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New Member

STUN with local ack

My situation is this:


router RTR2 with 'stun protocol-group basic' is now working fine. I want to use local-ack, but the documentation is confusing me.

I have the following questions:

- stun route address xx tcp .... xx is the sdlc address of FEPs at either end ?

- 'SDLC address xx echo' - command is needed when ?

- RTR1 should have 'STUN sdlc-role secondary', and also RTR2 should be the same ? since they both connect to FEPs. Is that correct ??

could someone help me out here, please..


Re: STUN with local ack

First, I would like to confirm the network topology. The connection between FEP1 and RTR1 is SDLC (serial), not T/R. Is that correct? STUN does not support token ring between the STUN router and FEP.

Assume that the connection betwen RTR1 and FEP1 is SDLC. The answer of your questions are as follows:

1. No. FEP to FEP uses dynamice SDLC address. The secondary FEP determines the polling address. You may need to use debug stun packet or debug sdlc packet to determine the polling address.

2. SDLC address xx echo is needed when

a. a FEP to FEP connection


b. you use STUN SDLC with local ack.

3. No. The FEP with the higher subarea number is SDLC primary. Thus, the router connects to the FEP with higher subarea number is secondary. The router connects to the FEP with lower subarea number is primary.

Please go to the following URL for details:

The URL does not have any STUN examples for PU4-PU4. However, it explains how FEP determines its polling address and SDLC role in a PU4-PU4 connection.

New Member

Re: STUN with local ack

Your assumption is correct. Connection between RTR1 and FEP1 is serial.

1. Since FEP is using dynamic addresses, will these addresses change on every IPL, or whenever ? If these addresses changed then STUN link would not come up.

2. Whats the purpose of SDLC address xx echo command ? Is it to force an address on the interface ? and then use that xx address in STUN route command ?


Re: STUN with local ack

The addresses chosen by the FEP are dependant on the position of the lines in the NCP gen so, so long as that does not change, the addresses will not change.

The echo option is because the FEP sets the echo bit (the first bit) in the SDLC address for return traffic.

You should be aware that local ack is only supported for modulo 8 traffic. Since many FEP to FEP connection use modulo 128, many sites continue to use STUN basic.

New Member

Re: STUN with local ack

thanks for the reply..

1. how do i find the address of each FEP in the debug output?

2. SDLC address command - Whats it for ? explanation in documentation is not clear at all.

3. How do i know if FEPs are using Modulo8 or 128 ?


Re: STUN with local ack

1. If you turn on debug sdlc packet, the first byte is the polling address. As this is a PU4-PU4 connection, you will see different polling address on NDI and SDI traffic. Pick the one with the lower value. For example, you will see 01 and 81. Pick sdlc address as 01.


3. You need to go to the NCP gen. Find the link for this particular remote NCP. Look for the value of MODULO.

New Member

Re: STUN with local ack

Thanks for your replies.. I want to get back to this issue now.

- Theres no 'Modulo' coded in my entire NCP except in an X25 node definition, however, NCP Manual suggests that for an SDLC PU modulo=8 is the default. So, I can use localACK ?

Cisco Employee

Re: STUN with local ack

Also check that the SDLC group/line and SDLCST groups have MODULO = 8 and MAXOUT = 7 configured. Then the answer is yes. You might also look into SDLC TG


Easiest to configure.

All retransmits and recovery done by the FEPs. Routers just encapsulate all frames and transport over IP.

Supports Modulo 128.

Supports remote NCP load and activation ( link attached NCP, not SNI ).

RRs get sent across WAN.

Good if clean link.

SDLC roles and SDLC address determined by FEPs and not configured in routers.


Provides local-ack so that RRs are not sent across WAN.

Must configure SDLC role and SDLC address

Modulo 8

Easier to configure than STUN SDLC


Same as above, but a little more difficult to configure, see link below:

I usually tell customers to bring it up in STUN BASIC, see which side sends SNRM. That will be the primary side and see what address the secondary NCP responds with. This will tell you what address to use. If the connection comes up in STUN BASIC, you know the two FEPs can talk to each other. If this doesn't work, it is not because you configured the wrong SDLC role or address. If this doesn't work, then it is usally a mismatch on defintions in the NCPs on the FEPs.

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