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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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Definition and Purpose

When multicast and an MPLS TE tunnel are deployed in a network, multicast may be affected by the TE tunnel, which causes multicast services to become unavailable.

To solve this problem, you can enable local multicast-topology (MT) for multicast packet forwarding.

Local MT

After IGP Shortcut is configured on a TE tunnel, the outbound interface of the route calculated by an IGP may not be the actual physical interface but a TE tunnel interface.

According to the unicast route to the multicast source address, a router sends a Join message through a TE tunnel interface. In this situation, routers spanned by the TE tunnel cannot detect the Join message, so they do not create any multicast forwarding entry.

As shown in Figure 5-18, RouterB spanned by the TE tunnel does not create any multicast forwarding entry.

Figure 5-18 OSPF Local MT

A TE tunnel is unidirectional, so multicast data packets sent by the multicast source are sent to the routers spanned by the tunnel through physical interfaces. These routers discard the multicast data packets, because they do not have any multicast forwarding entry. As a result, services become unavailable.

After local MT is enabled, if the outbound interface of the calculated route is a TE tunnel interface of IGP Shortcut type, the route management (RM) module creates a separate Multicast IGP (MIGP) routing table for the multicast protocol, calculates the actual physical outbound interface for the route, and then adds the route to the MIGP routing table. Multicast then uses routes in the MIGP routing table to forward packets.

In Figure 5-18, the packets requesting to join a multicast group is sent from RouterA to RouterB through GE 1/1/0. RouterB then can create the multicast forwarding table correctly.

Updated: 2019-12-27

Document ID: EDOC1000174069

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