Both unicast and multicast routing protocols forward data on interfaces from which they have received routing control information.
This model requires a bidirectional link. However, some network links are unidirectional. For networks that are unidirectional
(such as broadcast satellite links), a method of communication that allows for control information to operate in a unidirectional
environment is necessary. (Note that IGMP is not a routing protocol.)
Specifically, in unicast routing, when a router receives an update message on an interface for a prefix, it forwards data
for destinations that match that prefix out that same interface. This is the case in distance vector routing protocols. Similarly,
in multicast routing, when a router receives a Join message for a multicast group on an interface, it forwards copies of data
destined for that group out that same interface. Based on these principles, unicast and multicast routing protocols cannot
be supported over UDLs without the use of UDLR. UDLR is designed to enable the operation of routing protocols over UDLs without
changing the routing protocols themselves.
UDLR enables a router to emulate the behavior of a bidirectional link for IP operations over UDLs. UDLR has three complementary
mechanisms for bidirectional link emulation, which are described in the following sections:
UDLR Tunnel--A mechanism for routing unicast and multicast traffic.
Internet Group Management Protocol (IGMP) UDLR--Mechanism for routing multicast traffic. This method scales well for many
broadcast satellite links.
IGMP Proxy--Mechanism for routing multicast traffic.
You can use each mechanism independently or in conjunction with the others. IGMP proxy is described in the “ Customizing
IGMP ” module.