Configuring a T1 With PPP Between Cisco IOS and Adtran AOS

Recently I was tasked with configuring a T1 with PPP encapsulation on an Adtran router. Having never configured an Adtran router before, I did a little poking around in the documentation. Adtran routers run AOS, which is very similar contextually to Cisco IOS. However, the configuration of a T1 and PPP on AOS is different enough to warrant discussion. So, let's dive in.

Cisco IOS Side

IOS does not differentiate T1 interfaces from plain serial interfaces, so make sure you're working on the right hardware before attempting configuration.

Cisco# show interface s0/0
Serial0/0 is down, line protocol is down 
  Hardware is GT96K with integrated T1 CSU/DSU
  Internet address is 10.0.0.1/30
  MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Closed, loopback not set
  ...

The relevant IOS configuration for a T1 with PPP and CHAP is provided below. The username and password configured under global configuration are to be used to authenticate the Adtran router via CHAP.

username adtran password 0 foobar42
!
interface Serial0/0
 ip address 10.0.0.1 255.255.255.252
 encapsulation ppp
 ppp authentication chap
 ppp chap hostname cisco
 ppp chap password 0 foobar42

Adtran AOS Side

Compared to its IOS counterpart, the AOS configuration is rather verbose:

interface t1 0/1
  tdm-group 1 timeslots 1-24 speed 64
  no shutdown
!
interface ppp 1
  ip address  10.0.0.2  255.255.255.252 
  ppp authentication chap
  username cisco password foobar42
  ppp chap hostname adtran
  ppp chap password foobar42
  no shutdown
  cross-connect 1 t1 0/1 1 ppp 1

The most substantial difference on AOS is that the T1 and PPP connections are split into separate physical and logical interfaces, respectively. The T1 interface holds only layer two configuration, while IP addressing and authentication are configured on the PPP interface. It's possible to administratively shut down the PPP interface without affecting layer two connectivity across the T1.

On the T1 interface, we've set up a TDM group containing all twenty-four 64 kbps channels on the T1. On the PPP interface we've assigned an IPv4 address and configured PPP encapsulation with CHAP authentication. Notice that in AOS the CHAP username and password for the remote router are configured under the PPP interface, not under global configuration as on IOS.

The cross-connect command at the end is a bit of an oddball. It may be issued from global configuration mode, yet shows up under interface configuration in the running config. It feels like the AOS developers couldn't decide whether they wanted it to be a global command or an interface command; under interface configuration, the ppp 1 bit at the end seems redundant. At any rate, the cross-connect command simply binds the T1 TDM group to the PPP interface.

The configuration commands have colored below to show how they correspond with one another.

The following outputs are taken from the Adtran router. Once our configuration is complete, we should see both the T1 and PPP interfaces up and running.

TA908e# show interface t1 0/1
t1 0/1 is UP
  Receiver has no alarms
  T1 coding is B8ZS, framing is ESF
  FDL type is ANSI
  Line build-out is 0dB
  No remote loopbacks, No network loopbacks
  Acceptance of remote loopback requests enabled
  Tx Alarm Enable: rai
...
TA908e# show interface ppp 1
ppp 1 is UP
  Configuration:
    Keep-alive is set (10 sec.)
    No multilink
      MTU = 1500
    Local authentication is CHAP, 
      peer authentication is CHAP
    IP is configured
      10.0.0.2  255.255.255.252
...
TA908e# ping 10.0.0.1
Type CTRL+C to abort.
Legend: '!' = Success, '?' = Unknown host, '$' = Invalid host address
        '*' = Request timed out, '-' = Destination host unreachable
        'x' = TTL expired in transit, 'e' = Unknown error

Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/3 ms