IS-IS Multicast Topologies Overview
Most multicast routing protocols perform a reverse-path forwarding (RPF) check on the source of multicast data packets. If a packet comes in on the interface that is used to send data to the source, the packet is accepted and forwarded to one or more downstream interfaces. Otherwise, the packet is discarded and a notification is sent to the multicast routing protocol running on the interface.
In certain instances, the unicast routing table used for the RPF check is also the table used for forwarding unicast data packets. Thus, unicast and multicast routing are congruent. In other cases, where it is preferred that multicast routing be independent of unicast routing, the multicast routing protocols are configured to perform the RPF check using an alternate unicast routing table inet.2.
You can configure IS-IS to calculate an alternate IPv4 multicast topology, in addition to the normal IPv4 unicast topology, and add the corresponding routes to inet.2. The IS-IS interface metrics for the multicast topology can be configured independently of the unicast metrics. You can also selectively disable interfaces from participating in the multicast topology while continuing to participate in the regular unicast topology. This enables you to exercise control over the paths that multicast data takes through a network so that it is independent of unicast data paths. You can also configure IS-IS to calculate an alternate IPv6 multicast topology, in addition to the normal IPv6 unicast topology.
IS-IS only starts advertising the routes when the interface routes are in inet.2.
For the multicast metric commands, these are interface-specific settings, not global.
Table 1 lists the various IPv4 statements you can use to configure IS-IS topologies.
Statement |
Description |
|---|---|
|
Enables an alternate IPv4 multicast topology. |
|
Configures the multicast metric for an alternate IPv4 multicast topology. |
|
Excludes an interface from the IPv4 multicast topology. |
|
Excludes an interface from the IPv4 unicast topologies. |
Table 2 lists the various IPv6 statements you can use to configure IS-IS topologies.
Statement |
Description |
|---|---|
|
Enables an alternate IPv6 multicast topology. |
|
Enables an alternate IPv6 unicast topology. |
|
Configures the multicast metric for an alternate IPv6 multicast topology. |
|
Configures the unicast metric for an alternate IPv6 multicast topology. |
|
Excludes an interface from the IPv6 multicast topology. |
|
Excludes an interface from the IPv6 unicast topologies. |
For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.