Configuring T1 Interfaces

Problem

You want to configure the T1 interface on your router.

Solution

Use the following commands to configure a T1 interface:

[edit interfaces] aviva@RouterF# set t1-0/0/3 unit 0 family inet address 10.0.13.1/24 aviva@RouterF# set t1-0/0/3 description "J2300 T1 line in local office"

Discussion

The basic configuration to get a T1 interface is very straightforward. You just set the interface's IP address. T1 interfaces can handle 24 simultaneous connections, called channels or timeslots, running at a combined 1.544 Mbps. Each T1 or DS1 packet is 193 bits and consists of 24 8-bit frames that carry data, plus one framing bit. Each port on a T1 or DS1 PIC can have a maximum of 24 channels.

Once the interface is configured, check its status:

aviva@RouterF> show interfaces t1-0/0/3 Physical interface: t1-0/0/3, Enabled, Physical link is Up Interface index: 140, SNMP ifIndex: 38 Description: J2300 T1 line in local office Link-level type: PPP, MTU: 1504, Clocking: Internal, Speed: T1, Loopback: None, FCS: 16, Framing: ESF Device flags : Present Running Interface flags: Point-To-Point SNMP-Traps 16384 Link flags : Keepalives Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3 Keepalive: Input: 164568 (00:00:06 ago), Output: 164561 (00:00:03 ago) LCP state: Opened NCP state: inet: Opened, inet6: Not-configured, iso: Not-configured, mpls: Not-configured CHAP state: Not-configured CoS queues : 8 supported Last flapped : 2005-04-15 21:13:25 PDT (5w3d 11:12 ago) Input rate : 40 bps (0 pps) Output rate : 48 bps (0 pps) DS1 alarms : None DS1 defects : None Logical interface t1-0/0/3.0 (Index 69) (SNMP ifIndex 40) Flags: Point-To-Point SNMP-Traps Encapsulation: PPP Protocol inet, MTU: 1500 Flags: None Addresses, Flags: Is-Preferred Is-Primary Destination: 10.0.13/24, Local: 10.0.13.2, Broadcast: 10.0.13.255

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