Discussion

This output shows some of the default settings on T1 interfaces. The first highlighted
line shows that the connection is point to point (PPP), the MTU size is 1,504
bytes, the clocking is internal, and the interface speed is T1, or 1.544 Mbps. The second
line shows that loopback mode is disabled, the frame checksum size (FCS) is 16
bits, and the framing mode is extended superframe. Two other defaults not shown in
the output are a byte encoding of 8 bites per byte (nx64) and B8ZS line encoding. In
the last line of the output, you see the interface address that you configured.
To change the FCS to 32 bits to provide more reliable packet verification, use this
command:
[edit interfaces t1-0/0/3]
aviva@RouterF# set t1-options fcs 32
T1 has two framing modes, D4 super frame (SF) and extended super frame (ESF). An
SF frame consists of 192 data bits, arranged into 24 8-bit channels, and a single framing
bit. That single framing bit is used as part of a 12-bit framing sequence. ESF
extends the super frame from 12 frames to 24 frames. To change the framing mode
to SF, use this command:
[edit interfaces t1-0/0/3]
aviva@RouterF# set t1-options framing sf
7.17 Performing a Loopback Test on a T1 Interface
Problem
A new T1 interface has been installed and configured, and you want to perform a
loopback test to make sure the interface is operating properly.
Solution
Set up a T1 loopback configuration:
[edit interfaces t1-0/0/3]
aviva@RouterF# set t1-options loopback local
aviva@RouterF# set no-keepalives
aviva@RouterF# set encapsulation cisco-hdlc
aviva@RouterF# commit
Make sure the interface is up:
aviva@RouterF> show interfaces t1-0/0/3 terse
Interface Admin Link Proto Local Remote
t1-0/0/3 up up
Then zero all the statistics on the interface:
aviva@RouterF> clear interfaces statistics t1-0/0/3
226 | Chapter 7: Router Interfaces
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