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Why Frame Relay? 351
Frame Relay Configuration Parameters
This section discusses some common Frame Relay configuration parameters. Some common
parameters were used in the mid-to-late '90s and might not be needed by carriers as much as
they were before. Switches in the cloud have progressed to such a speed that it takes carriers
more time to filter for FECN and BECN frames than it is worth. It is actually faster now to not
look for them due to the fast switching equipment. But they might still be used in some places
and are often the subject matter of tests, so it is still necessary to mention them.
Forward and Backward Explicit Congestion Notification
The carrier sets FECNs and BECNs. This means that your Frame Relay packets are running into
congestion somewhere in your provider's network. These bits can be set in the Frame Relay
header. The idea is that when a frame encounters congestion, the Frame Relay switch can set
the FECN bit to notify the destination end that it encountered congestion. Hopefully the
destination end notified the original sender that things are busy in the cloud and told it to please
slow down.
If this congestion gets severe enough (each provider defines what severe is), your provider starts
discarding packets that exceed the Committed Information Rate (CIR) and are marked discard-
eligible. If the congestion gets even more severe (again, this is carrier-dependent), the provider's
network might discard all packets regardless of whether they exceed the customer's CIR.
To test whether any traffic is getting lost, do an extended ping from the router when you see
FECNs and BECNs incrementing on your router. This will tell you if all the pings get through.
Other than this, you are at your provider's mercy, and you must go through them to get info on
what is going on inside the Frame Relay cloud.
Extended ping is a very valuable tool for the CCDP to test the network design. By setting
variables, you can conduct tests to ensure that not only the smaller 32-byte pings get through,
but much larger packet sizes as well. Example 10-4 shows an example of extended ping. Simply
type ping at the router prompt. Remember to answer yes to "Extended commands."
Example 10-4
Extended Ping
Router#ping
----------Just type ping at the command prompt and press Enter
Protocol [ip]:
------------Select the protocol
Target IP address: 192.168.100.1
---------Select the target IP address
Repeat count [5]:
-------How many packets to send?
Datagram size [100]:
----- The datagram will be 100 bytes
Timeout in seconds [2]:
The router will time out after two seconds, and a dot
is echoed back to the user.
Extended commands [n]: yes (these are more detailed commands).
Source address or interface: 192.168.100.2
------Select which IP address will be
the source address for the ping
Type of service [0]:
--- Select type of service
Set DF bit in IP header? [no]:
-Select "Don't fragment" bit, which means do not
fragment this packet.
Validate reply data? [no]:
continues
87200333.book Page 351 Wednesday, August 22, 2001 2:53 PM