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348 Chapter 10: X.25/Frame Relay Topologies

Notice the local loop in Figure 10-13. This can be viewed as the wire from the central office to the
local building. With most phone companies, this is a flat fee. In other words, no matter how far
from the central office a company is, this will not cost any more money. So if you are 10 miles
from the local central office, there's no extra fee. Frame Relay can be called "distance insensitive."

This isn't the case with dedicated leased lines. Leased lines are priced per mile and are quite
expensive. The farther you are from the central office, the more it costs per month for the local
loop. So now you might want to add another port only every 10 to 20 circuits or so, depending
on oversubscription (see the later section "Oversubscription").

NOTE

Some states are considering subsidizing businesses that might want to build in a remote area of
the state, where property is cheaper. But T3s and T1s are more expensive in those areas because
the central office is some distance away, so they are hesitant to do so. Therefore, governors are
offering subsidies to offset those costs.

Frame Relay Access Devices (FRADs)

Frame Relay Access Devices (FRADs) aren't often a test topic but are important nonetheless.
There are many different ways of accessing Frame Relay networks, not just by Cisco routers.
There are LAN FRADs and wide-area FRADs. Wide-area FRADs take serial line data in, tunnel
it across the frame link, and deliver it out the other end as a serial line. LAN FRADs take an
Ethernet frame, bridge it on a Frame Relay link, and bridge it out the other side as Ethernet. The
idea behind FRADs is that they are low-cost Frame Relay access devices. You have to check
each vendor specification to find out what they can or cannot do.

Remember that low-end Cisco routers are very cost-competitive. When interfacing FRADs
with Cisco routers, use the IETF standard for signaling, because you will be intermixing
different vendors:

frame-relay map ip 10.10.10.2 16 broadcast ietf

DLCI

Data Link Connection Identifier (DLCI) is the virtual circuit used to communicate with the
Frame Relay switch. The DLCI is a value that specifies a PVC or SVC in the Frame Relay
network. This value is configured on the Frame Relay switch and is obtained by the router via
LMI signaling. When the router is powered up, it performs a full status inquiry to the Frame
Relay switch and obtains a list of DLCIs assigned to the router's serial port. After this
information is obtained by the router, there are four possible states the DLCI could be in:

Deleted--The switch does not recognize the DLCI as a valid circuit. Look at it as though
the switch in the provider's network is not sending a DLCI to the local router, or that the
DLCI you configured under the subinterface on the router (such as DLCI 100) is not
configured on the switch.

87200333.book Page 348 Wednesday, August 22, 2001 2:53 PM