Frame Relay provides a means for
statistically multiplexing many logical data conversations (referred to
as virtual circuits [VCs]) over a single physical transmission link by
assigning connection identifiers to each pair of DTE devices. The
service provider's switching equipment constructs a table that maps
connection identifiers to outbound ports. When a frame is received, the
switching device analyzes the connection identifier and delivers the
frame to the pre-established associated outbound port.
The VCs can be either permanent VCs (PVCs) or
switched VCs (SVCs). PVCs are permanently established connections that
are used when there is frequent and consistent data transfer between DTE
devices across a Frame Relay network.
With ANSI T1.617, ITU-T Q.933 (Layer 3), and Q.922
(Layer 2), Frame Relay now supports SVCs. SVCs are temporary
connections, used when there is only sporadic data transfer between DTE
devices across the Frame Relay network. Because they are temporary, SVC
connections require call setup and termination for each connection.
Cisco IOS® Release 11.2 or later supports Frame Relay SVCs. You will
need to determine whether your carrier supports SVCs before implementing
them.
A data-link connection identifier (DLCI)
identifies the logical VC between the CPE and the Frame Relay switch.
The Frame Relay switch maps the DLCIs between each pair of routers to
create a PVC. DLCIs have local significance in that the identifier
references the point between the local router and the Frame Relay switch
to which it is connected. Your Frame Relay provider sets up the DLCI
numbers to be used by the routers for establishing PVCs.
Local Significance
DLCIs have local significance; that is,
the end devices at two different ends of a connection may use a
different DLCI to refer to that same connection. The Figure provides an
example of one VC identified at each end by two different DLCI numbers.
Because the DLCIs have only local
significance, the only real restriction on the use of DLCIs is that they
are not used for more than one destination from the same port.
You configure an available DLCI number to
map this provided Frame Relay number to a network address. For example,
an administrator might map to an IP address of the interface on Router A
in the Figure. This mapping in the router points to a static route,
which is the PVC to that remote router. For example, the administrator
can configure a Frame Relay map for 172.16.11.3 by using the PVC
identified as DLCI 500. Frame Relay uses VCs identified with DLCI
numbers. The DLCI numbers have only local significance.
The address mapping can be either configured
manually or dynamically. With dynamic address mapping, Frame Relay
Address Resolution Protocol (ARP) provides a given DLCI and requests
next-hop protocol addresses for a specific connection. The router then
updates its mapping table and uses the information in the table to
forward packets on the correct route. Frame Relay ARP is commonly known
as inverse ARP.
When packets are sent across the network,
the intermediate switches will look up the DLCI in the map table and do
the following:
DLCI Numbering Scheme
The Frame Relay service provider will assign the
DLCI numbers for your WAN. Usually, DLCIs 0 to 15 and 1008 to 1023 are
reserved for special purposes. Therefore, service providers
typically assign DLCIs in the range of 16 to 1007.
Multicasts can use DLCI 1019 and 1020. Cisco
Local Management Interface (LMI) uses DLCI 1023 and ANSI/ITU-T uses DLCI
0 (there is a discussion on LMI in the following section).