast and multicast) address family is currently supported. MTR supports up to 32 unicast topologies (including the base topology) and a separate multicast topology. A topology can overlap with another or share any subset of the underlying network. Each topology is configured with a unique topology ID. The topology ID is configured under the routing protocol and is used to identify and group NLRI for each topology in updates for a given protocol.
Unicast Topology Configuration for MTR
Up to 32 unicast topologies can be configured on each router. The topology is first defined by entering the global-address-family command in global configuration mode. The address family and optionally the subaddress family are specified in this step. The topology subcommand is then entered in global address family configuration mode. This command places the router in address family topology configuration mode. The following global topology configuration parameters are applied in this mode:
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Multicast Topology Configuration for MTR
Cisco IOS software supports legacy (pre-MTR) IP multicast behavior by default. MTR support for IP multicast must be explicitly enabled. Legacy IP multicast uses reverse path forwarding on routes in the unicast RIB (base unicast topology) to build multicast distribution trees (MDTs).
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MTR introduces a multicast topology that is completely independent from the unicast topology. MTR integration with multicast will allow the user to control the path of multicast traffic in the network.
The multicast topology maintains separate routing and forwarding tables. The following list summarizes MTR multicast support that is integrated into Cisco IOS software:
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Multicast support is enabled by configuring the ip multicast-routing command in global configuration mode, as in pre-MTR software. MTR support for multicast is enabled by configuring the ip multicast rpf multitopology command. The global-address-family command is entered with the IPv4 address family and multicast subaddress family. The topology command is then entered with the base keyword. The following global topology configuration parameters are applied in this mode:
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Routing Protocol Support for MTR
IP routing must be enabled on the router in order for MTR to operate. MTR supports static and dynamic routing in Cisco IOS software. Dynamic routing can be enabled per-topology to support inter-domain and intra-domain routing. Route calculation and forwarding are independent for each topology. MTR support is integrated into Cisco IOS software for the following protocols:
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Per-topology configuration is applied under the router address family configuration of the global routing process (router configuration mode). The address family and subaddress family are specified when entering address-family configuration mode. The topology name and topology ID are specified under the address-family configuration by entering the topology command.
Each topology is configured with a unique topology ID. The topology ID is configured under the routing protocol and is used to identify and group NLRI for each topology in updates for a given protocol. In OSPF, EIGRP, and IS-IS, the topology ID is entered during the first configuration of the topology command for a class-specific topology. In BGP, the topology ID is configured by entering the bgp tid command under the topology configuration.
Class-specific topologies can be configured with different metrics than the base topology. Interface metrics configured on the base topology can be inherited by the class-specific topology. Inheritance occurs if no explicit inheritance metric is configured in the class-specific topology.
BGP support is configured only in router configuration mode. IGP support is configured in router configuration mode and/or interface configuration mode.
By default, interfaces are not included in non-base topologies. For routing protocol support for EIGRP, IS-IS, and OSPF, explicit configuration of a non-base topology on an interface is required. The default behavior can be overridden by using the all-interfaces command in address family topology configuration mode. The all-interfaces command causes the non-base topology to be configured on all interfaces of the router that are part of the default address space or the VRF in which the topology is configured.
BGP Routing Protocol Support for MTR
Before using BGP to support MTR, you should be familiar with the following concepts:
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BGP Network Scope
A new configuration hierarchy, named scope, has been introduced into the BGP protocol. To implement MTR for BGP, the scope hierarchy is required, but the scope hierarchy is not limited to MTR use. The scope hierarchy introduces some new configuration modes such as router scope configuration mode. Router scope configuration mode is entered by configuring the scope command in router configuration mode, and a collection of routing tables is created when this command is entered. BGP commands configured under the scope hierarchy are configured for a single network (globally), or on a per-VRF basis, and are referred to as scoped commands. The scope hierarchy can contain one or more address families.
MTR CLI Hierarchy Under BGP
The BGP CLI has been modified to provide backwards compatibility for pre-MTR BGP configuration and to provide a hierarchical implementation of MTR. Router configuration mode is backwards compatible with the pre-address family and pre-MTR configuration CLI. Global commands that affect all networks are configured in this configuration mode. For address-family and topology configuration, general session commands and peer templates can be configured to be used in the address-family or topology configuration modes.
After any global commands are configured, the scope is defined either globally or for a specific VRF. Address family configuration mode is entered by configuring the address-family command in router scope configuration mode or router configuration mode. Unicast is the default address family if no subaddress family (SAFI) is specified. MTR supports only the IPv4 address family with a SAFI of unicast or multicast. Entering address family configuration mode from router configuration mode configures BGP to use pre-MTR-based CLI. This configuration mode is backwards compatible with pre-existing address family configurations. Entering address family configuration mode from router scope configuration mode configures the router to use the hierarchical CLI that supports MTR. Address family configuration parameters that are not specific to a topology are entered in this address family configuration mode.
BGP topology configuration mode is entered by configuring the topology (BGP) command in address family configuration mode. Up to 32 topologies (including the base topology) can be configured on a router. The topology ID is configured by entering the bgp tid command. All address family and subaddress family configuration parameters for the topology are configured here.
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The following shows the hierarchy levels that are used when configuring BGP for MTR implementation:
router bgp <autonomous-system-number> ! Global commands scope {global | vrf <vrf-name>} ! Scoped commands address-family {<afi>} [<safi>] ! Address family specific commands topology {<topology-name> | base} ! topology specific commandsBGP Sessions for Class-Specific Topologies
MTR is configured under BGP on a per-session basis. The base unicast and multicast topologies are carried in the global (default) session. A separate session is created for each class-specific topology that is configured under a BGP routing process. Each session is identified by its topology ID. BGP performs a best-path calculation individually for each class-specific topology. A separate RIB and FIB are maintained for each session.
Topology Translation Using BGP
Depending on the design and policy requirements for your network, you may need to install routes from a class-specific topology on one router in a class-specific topology on a neighboring router. Topology translation functionality using BGP provides support for this operation. Topology translation is BGP neighbor-session based. The neighbor translate-topology command is configured using the IP address and topology ID from the neighbor.
The topology ID identifies the class-specific topology of the neighbor. The routes in the class-specific topology of the neighbor are installed in the local class-specific RIB. BGP performs a best-path calculation on all installed routes and installs these routes into the local class-specific RIB. If a duplicate route is translated, BGP will select and install only one instance of the route per standard BGP best-path calculation behavior.
Topology Import Using BGP
Topology import functionality using BGP is similar to topology translation. The difference is that routes are moved between class-specific topologies on the same router using BGP. This function is configured by entering the import topology command. The name of the class-specific topology or base topology is specified when entering this command. Best-path calculations are run on the imported routes before they are installed into the topology RIB. This command also includes a route-map keyword to allow you to filter routes that are moved between class-specific topologies.
MTR Traffic Classification
MTR cannot be enabled on a router until traffic classification has been configured, even if only one class-specific topology has been configured. Traffic classification is used to configure topology specific forwarding behaviors when multiple topologies are configured on the same router. Traffic classification must be applied consistently throughout the network. Class-specific packets are associated with the corresponding topology table forwarding entries.
Traffic classification is configured using the Modular QoS CLI (MQC). MTR traffic classification is similar to QoS traffic classification. However, there is an important distinction. MTR traffic classification is defined globally for each topology, rather than at the interface level as in Quality of Service (QoS).
A subset of DSCP bits is used to encode classification values in the IP packet header. A class map is configured to define the traffic class by entering the class-map command in global configuration mode. Only the match-any keyword is supported for MTR. The traffic class is associated with a policy by configuring the policy-map type class-routing ipv4 unicast command in global configuration mode. The policy is activated for the topology by configuring the service-policy type class-routing command in global address family configuration mode. When configured, the service policy is associated with all interfaces on the router.
Some of the same goals can be achieved through QoS configuration, to which MTR provides a more powerful and flexible alternative. MTR traffic classification and IP Differentiated Services or IP Precedence-based traffic classification can be configured in the same network. However, MTR requires exclusive use of some subset of the DSCP bits in the IP packet header for specific topology traffic. In a network where MTR and QoS traffic classification are configured, simultaneous configuration must be carefully coordinated.
Network Management Support for MTR
Standard network management utilities, such as ping and traceroute, have been enhanced to support MTR. You can configure a standard or extended ping using the topology name in place of a hostname or IP address. Traceroute has been similarly enhanced. Context-based Simple Network Management Protocol (SNMP) functionality has been integrated into Cisco IOS software and can be used to support MTR.
ISSU—MTR
All protocols and applications that support MTR and that also support In Service Software Upgrade (ISSU) have extended their ISSU support to include the MTR functionality. See the Cisco IOS In Service Software Upgrade Configuration module for information on ISSU-capable protocols and applications.
ISSU allows a high-availability (HA) system to run in Stateful Switchover (SSO) mode even when different versions of Cisco IOS software are running on the active and standby Route Processors (RPs). This feature allows the system to switch over to a secondary RP that is running upgraded (or downgraded) software and to continue forwarding packets without session loss and with minimal or no packet loss.
This feature is enabled by default.
MTR Deployment Models
The base topology is the superset of all topologies in the network. It is defined by NLRI for all reachable routers regardless of the deployment model that is used. MTR can be deployed using the service separation MTR model shown in Figure 6, or it can deployed using the overlapping MTR model shown in Figure 7. Each of these models represent a different approach to deploying MTR. However, these models are not mutually exclusive. Any level of variation of a combined model can be deployed.
Service Separation MTR Model
Figure 6 shows the service separation model where no colored topologies (except for the base) overlap with each other. In the service separation model, each class of traffic is constrained to its own exclusive topology. This model restricts the given class of traffic to a subset of the network. This model is less configuration intensive because no topology-specific metrics need to be configured.
Figure 6 Service-Separation MTR Model
Overlapping MTR Model
In the overlapping MTR model, all topologies are configured to run over all routers in the network. This model provides the highest level of redundancy. All classes of traffic may use all links. Per-topology metrics are then configured to bias different classes of traffic to use different parts of the network. The redundancy that this model provides, however, makes it more configuration intensive. Figure 7 shows the red and gray topologies. All topologies are configured to run over all network routers. In this model, per-topology metrics are configured to bias the preferred routes for each topology.
Figure 7 Overlapping MTR Model
MTR Deployment Configuration
MTR supports both full and incremental deployment configurations. To support these options, MTR provides two different, configurable forwarding rules. For full deployment, MTR supports a (default) longest-match lookup in only the forwarding table of the corresponding class-specific topology. If no route is found, the packet is dropped. For incremental deployment, MTR supports a longest- match lookup first in the forwarding table for the corresponding class-specific topology, and subsequently, in the base topology if no class-specific entry is found. The former forwarding rule is known as "strict mode," the latter as "incremental mode."
Full Deployment
Strict forwarding mode is the default forwarding mode in MTR. In this mode, the router will look for a forwarding route only in the class-specific FIB. If no forwarding route is found, the packet is dropped. In this mode, the router performs a longest match look up for the topology FIB entry. This mode is designed for full deployment, where MTR is enabled on every router in the network or every router in the topology. Strict forwarding mode should be enabled after an incremental deployment transition has been completed or when all routers in the network or topology are MTR enabled. Strict forwarding mode can be enabled after incremental forwarding mode by entering the no form of the forward-base command.
Incremental Deployment
Incremental forwarding mode is designed to support transitional or incremental deployment of MTR, where there are routers in the network that are not MTR enabled. In this mode, the router will look for a forwarding entry first in the class-specific FIB. If an entry is not found, the router will then look for the longest match in the base topology FIB. If an entry is found in the base topology FIB, the packet will be forwarded on the base topology. If a forwarding entry is not found in the base topology FIB, the packet is dropped.
This mode is designed to preserve connectivity during an incremental deployment of MTR and is recommended to be used only during migration (the transition from a non-MTR to MTR enabled network). Class-specific traffic for a given destination is forwarded over contiguous segments of the class-specific topology containing that destination; otherwise it is forwarded over the base topology.
This forwarding mode can also be enabled to support mixed networks where some routers are not configured to run MTR. Incremental forwarding mode is enabled by entering the forward-base command in address family topology configuration mode.
Guidelines for Enabling and Disabling MTR
The section provides guidelines and procedures for enabling or disabling MTR in a production network. These guidelines assume that all participating networking devices are running a software image that supports MTR. They are designed to prevent major traffic interruptions due to misconfiguration and to minimize temporary transitional effects that can occur when introducing or removing a topology from a network. The procedures described below must be implemented in the order that they are described.
First, create a class-specific topology on all networking devices and enable incremental forwarding mode by entering the forward-base command in the address family topology configuration. Incremental forwarding should be configured whenever a topology is introduced or removed from the network. The topology is defined as a global container at this stage. No routing or forwarding can occur within the topology. Routing protocol support should not be configured.
Second, configure classification rules for the class-specific topology. Classification must be consistently applied on all routers in the topology; each router has identical classifier configuration. The topology is activated when a valid classification configuration is attached to the global topology configuration. Reachability can be verified, for interfaces and networking devices that are in the same topology and configured with identical classification, using ping and trace route.
Third, configure routing protocol support and/or static routing. The routers in the topology should be configured one at a time. This configuration includes interface, router process, and routing protocol-specific metrics and filters.
You should enable routing in the topology using a physical pattern in a contiguous manner relative to a single starting point. For example, you should configure all interfaces on a single router, and then all interfaces on each adjacent router. You should follow this pattern until the task is complete. The starting point can be on the edge or core of the network. This recommendation is designed to increase the likelihood that class-specific traffic is forwarded on the same paths in the incremental topology during as it is on the full topology when MTR is completely deployed.
Incremental forwarding should be disabled (if your network design requires strict forwarding mode) only after routing has been configured on all routers in a given topology. At this stage, MTR is fully operational. Class-specific traffic is forwarded only over devices within the topology. Traffic that is not classified or destined for the topology is dropped.
When disabling a topology, you should reenable incremental forwarding mode. You should remove custom route configuration, such as route summarization and default routes before disabling a topology, and you should reapply custom route configuration only after the topology is reenabled. This recommendation is designed to prevent traffic interruption, as some destinations may be obscured during the transition. Custom route configuration is most useful when all of the more specific routes are available in the routing table of the topology.
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How to Configure Multi-Topology Routing
This section contains the following tasks:
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Configuring a Unicast Topology for MTR
Perform this task to configure a unicast topology. Only Steps 1 through 4 are required to complete this task. The remaining steps are optional.
MTR Scaling Characteristics
For each new topology that you configure on a router, you increase the total number of routes from the global routing table by the number of routes that are in each new topology [base+topology(n)]. If the router carries a large global routing table, and you plan to add a significant number of routes through MTR topology configuration, you can configure the maximum routes (MTR) command in address family topology configuration mode to limit the number of routes that the router will accept for a given topology and install into the corresponding RIB.
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What to Do Next
Repeat this task for each unicast topology instance that you need to create. Proceed to "Configuring a Multicast Topology for MTR" section to configure a multicast topology.
Configuring a Multicast Topology for MTR
Cisco IOS software supports legacy (pre-MTR) multicast behavior by default. Perform this task to configure a multicast topology. Only Steps 1 through 6 are required to complete this task. The remaining steps are optional.
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What to Do Next
The topology is not activated until classification is configured. Proceed to the "Configuring MTR Traffic Classification" section to configure classification for a class-specific topology.
Configuring MTR Traffic Classification
Perform this task to define MTR traffic classification. Traffic classification is used to associate different classes of traffic with different topologies when multiple topologies are configured on the same router. MTR traffic classification is similar to QoS traffic classification and is configured using the MQC.
The service-policy type class-routing command is used to attach a service policy to a policy map for topology traffic classification. The service policy is activated for the topology after the service-policy type class-routing command is entered, enabling traffic classification. Following the correct order of the commands in this task is very important. Ensure that all configuration that affects traffic classification is complete before entering the service-policy type class-routing command.
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It is also important that all routers throughout the network have the same definition of classifiers and the same sequencing of classifiers.
MTR and QoS Traffic Classification in the Same Network
MTR traffic classification and IP Differentiated Services or IP Precedence based traffic classification can be configured in the same network. However, MTR requires exclusive use of the DSCP bits in the IP packet header for specific topology traffic. In a network where MTR and QoS traffic classification is configured, simultaneous configuration must be carefully coordinated.
Before configuring MTR traffic classification, you should be familiar with all the concepts documented in the "MTR Traffic Classification" section.
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enable
Example:Router> enable
Enables privileged EXEC mode.
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configure terminal
Example:Router# configure terminal
Enters global configuration mode.
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class-map match-any class-map-name
Example:Router(config)# class-map match-any VOICE-CLASS
Creates a class map to be used for matching packets to a specified class and enters class-map configuration mode.
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match [ip] dscp dscp-value [dscp-value
dscp-value dscp-value dscp-value dscp-value dscp-value dscp-value]Example:Router(config-cmap)# match ip dscp 9
Identifies a DSCP value as a match criteria.
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exit
Example:Router(config-cmap)# exit
Exits class-map configuration mode, and enters global configuration mode.
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policy-map type class-routing ipv4 unicast
policy-map-nameExample:Router(config)# policy-map type class-routing ipv4 unicast VOICE-CLASS-POLICY
Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and enters policy-map configuration mode.
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class {class-name | class-default}
Example:Router(config-pmap)# class VOICE-CLASS
Specifies the name of the class whose policy you want to create or change or specifies the default class and enters policy-map class configuration mode.
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select-topology topology-name
Example:Router(config-pmap-c)# select-topology VOICE
Attaches the policy map to the topology.
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exit
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