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You must enable IP routing on the device for Multitopology Routing (MTR) to operate. MTR supports static and dynamic routing in Cisco software. You can enable dynamic routing per topology to support interdomain and intradomain routing. Route calculation and forwarding are independent for each topology. MTR support is integrated into Cisco software for the following protocols:

• Border Gateway Protocol (BGP)

• Enhanced Interior Gateway Routing Protocol (EIGRP)

• Integrated Intermediate System-to-Intermediate System (IS-IS)

• Open Shortest Path First (OSPF)

You apply the per-topology configuration in router address family configuration mode of the global routing process (router configuration mode). The address family and subaddress family are specified when the device enters address family configuration mode. You specify the topology name and topology ID by entering the topology command in address family configuration mode.

You configure each topology with a unique topology ID under the routing protocol. The topology ID is used to identify and group Network Layer Reachability Information (NLRI) for each topology in updates for a given protocol. In OSPF, EIGRP, and IS-IS, you enter the topology ID during the first configuration of the topology command for a class-specific topology. In BGP, you configure the topology ID by entering the bgp tid command under the topology configuration.

You can configure class-specific topologies 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.

You configure BGP support only in router configuration mode. You configure Interior Gateway Protocol (IGP) support in router configuration mode and in interface configuration mode.

By default, interfaces are not included in nonbase topologies. For routing protocol support for EIGRP, IS-IS, and OSPF, you must explicitly configure a nonbase topology on an interface. You can override the default behavior by using the all-interfaces command in address family topology configuration mode. The all-interfaces command causes the nonbase topology to be configured on all interfaces of the device that are part of the default address space or the virtual routing and forwarding (VRF) instance in which the topology is configured.

To implement Border Gateway Protocol (BGP) support for Multitopology Routing (MTR), the scope hierarchy is required, but the scope hierarchy is not limited to MTR use. The scope hierarchy introduces new configuration modes such as router scope configuration mode. The device enters router scope configuration mode when you configure the scope command in router configuration mode. When this command is entered, a collection of routing tables is created.

You configure BGP commands under the scope hierarchy for a single network (globally), or on a per-virtual routing and forwarding (VRF) basis; these configurations are referred to as scoped commands. The scope hierarchy can contain one or more address families.

The Border Gateway Protocol (BGP) CLI provides backward compatibility for pre-Multitopology Routing (MTR) BGP configuration and provides a hierarchical implementation of MTR. Router configuration mode is backward 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, you configure general session commands and peer templates to be used in address family configuration mode or in topology configuration mode.

After configuring any global commands, you define the scope either globally or for a specific virtual routing and forwarding (VRF) instance. The device enters address family configuration mode when you configure the address-family command in router scope configuration mode or in router configuration mode. Unicast is the default address family if no subaddress family identifier (SAFI) is specified. MTR supports only the IPv4 address family with a SAFI of unicast or multicast.

When the device enters address family configuration mode from router configuration mode, the software configures BGP to use pre-MTR-based CLI. This configuration mode is backward compatible with pre-existing address family configurations. Entering address family configuration mode from router scope configuration mode configures the device 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.

The device enters BGP topology configuration mode when you configure the topology command in address family configuration mode. You can configure up to 32 topologies (including the base topology) on a device. You configure the topology ID by entering the bgp tid command. All address family and subaddress family configuration parameters for the topology are configured here.

Note	Configuring a scope for a BGP routing process removes CLI support for pre-MTR-based configuration.

The following example shows the hierarchy levels that are used when you configure 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 commands

Multitopology Routing (MTR) is configured under the Border Gateway Protocol (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 Routing Information Base (RIB) and Forwarding Information Base (FIB) are maintained for each session.

Depending on the design and policy requirements for your network, you might need to install routes from a class-specific topology on one device in a class-specific topology on a neighboring device. Topology translation functionality using the Border Gateway Protocol (BGP) provides support for this operation. Topology translation is BGP neighbor-session based. You configure the neighbor translate-topology command by 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 Routing Information Base (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 selects and installs only one instance of the route per standard BGP best-path calculation behavior.

Importing topologies using the Border Gateway Protocol (BGP) is similar to topology translation. The difference is that routes are moved between class-specific topologies on the same device. You configure this function by entering the import topology command and specify the name of the class-specific topology or base topology. Best-path calculations are run on the imported routes before they are installed into the topology Routing Information Base (RIB). This import topology command also includes a route-map keyword to allow you to filter routes that are moved between class-specific topologies.

Perform this task to activate a Multitopology Routing (MTR) topology inside an address family by using the Border Gateway Protocol (BGP). This task is configured on Device B in the figure below and must also be configured on Device D and Device E. In this task, a scope hierarchy is configured to apply globally, and a neighbor is configured in router scope configuration mode. Under the IPv4 unicast address family, an MTR topology that applies to video traffic is activated for the specified neighbor. There is no interface configuration mode for BGP topologies.

Figure 1. BGP Network Diagram

1.    enable


2.    configure terminal


3.    router bgp autonomous-system-number


4.    scope {global | vrf vrf-name}


5.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number


6.    neighbor {ip-address | peer-group-name} transport {connection-mode {active | passive} | path-mtu-discovery | multi-session | single-session}


7.    address-family ipv4 [mdt | multicast | unicast]


8.    topology {base | topology-name}


9.    bgp tid number


10.    neighbor ip-address activate


11.    neighbor {ip-address | peer-group-name} translate-topology number


12.    end


13.    clear ip bgp topology {* | topology-name} {as-number | dampening [network-address [network-mask]] | flap-statistics [network-address [network-mask]] | peer-group peer-group-name | table-map | update-group [number | ip-address]} [in [prefix-filter] | out | soft [in [prefix-filter] | out]]


14.    show ip bgp topology {* | topology} summary

Device> enable 

Device# configure terminal 

Device(config)# router bgp 45000 

Device(config-router)# scope global 

Device(config-router-scope)# neighbor 172.16.1.2 remote-as 45000

Device(config-router-scope)# neighbor 172.16.1.2 transport multi-session

Device(config-router-scope)# address-family ipv4 

Device(config-router-scope-af)# topology VIDEO 

Device(config-router-scope-af-topo)# bgp tid 100

Device(config-router-scope-af-topo)# neighbor 172.16.1.2 activate

Device(config-router-scope-af-topo)# neighbor 172.16.1.2 translate-topology 200 

Device(config-router-scope-af-topo)# end 

Device# clear ip bgp topology VIDEO 45000

Device# show ip bgp topology VIDEO summary

1.    enable


2.    configure terminal


3.    ip prefix-list list-name [seq number] {deny | permit} network/length [ge ge-length] [le le-length]


4.    route-map map-name [permit | deny] [sequence-number]


5.    match ip address {access-list-number [access-list-number ... | access-list-name...] | access-list-name [access-list-number ... | access-list-name] | prefix-list prefix-list-name [prefix-list-name...]}


6.    exit


7.    router bgp autonomous-system-number


8.    scope {global | vrf vrf-name}


9.    address-family ipv4 [mdt | multicast | unicast]


10.    topology {base | topology-name}


11.    import topology {base | topology-name} [route-map map-name]


12.    end

Device> enable 

Device# configure terminal 

Device(config)# ip prefix-list TEN permit 10.2.2.0/24

Device(config)# route-map 10NET

Device(config-route-map)# match ip address prefix-list TEN

Device(config-route-map)# exit

Device(config)# router bgp 50000 

Device(config-router)# scope global 

Device(config-router-scope)# address-family ipv4 

Device(config-router-scope-af)# topology VIDEO 

Device(config-router-scope-af-topo)# import topology VOICE route-map 10NET

Device(config-router-scope-af-topo)# end