IP Multicast Stub Routing in the AOS
IP Multicast Stub Routing in AOS
10
Copyright © 2005 ADTRAN, Inc.
61200890L1-29.3A
IP Multicast Stub Routing in the AOS
In Release 7.1, AOS introduced IP multicast support in the form of IP Multicast Stub Routing using IGMP
Forwarding. Note the following key points for this release:
•
AOS command syntax differs from some other vendors when configuring this feature. In other
vendor implementations, multicast forwarding, PIM, and IGMP functions are intertwined. By
enabling PIM on an interface, you also enable IGMP and multicast forwarding. Even when
configuring multicast stub operation, PIM commands are used and the PIM process is running on
the stub router. In contrast, AOS performs a pure IGMP forwarding operation with no multicast
routing protocol process. A unique command set was developed for AOS to clearly indicate these
differences in architecture.
•
IGMP forwarding is based on the Internet Draft
draft-ietf-magma-igmp-proxy-06.txt
,
but some
differences exist. Most notably, if there is a multicast source on a stub router’s downstream
interface, it may be received by group members on other downstream interfaces of the same router,
but it will not be forwarded to the upstream interface. Therefore, applications requiring a multicast
source located on an AOS stub network with receivers outside of that stub network are not
supported. For instance, in Figure 2 on page 5, if
PC3
were acting as a media server or some other
form of multicast source,
PC1
and
PC2
could join the group and receive the stream. That stream
will never be sent to the backbone, so no other part of the multicast network can receive the stream.
•
While multiple upstream interfaces may be configured, only one will be selected to serve as the
current upstream interface for IGMP Forwarding. This works well in situations such as dial backup
when either the primary or the backup interface is operational at any one time. Topologies with
multiple operational upstream interfaces (such as dual PVCs or PPP links) to the multicast network
require special care since there is no automatic protection from multicast loops and no dynamic
best-path selection. Dual active links can be used, but careful network design is required.
AOS Multicast Stub Routing Feature Details and Operation
Figure 3 on page 12 shows a stub network connected to a larger multicast-enabled network via an
AOS-based stub router.
•
Referring to Figure 3 on page 12, interfaces
eth 0/1
and
eth 0/2
on the
Stub Router
are configured
as multicast stub downstream interfaces (using the
ip mcast-stub downstream
command). These
interfaces run IGMP in router mode and can support IGMP V1 or V2. Most applications will use
V2. Both interfaces are configured with IGMP helper enabled (using the
ip mcast-stub
helper-enable
command).
•
Interface
fr 1.1
is the primary WAN link to the multicast network. Interface
ppp 1
is the dial backup
for the primary link. Both are configured as multicast stub upstream interfaces.
•
An IGMP helper address is defined as a global parameter for the router (using the
ip mcast-stub
helper-address
a.b.c.d command). The router will use the unicast routing table to select the best
interface to reach the specified address. If the selected interface is an IP multicast upstream
interface, that interface will become the IGMP forwarding interface and will run the IGMP client
process. This interface will perform IGMP forwarding (proxying) for any downstream interface that
has IGMP forwarding enabled (using the
ip mcast-stub helper-enable
command). In this
example, the helper address is the address of the media server.
