IP Multicast Stub Routing in AOS
IP Multicast Stub Routing in the AOS
61200890L1-29.3A
Copyright © 2005 ADTRAN, Inc.
11
Helper Address Technical Note
The helper address can be any address on the path from the desired upstream interface to the
multicast-source device. The choice depends on several network design parameters such as:
•
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 mcast-stub helper-enable
command),
interface
fr1.1
(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 signals towards the media server, and the stream begins to flow to
PC1
.
•
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.
