Serial Tunneling FAQ

Q. What address should I use to define the Serial Tunneling (STUN) peer-name statement?

A. You may use any IP address of an active interface in the router. You should, however, use the most stable interface IP address, which is the loopback address.

Q. Why is my Serial Tunneling (STUN) peer name closed?

A. Your STUN peer name is closed because no data was exchanged.

• If using direct encapsulation, then your interface is down.

• If using IP encapsulation, then the IP connection between the two peers is not up either because there is no IP connectivity or because neither device has attempted to send data over the pipe.

Q. Why and when are different Serial Tunneling (STUN) groups used?

A. Use different STUN groups to differentiate the traffic from front-end processors (FEPs) that have controllers with the same address.

Q. After configuring Serial Tunneling (STUN) encapsulation on the interface, how do I remove this feature? The ???no stun encapsulation??? command does not work.

A. Issue the hdlc encapsulation command, which will set the interface back to its default encapsulation.

Q. Why do we have to use a break-out box to tie Request To Send (RTS) and data terminal ready (DTR) together to keep the Serial Tunneling (STUN) interface up?

A. Unless you have the new half-duplex nonreturn to zero inverted (NRZI), STUN only supports full duplex; by this convention, ???full duplex??? means RTS and Clear To Send (CTS) are always high. Strapping the RTS and DTR pins together will keep RTS high at all times.

Q. How should I prioritize Serial Tunneling (STUN) traffic?

A. Prioritize STUN traffic for Cisco IOS Software Release 9.1 and later, as shown below.

• Use the procedure below for simple serial encapsulation.

  1. Issue the following command:

     priority-list x stun {high|medium|normal|low} 
          address stun_group controller_address
     
     
     !--- The above command is entered on one line.
     
     

  2. Assign the priority group to the output interface.

• For TCP encapsulation there are now four ports, listed below, instead of one port, as in the previous software.

  1994 : high priority
  1990 : medium priority
  1991 : normal priority
  1992 : low priority

  So to prioritize the traffic, code the STUN port first, and then use the priority-list command to assign the prioritization. For example, set up the priority list for STUN on interface serial 1 with controller address C1 as shown below.

  priority-list 1 protocol ip high tcp 1994
  priority-list 1 protocol ip medium tcp 1990
  priority-list 1 protocol ip normal tcp 1991
  priority-list 1 protocol ip low tcp 1992
  priority-list 1 stun high address 1 C1
  interface s 1
  encapsulation stun
  stun group 1
  stun route address C1 tcp 131.108.64.250
       local-ack priority
  
  !--- The above command is entered on one line.
  
  interface serial 2
  priority-group 1
  
  !--- Note: This is the WAN interface.
  
  

Note: The priority-group statement is applied to the WAN interface over which the STUN traffic is tunnelled, not to the STUN interface itself.

Q. Can Serial Tunneling (STUN) work over a Switched Multimegabit Data Service (SMDS), Frame Relay, or X.25 cloud?

A. Yes, if using TCP encapsulation. After the encapsulation, the packet will be just like an ordinary IP packet, and it will be routed over X.25, Frame Relay, or SMDS as normal IP traffic.

Q. Why is slow switching between 56 kbps links recommended over fast switching?

A. In most cases, fast switching puts the packets into the output queue too quickly for the 56 kbps link, and the packets are dropped if no output buffer can be allocated. When a packet drops, TCP tries to retransmit the packet, which uses a lot of CPU cycles. Therefore, with any link that is 56 kbps or slower, it is usually recommended that you turn off fast switching.

Q. In the output of a debug stun packet command, what do SDI and NDI mean?

A. Refer to the Explanation of SDI and NDI from a debug stun packet Command for the meanings of Serial Data Input (SDI) and Network Data Input (NDI).

Related Information

• Technical Support - Cisco Systems