Multitopology Routing

Use Multitopology Routing (MTR) to configure service differentiation through class-based forwarding. Two primary components comprise MTR configuration: independent topology configuration and traffic classification configuration.

A topology is defined as a subset of devices 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 Network Layer Reachability Information (NLRI) that a device uses to calculate the global routing table to make routing and forwarding decisions. 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 device to perform independent route calculation and forwarding for each topology.

MTR creates a selection of routes within a given device 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 devices and interfaces in the network. The figure below shows a base topology, which is a superset of the red, blue, and green topologies.

The figure below 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.

The figure below 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.

The same topology can have configured backup paths. In the figure below, the preferential path for the voice topology is represented by the solid blue line. In case this path becomes unavailable, you can configure MTR to choose the voice backup path represented by the dotted blue line. Both of these paths represent the same topology and none overlap.

The figure below 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.

MTR is implemented in Cisco software according to a address family and subaddress family basis. 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. You configure each topology with a unique topology ID. You configure the topology ID under the routing protocol, and the ID is used to identify and group NLRI for each topology in updates for a given protocol.

You can configure up to 32 unicast topologies on each device. You first define the topology by entering the global-address-family command in global configuration mode. The address family and optionally the subaddress family are specified in this step. You then enter the topology command in global address family configuration mode. This command places the device in address family topology configuration mode, and the global topology configuration parameters are applied in this mode.

For each new topology that you configure on a device, 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 device carries a large global routing table, and you plan to add a significant number of routes through the Multitopology Routing (MTR) topology configuration, you can configure the maximum routes command in address family topology configuration mode to limit the number of routes that the device accepts for a given topology and installs into the corresponding Routing Information Base (RIB).

Note	Per-interface topology configuration parameters override configurations applied in global address family topology configuration mode and router address family topology configuration mode.

The configuration of a Multitopology Routing (MTR) topology in interface configuration mode allows you to enable or disable MTR on a per-interface basis. By default, a class-specific topology does not include any interfaces.

You can include or exclude individual interfaces by configuring the topology interface configuration command. You specify the address family and the topology (base or class-specific) when entering this command. The subaddress family can be specified. If no subaddress family is specified, the unicast subaddress family is used by default.

You can include globally all interfaces on a device in a topology by entering the all-interfaces command in routing topology configuration mode. Per-interface topology configuration applied with the topology command overrides global interface configuration.

The interface configuration support for MTR has these characteristics:

• Per-interface routing configuration: Interior Gateway Protocol (IGP) routing and metric configurations can be applied in interface topology configuration mode. Per-interface metrics and routing behaviors can be configured for each IGP.
• Open Shortest Path First (OSPF) interface topology configuration: Interface mode OSPF configurations for a class-specific topology are applied in interface topology configuration mode. In this mode, you can configure an interface cost or disable OSPF routing without removing the interface from the global topology configuration.
• Enhanced Interior Gateway Routing Protocol (EIGRP) interface topology configuration: Interface mode EIGRP configurations for a class-specific topology are applied in interface topology configuration mode. In this mode, you can configure various EIGRP features.
• Intermediate System-to-Intermediate System (IS-IS) interface topology configuration: Interface mode IS-IS configurations for a class-specific topology are applied in interface topology configuration mode. In this mode, you can configure an interface cost or disable IS-IS routing without removing the interface from the global topology configuration.

The base topology is the superset of all topologies in the network. It is defined by Network Layer Reachability Information (NLRI) for all reachable devices regardless of the deployment model that is used. Multitopology Routing (MTR) can be deployed using the service separation MTR model, or it can deployed using the overlapping MTR model. Each model represents 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
• Overlapping MTR Model

Multitopology Routing (MTR) supports both full and incremental deployment configurations. To support these options, MTR provides two different, configurable forwarding rules: strict forwarding mode for full deployment and incremental forwarding mode for an incremental deployment.

• Strict Forwarding Mode for Full Deployment of MTR
• Incremental Forwarding Mode for Incremental Deployment of MTR

The section provides guidelines and procedures for enabling or disabling Multitopology Routing (MTR) in a production network. These guidelines assume that all participating networking devices are running a software image that supports MTR. The guidelines are designed to prevent major traffic interruptions due to misconfiguration and to minimize temporary transitional effects that can occur when you introduce or remove a topology from a network. The following guidelines 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 address family topology configuration mode. Configure incremental forwarding 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. You must consistently apply classification on all devices in the topology; each device has identical classifier configuration. You activate the topology when you attach a valid classification configuration to the global topology configuration. You can use ping and traceroute commands to verify reachability for interfaces and networking devices that are in the same topology and configured with identical classification.

Third, configure routing protocol support and static routing. Configure the devices in the topology one at a time. This configuration should include an interface, router process, and routing protocol-specific metrics and filters.

Enable routing in the topology by using a physical pattern in a contiguous manner relative to a single starting point. For example, configure all interfaces on a single device, and then all interfaces on each adjacent device. 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 as it is on the full topology when MTR is completely deployed.

If your network design requires strict forwarding mode, you should disable incremental forwarding only after you configure routing on all devices 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, reenable incremental forwarding mode. Remove custom route configuration, such as route summarization and default routes before disabling a topology, and reapply custom route configuration only after the topology is reenabled. This recommendation is designed to prevent traffic interruption because some destinations might 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.

Note	These guideliens apply only when a given classifier is enabled or disabled for a given topology. All other MTR configuration, including interface and routing protocol-specific configuration (other than the topology ID) can be modified dynamically as necessary.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    global-address-family ipv4 [multicast | unicast]

4.    topology {base | topology-name}

5.    all-interfaces

6.    forward-base

7.    maximum routes number [threshold [reinstall threshold] | warning-only]

8.    shutdown

9.    end

10.    show topology [cache [topology-id] | ha [[detail | interface | lock | router] [all | ipv4 | ipv6 | vrf vpn-instance]]]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Device> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Device# configure terminal	Enters global configuration mode.
Step 3	global-address-family ipv4 [multicast | unicast] Example: Device(config)# global-address-family ipv4	Enters global address family topology configuration mode to configure the global topology. The address family for the class-specific topology is specified in this step. The subaddress family can be specified. Unicast is the default if no subaddress family is entered.
Step 4	topology {base | topology-name} Example: Device(config-af)# topology VOICE	Configures the global topology instance and enters address family topology configuration mode. The base keyword is used to configure the base topology or a multicast topology. The topology-name argument is entered to label a class-specific topology. Topology names are case-sensitive. For example, VOICE and voice identify two different topologies. Multitopology Routing (MTR) supports 32 unicast topologies including the base topology.
Step 5	all-interfaces Example: Device(config-af-topology)# all-interfaces	(Optional) Configures the topology instance to use all interfaces on a device. By default, no interfaces are used. Note    The configuration of this command does not override the topology configuration applied in interface configuration mode.
Step 6	forward-base Example: Device(config-af-topology)# forward-base	(Optional) Configures the forwarding mode under a topology instance. Strict mode (default) configures the device to look for forwarding entries only in the topology-specific Forwarding Information Base (FIB). If an entry is not found, the packet is dropped. Incremental mode (enable form) configures the device to look first in the class-specific topology FIB. If a forwarding route is not found, then the device looks in the base topology FIB.
Step 7	maximum routes number [threshold [reinstall threshold] | warning-only] Example: Device(config-af-topology)# maximum routes 1000 warning-only	(Optional) Configures the maximum number of routes that a topology instance accepts and installs into the RIB. Use the warning-only keyword to generate only a warning, to set an upper limit, and to set a lower limit (low-water mark) for reinstalling routes after the maximum limit has been exceeded.
Step 8	shutdown Example: Device(config-af-topology)# shutdown	(Optional) Temporarily disables a topology instance without removing the topology configuration (while other topology parameters are configured and other devices are configured with MTR).
Step 9	end Example: Device(config-af-topology)# end	(Optional) Exits address family topology configuration mode and enters privileged EXEC mode.
Step 10	show topology [cache [topology-id] | ha [[detail | interface | lock | router] [all | ipv4 | ipv6 | vrf vpn-instance]]] Example: Device# show topology	(Optional) Displays information about class-specific and base topologies.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    topology ipv4 [multicast | unicast] {topology-name [disable] | base}

5.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Device> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Device# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Device(config)# interface Ethernet 0/0	Specifies the interface type and number, and enters interface configuration mode.
Step 4	topology ipv4 [multicast | unicast] {topology-name [disable] | base} Example: Device(config-if)# topology ipv4 VOICE	Enters interface topology configuration mode to configure a Multitopology Routing (MTR) topology name on an interface. Use the disable keyword to disable the topology instance on the interface. This form is used to exclude a topology configuration from an interface. If the no form of this command is used, the topology interface configuration is removed. If the no form of this command is used with the disable keyword, the topology instance is enabled on the interface.
Step 5	end Example: Device(config-if-topology)# end	Exits interface topology configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    global-address-family ipv4 [multicast | unicast]

4.    topology accounting

5.    exit

6.    interface type number

7.    ip topology-accounting

8.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Device> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Device# configure terminal	Enters global configuration mode.
Step 3	global-address-family ipv4 [multicast | unicast] Example: Device(config)# global-address-family ipv4	Enters global address family configuration mode.
Step 4	topology accounting Example: Device(config-af)# topology accounting	Enables topology accounting on all interfaces in the global address family for all IPv4 unicast topologies in the default virtual routing and forwarding (VRF) instance.
Step 5	exit Example: Device(config-af)# exit	Exits global address family configuration mode.
Step 6	interface type number Example: Device(config)# interface FastEthernet 1/10	Specifies the interface type and number, and enters interface configuration mode.
Step 7	ip topology-accounting Example: Device(config-if)# ip topology-accounting	Enables topology accounting for all IPv4 unicast topologies in the VPN VRF associated with the specified interface. This topology accounting is supported only for the default VRF.
Step 8	end Example: Device(config-if)# end	Exits interface configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    show ip interface [type number] [topology {name | all | base}] [stats]

3.    show ip traffic [topology {name | all | base}]

4.    clear ip interface type number [topology {name | all | base}] [stats]

5.    clear ip traffic [topology {name | all | base}]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Device> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	show ip interface [type number] [topology {name | all | base}] [stats] Example: Device# show ip interface FastEthernet 1/10 stats	(Optional) Displays IP traffic statistics for all interfaces or statistics related to the specified interface. If you specify an interface type and number, information for that specific interface is displayed. If you specify no optional arguments, information for all the interfaces is displayed. If the topology name keyword and argument are used, statistics are limited to the IP traffic for that specific topology. The base keyword displays the IPv4 unicast base topology.
Step 3	show ip traffic [topology {name | all | base}] Example: Device# show ip traffic topology VOICE	(Optional) Displays global IP traffic statistics (an aggregation of all the topologies when MTR is enabled) or statistics related to a particular topology. The base keyword is reserved for the IPv4 unicast base topology.
Step 4	clear ip interface type number [topology {name | all | base}] [stats] Example: Device# clear ip interface FastEthernet 1/10 topology all	(Optional) Resets interface-level IP traffic statistics. If the topology keyword and a related keyword are not used, only the interface-level aggregate statistics are reset. If all topologies need to be reset, use the all keyword as the topology name.
Step 5	clear ip traffic [topology {name | all | base}] Example: Device# clear ip traffic topology all	(Optional) Resets IP traffic statistics. If no topology name is specified, global statistics are cleared.

SUMMARY STEPS

1.    enable

2.    ping [vrf vrf-name | topology topology-name] protocol [target-address] [source-address]

3.    traceroute [vrf vrf-name | topology topology-name] [protocol] destination

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Device> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	ping [vrf vrf-name | topology topology-name] protocol [target-address] [source-address] Example: Device# ping topology VOICE ip	Configures the device to transmit ping messages to the target host in a topology. An extended ping is configured by entering this command with only the topology name.
Step 3	traceroute [vrf vrf-name | topology topology-name] [protocol] destination Example: Device# traceroute VOICE	Configures the device to trace the specified host in a topology. An extended trace is configured by entering this command with only the topology name. If the vrf vrf-name keyword and argument are used, the topology option is not displayed because only the default virtual routing and forwarding (VRF) instance is supported. The topology topology-name keyword and argument and the differentiated services code point (DSCP) option in the extended traceroute system dialog are displayed only if there is a topology configured on the device.