Multitopology Routing Overview

Multitopology Routing enables you to configure class-based forwarding for different types of traffic, such as voice, video, and data. Each type of traffic is defined by a topology that is used to create a new routing table for that topology. Multitopology Routing provides the ability to generate forwarding tables based on the resolved entries in the routing tables for the custom topologies you create. In this way, packets of different classes can be routed independently from one another.

This chapter discusses the following topics that provide background information about Multitopology Routing:

Routing Table Naming Conventions for Multitopology Routing

Each routing protocol creates a routing table based on the topology name, the instance name, and the purpose of the table. A routing table for each topology uses the following format:

logical-system-name/routing-instance-name:topology-name.protocol.identifier

The routing instance string is included only if the instance is not the master. The logical system string is included only if the logical system identifier has a value other than 0 (zero). Each routing table for a topology includes a colon (:) before the topology name that also separates the routing-instance name from the topology name. protocol is the protocol family, which can be inet or inet6. identifier is a positive integer that specifies the instance of the routing table. Table 6 shows specific examples of routing tables for various topologies.

Table 6: Examples of Routing Tables for Custom Topologies

Name of Routing Table	Description
:voice.inet.0	Master instance, voice topology, unicast IPv4 routes
:voice.inet6.0	Master instance, voice topology, unicast IPv6 routes
:voice.inet.3	Master instance, voice topology, ingress label-switched paths (LSPs)
private_1/:voice.inet.0	Logical system private, voice topology, unicast IPv4 routes
customer-A:voice.inet.0	Virtual-router customer-A, voice topology, unicast IPV4 routes
customer-B:voice.inet.3	Virtual-router customer-B, voice topology, ingress LSPs
customer-A:voice.mpls.0	Virtual-router customer-A, voice topology, unicast carrier-of-carriers IPV4 routes

Routing Protocol Support for Multitopology Routing

To run Multitopology Routing, you must configure IP routing. Multitopology Routing supports OSPF version 2 (OSPFv2), static routes, and BGP. You must configure an interior gateway protocol (IGP), such as OSPFv2 or static routing. Configure BGP to add routes learned through BGP to the appropriate custom topologies.

OSPF in Multitopology Routing uses a single instance of OSPF to carry connectivity and IP reachability information for different topologies. That information is used to calculate shortest-path-first (SPF) trees and routing tables. OSPF in Multitopology Routing supports protocol extensions that include metrics that correspond to different topologies for link and prefix reachability information. The type-of-service (TOS) metric field is used to advertise the topology-specific metric for links and prefixes belonging to that topology. The TOS field is redefined as MT-ID in the payload of router, summary, and Type 5 and Type 7 autonomous-system-external link-state advertisements (LSAs).

BGP in Multitopology Routing provides the ability to resolve BGP routes against configured topologies. An inbound policy is used to select routes for inclusion in the appropriate routing tables for the topologies.

Note: Multitopology Routing is also supported on logical systems and the virtual router routing instance. No other routing instance type is supported on Multitopology Routing. For more information about configuring logical systems, see Logical Systems Overview. For more information about configuring routing instances see, Complete Routing Instances Configuration Statements. For more information about configure a virtual router instance, see the Junos VPNs Configuration Guide.

Filter-Based Forwarding Support

By default, the ingress interface forwards traffic to the default topology for each configured routing instance. Multitopology Routing supports filter-based forwarding, which enables you to match traffic on the ingress interface with a specific type of forwarding class and then forward that traffic to the specified topology. You can further define how traffic is handled for each forwarding class by configuring additional firewall filters that match traffic for such values as the IP precedence field or the Differentiated Services code point (DSCP).