IP Multicast Stub Routing in AOS | IP Multicast Stub Routing in the AOS |
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Helper Address Technical Note
The helper address can be any address on the path from the desired upstream interface to the
•Where the upstream interface(s) connect in the multicast network.
•The number and location of sources in the multicast network.
•The granularity of network routes in the stub router’s unicast routing table.
If using default routes, the helper address can be a dummy address since it will resolve to the current default route interface. If using highly granular routes (and if more than one upstream interface can be simultaneously operational), it should be a point in the multicast network that is common to the paths used by all upstream interfaces to reach all sources.
In the example of Figure 3 on page 12, the primary and backup interfaces connect to different locations in the multicast network. Since there is only one Multicast Source, an address common to the primary and backup interfaces in reaching the source would be the best choice. This setup would eliminate concern of route granularity and of whether one or more upstream interfaces is ever up simultaneously (assuming routing is properly weighted and symmetrical). In this case the address of the Central Router, Router 3, or even the address of the source itself would be ideal. In a scenario with a single interface to the multicast network, the helper address could simply be that of the next hop toward the source.
Feature Operation
•The helper address is set to the address of the media server, and the primary and dial backup interfaces are both configured as upstream interfaces. Therefore, when the primary interface is up, the backup interface is down and the primary is selected as the IGMP forwarding interface and performs the IGMP host function.
•When PC1 wishes to receive the media server stream being transmitted on group address 224.1.1.1, it sends an IGMP message on its segment indicating it wishes to join that group. The AOS router registers that group address on interface eth 0/1.
•Since eth 0/1 is set with IGMP helper enabled (using the ip
•If the primary link has a failure, the dial backup interface is activated and connects. The unicast route table establishes a new best path toward the specified helper address, and the ppp 1 interface becomes the IGMP forwarding interface, taking over the IGMP host function. Since eth 0/1 is still a member of group 224.1.1.1, interface ppp 1 (acting as an IGMP host) sends an IGMP message toward the multicast network indicating that it wishes to join group address 224.1.1.1. The peer multicast router registers that group address on its interface toward the stub. Using its multicast routing protocol, the multicast router resolves the path, and the stream again begins to flow to PC1 via the dial backup interface.
•When the primary link is restored, the process reverses and the primary interface is used once again.
•When PC1 signals it is leaving group 224.1.1.1, interface eth 0/1 is unregistered as a group member. The IGMP forwarding interface signals upstream that it is leaving the group, and the stream is no longer forwarded to the stub.
•Should PC4 become a multicast source, PC1 can join the group and the router will forward the stream toward PC1, but not toward the multicast network.
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