Cisco Cisco IOS Software Release 12.0(17)S

The following sections provide additional information about the multilink (FRF.16) feature:

The Multilink Frame Relay feature enables you to create a virtual interface called a bundle or bundle 
interface. The bundle interface serves as the Frame Relay data link and performs the same functions as 
a physical interface. 

The bundle is made up of physical serial links, called bundle links. The bundle links within a bundle 
function as one physical link. Bundle links are invisible to the Frame Relay data-link layer, so Frame 
Relay functionality cannot be configured on these interfaces. Functionality that you want to apply to 
these links must be configured on the bundle interface. Bundle links are visible to peer devices. The local 
router and peer devices exchange link integrity protocol control messages to determine which bundle 
links are operational and to synchronize which bundle links should be associated with which bundles. 

Both ends of a bundle link send out hello messages at regular intervals. When a peer device receives a 
hello message, it responds by sending an acknowledgment message. This exchange of hello messages 
and acknowledgments serve as a keepalive mechanism for the link. If a router is sending hello messages 
but not receiving acknowledgments, it will resend the hello message up to a configured maximum 
number of times. If the router exhausts the maximum number of retries, the bundle link line protocol is 
considered down (unoperational). 

The bundle link line protocol is considered up (operational) when the peer device acknowledges that it 
will use the same link for the bundle. The line protocol remains up when the peer device acknowledges 
the hello messages from the local router. 

The bundle line protocol is considered up when the Frame Relay data-link layer at the local router and 
peer device synchronize using the Local Management Interface (LMI). The bundle line protocol remains 
up as long as the LMI keepalives are successful. FRF2.1 Annex 1 is supported as an alternative to LMI 
on NNI-type bundle interfaces. The bundle interface is considered up when the line protocol of at least 
one of its associated bundle links is up.

By combining multiple physical interfaces into a bundle, you can design a Frame Relay interface with 
more bandwidth than is available from any single physical interface. For example, many new network 
applications require more bandwidth than is available on a T1 line. One option is to invest in a T3 line;