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New Member

Migration from ISL to 802.1q and STP loops

Hi, I have a customer with 2 Catalyst 6500 connected each other and also to 20 or 30 Catalyst 2924 all trunking in ISL, and now the customer just bougth new LAN switches and all are 2950 and this new switches only support DOT1Q, so I now have trunks with the 2 types of encapsulation and they have been working OK, but now we are implementing redundant links and when we tried to implemet the first one it creat a STP loop, and I was wondering if this has something to do with the 2 types of encap living togheter?

I had implement redundant links before, but only with one type of encapsulation running on the network and we just connect the switches and let STP to do the rest of the work.

Can anyone give me advice on this problem...........



Cisco Employee

Re: Migration from ISL to 802.1q and STP loops

Make sure you are using the lastest code on the 2950. I seem to remember seeing some bugs with STP and early code. Make sure you are using trunk mode "desirable". It is a best practices:

But, to answer to your orginal question. ISL and DOT1Q can reside on the same switch. For example, you may have a 6509 that does dot1q to a 2950, then on the same switch (6509) have a link that is ISL to another switch.

New Member

Re: Migration from ISL to 802.1q and STP loops

We have a large environment and seen similar issues which we believe are attributable to the 2950s. I understand that the CCOs recommendation is to set to trunks to "desirable". I thought that DTP was not supported on the 2900/2950s and in desirable mode the 6509 would still send out DTP. Surely nonegotiate is more appropriate?

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