debug dtp packets on 3550

[ciscoforum]

port 0/13 on 3550 connects to a 2600 router with the following related configuration on the switch:

interface FastEthernet0/13

switchport mode dynamic desirable

no ip address

2600 router is just a regular port(not isl),I turn on the debug dtp packets.I got question about the debug output

1. what is tos/tas and tot/tat

2. why on tot/tat is shows isl/negotiate.

If i show in trunk, 0/13 is not trunk for sure.

Thanks

[Georg Pauwen]

Hello,

when the port is configured as ´switchport mode dynamic desirable´, the interface actively attempts to convert the link to a trunk link, that is why you see the DTP packets being sent. If the neighboring interface is set to trunk mode, desirable mode, or auto mode, the interface would become a trunk.

Not sure what the various counters mean, I guess they have to do with the DTP negotiation attempts.

HTH,

Georg

[aashish.c]

Hi,

ISL is cisco`s default trunking protocol. So 2 devices which are trunking capable are connected in desirable and auto mode then they would form a ISL trunk, not a dot1q trunk. so i think it is trying to negotiate a ISL trunk with router`s interface.

on switchport apply the command :

switch(config-if)#switchport mode access

then cehck the dtp output, you should not see any trunking packets going across the link. By default on 3550 port stays in dynamic desirable for trunking. you have to manually make them access links by applying above command.

Regards

Aashish C