| Junos Router | Serial Interface with PPP | | Print | |
A serial interface can come in a variety of different physical forms, such as V.35, X.21, and EIA-530. The choice of physical media often depends on geographical location; V.35 is the most common choice in the United States and Europe, and X.21 is more common in Japan. Regardless of physical media, all serial interfaces have the same idea of defining a data circuit-terminating equipment (DCE) device and a data terminal equipment (DTE) device. The DTE device is the end unit that receives data encoding, clocking, and signal conversion from the DCE device. In modern communications, the DCE device often takes the form of a channel service unit/data service unit (CSU/DSU) or a modem; however, when connecting two routers in a back-to-back fashion, one of the routers takes the role of a DCE.
Router Ale and router Bock have a back-to-back serial connection using V.35 with the default encapsulation of PPP. Normally, a router will default to DTE mode, but in this case, Ale is automatically chosen as the DCE based on the detection of a DCE cable. You can observe this detection in the Local mode field of the show interfaces command:
root@ale# run show interfaces se-1/0/0 extensive | find "serial media"
Serial media information:
Line protocol: v.35
Resync history:
Sync loss count: 0
Data signal:
Rx Clock: OK
Control signals:
Local mode: DCE
To DTE: CTS: up, DCD: up, DSR: up
From DTE: DTR: up, RTS: up
DCE loopback override: Off
Clocking mode: internal
Clock rate: 8.0 MHz
Loopback: none
Tx clock: non-invert
Line encoding: nrz
Since one of the roles of the DCE is to provide clocking to the DTE, an internal clocking mode needs to be configured on Ale. This allows Ale to generate a clocking signal toward Bock using the internal clock with a default clock rate of 8 MHz:
[edit interfaces]
root@ale# show se-1/0/0
serial-options {
clocking-mode internal;
}
unit 0 {
family inet {
address 172.16.1.1/30;
}
}
Bock has no clocking mode configured and takes the default clock mode of loop-timed, which takes the transmitted clock from Ale. Bock could also have been configured for DCE mode, which would have the same result in this case. Here is the Bock configuration:
[edit interfaces se-1/0/1]
root@Bock# show
unit 0 {
family inet {
address 172.16.1.2/30;
}
}
You can verify the local mode, clocking mode, and clock rate on Bock by using the show interfaces command:
[edit interfaces se-1/0/1]
root@Bock# run show interfaces se-1/0/1 extensive | find "serial media"
Serial media information:
Line protocol: v.35
Resync history:
Sync loss count: 0
Data signal:
Rx Clock: OK
Control signals:
Local mode: DTE
To DCE: DTR: up, RTS: up
From DCE: CTS: up, DCD: up, DSR: up
Clocking mode: loop-timed
Clock rate: 8.0 MHz
Loopback: none
Tx clock: non-invert
Line encoding: nrz
Clocking can often be a confusing topic for many users. For back-to-back router connections, Juniper made it simple by allowing multiple different clocking modes to be configured and still "do the right thing." The only combinations that will not work for back-to-back connections are the DCE in loop mode and the DTE in loop or DCE mode. However, when connecting to a CSU/DSU or a modem, proper care must be taken to configure the correct clock mode.
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