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New Member

does Catalyst 2950 support SNA traffic ?


we are trying to integrate some legacy sna hosts

to switching enviromnent.

we are faced to a strange symptom when legacy hosts

are connected to 2950 switch no traffic sna pass thru

we checked vlan,ports ,STP, but didnt found any hints

removing 2950 from configuration clear out the problem.

so simple diagram

7206 ==wan===7206=ethernet HUB=legacy sna host OK

----DLSW----> -------SNA------>

7206==wab===7206==CAT2950===legacy sna host NO OK

-----DLSW----> ---------SNA----->

are they any issue on 2950 for SNA traffic .

thanks in advance


Cisco Employee

Re: does Catalyst 2950 support SNA traffic ?


in general, No. There are no general issues with sna and a cat2950. The switch doesnt care what type of traffic runs through.

Besides the obviouos, the client and the dlsw router

must be in the same vlan.

There are sometimes problems with a switched environment. I.e if you do trunking with dot1q between the switch and the dlsw router and you use the switch in a mode where it tries to autonegotiate the trunk. Dlsw forwards some of the negotiation packets which leaves the switch in a state where the connections dont work.

We have also seen a problem in some situations where a dot1q trunk is used when the switch is configured for PVSTP, per vlan stp. In that case a special type of bpdu's is send which are also get forwarded by dlsw and can cause the switch on the other end to go to blocking ect.

All of those problems can be fixed by either putting a sap filter onto the bridge group on the dlsw router and only allow the sna saps you need or adopt the switch configuration. I.e. turn of autonegotiation of the trunk mode ect.

The issue needs to be looked at individualy and i would advice to open a case with the tac and work on this problem. So that we can understand what the problem is and fix it.



New Member

Re: does Catalyst 2950 support SNA traffic ?

thanks for these infos, both are in same vlans

we'll checking trunking attributes now


New Member

Re: does Catalyst 2950 support SNA traffic ?

If you are running DLSW+, the problem is with the 802.1q trunk. DLSW does not work over an 802.1q trunk so you will have to use an older 2900 series switch that supports ISL trunking. We ran into the same problem. I do not know why Cisco is removing the isl trunking feature from newer switch. It might be due to the new enhancements of 802.1q with Mono Spanning Tree where it acts like ISL.


Re: does Catalyst 2950 support SNA traffic ?

You may need to config the router to use ieee 802.1q encapsulation, if the router port on the switch is defined as a trunk port.

One note about trunking with ios routers: IOS code will assume that vlan 1 is the native vlan on the trunk, and if it is not, then define a vlan 1 interface on the router and then shut it down; there is no need for ip address assignment.