Multiple Topology Overview

Separate routing topologies provide the capability to route different types of traffic on different paths through the network. This capability can be used, for example, to route voice and video traffic differently: video traffic could be routed using 10 Gb connections, while voice traffic might be routed on 1 Gb connections.

The S-Series router software has the capability to accept user-defined topologies for select routing purposes, such as routing of multicast traffic. A separate multicast topology can be configured for PIM to use a specific routing protocol such as BGP, OSPF, or OSPFv3. As well as the default “base” topology, two additional multicast topologies (one for each address family, IPv4 and/or IPv6) may be configured for each VRF on the device. A separate multicast topology has route tables for all interfaces configured for direct connection to the VRF, as well as any routes provided by the routing protocol(s) activated in that topology. Additional topologies, such as a unicast or multiple multicast topologies, cannot be configured in any VRF.

Each VRF on a device has a base topology that:

• is always present
• cannot be removed
• routes to all active interfaces in the VRF
• includes additional topologies as subsets of the base

A multicast topology is therefore a subset of the base topology and is not completely separate from it. One multicast topology may be configured per address family (IPv4, IPv6), per VRF. When a new topology is created, a separate route table and route table manager (RTM) instance is created for it. Within a multicast topology, PIM interacts only with the route table in that VRF for the configured address family. Once a topology has been created, it is available for use by all protocols that support multi-topology (besides PIM, currently limited to BGP, OSPF and OSPFv3). Each protocol must be separately configured to use the new multicast topology.

Configured OSPF and OSPFv3 parameters (such as timer values and authentication) apply to all instances of that protocol active on the interface for which they are configured, and override any values set in the global configuration of the protocol.

A multicast topology must be created before any routing protocol is configured to use it. By default, routing protocols operate in the base topology and must be explicitly configured to participate in a multicast topology. For multiple topologies using OSPF protocols, it is necessary to configure a separate instance of the protocol for each topology. When an instance of an OSPF protocol is activated, it joins the base topology by default. It remains joined to the base topology even when joined to a multicast topology, unless explicitly removed.

In any VRF there are only the base topology and an IPv4 multicast and/or IPv6 multicast topologies. If you create a process and specify a new multicast topology for it (for example, “address-family ipv4 multicast"), it is then in both unicast and multicast topologies. This is because its route table is still joined to the base (unicast), but now is configured to receive multicast routes from the base as well. To make the process be in the multicast topology only, you must also enter a "no address-family ipv4 unicast" command within that process. Once a multicast topology has been created, you can configure BGP and OSPF or OSPFv3 to join that topology.

In regards to OSPF use in Multi-Topology: you can run a single instance of OSPF connected to the unicast and multicast topology, or you can run multiple instances of OSPF each connected to either a unicast or multicast topology. Since unicast is the default topology you must explicitly disconnect from it if you wish to have an instance connected only to the multicast. It is also possible to have the same interface connect to multiple OSPF processes with the addition of an instance number and the OSPF configuration done under the interface.

interface vlan.0.100
ip ospf 1 area 0.0.0.1 instance 1
ip ospf 2 area 0.0.0.0 instance 2
exit

Redistribution and distance commands under router ospf apply to the base topology.

Redistribution and distance commands under the address-family command under router ospf apply to the multicast topology.