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IPC-505
Cisco IOS IP Configuration Guide
Configuring Unidirectional Link Routing
This chapter describes the unidirectional link routing (UDLR) feature. UDLR provides mechanisms for
a router to emulate a bidirectional link to enable the routing of unicast and multicast packets over a
physical unidirectional interface, such as a broadcast satellite link. However, there must be a back
channel or other path between the routers that share a physical unidirectional link (UDL). A UDLR
tunnel is a mechanism for unicast and multicast traffic; Internet Group Management Protocol (IGMP)
UDLR and IGMP Proxy are mechanisms for multicast traffic.
For information about tunnel interfaces, refer to the “Configuring Logical Interfaces” chapter in the
Cisco IOS Interface Configuration Guide. For information about IGMP, refer to the chapter
“Configuring IP Multicast Routing” in the Cisco IOS IP Configuration Guide.
For a complete description of the UDLR commands used in this chapter, refer to the “Unidirectional
Link Routing Commands” chapter in the Cisco IOS IP Command Reference, Volume 3 of 3: Multicast.
To locate documentation of other commands that appear in this chapter, use the command reference
master index, or search online.
To identify the hardware platform or software image information associated with a feature, use the
Feature Navigator on Cisco.com to search for information about the feature or refer to the software
release notes for a specific release. For more information, see the “Identifying Supported Platforms”
section in the “Using Cisco IOS Software” chapter.
UDLR Overview
Both unicast and multicast routing protocols forward data on interfaces from which they have received
routing control information. This model works only on bidirectional links for most existing routing
protocols. However, some networks use broadcast satellite links, which are unidirectional. For networks
that use broadcast satellite links, accomplishing two-way communication over broadcast satellite links
presents a problem in terms of discovering and sharing knowledge of a network topology.
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, existing unicast and multicast routing protocols cannot be supported over
UDLs. UDLR is designed to enable the operation of routing protocols over UDLs without changing the
routing protocols themselves.