DTP - mapping extended VLANS when using ISL

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Dec 6th, 2009
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Hello people!


Today I was reviwing the DTP and some statements were not clear for me. It is related to the DTP mappings when using extended VLANs and ISL on the trunk.


Reference: BCMSN Self-Study - 4th edition - pages 178 and 179


802.1Q allows the extended vlans on the trunk

ISL permits only vlan 1 to 1000 on the trunk


If I have two switches with, for example, VLAN 3001, and both switches are directly connected to one another and they are using DTP and, for some reason, it chose ISL and the trunk was established.


As the ISL does not accept VLANs above 1000, I readed that the DTP is able to map these VLANs to another VLAN less than 1000 so the traffic can pass thru the trunk. E.g.: VLAN 3001 could be mapped to VLAN 301 amd then, the switch can send that traffic to the trunk.


In this scenario, consider it is using VTP v3 in order to support extended VLANs.


As I told, this is not clear to me so my questions are:


1 - How does the DTP make that mapping and how does it work?

2 - Is possible to make that mappings manually?

3 - Considering the example above, DTP mapped vlan 3001 to vlan 301. The trunk accepts the traffic. When the traffic arrives on the other switch, how does the switch treat that packets? Does It just back to the original VLAN (3001) or does it keep that as 301?

4 - Still in our example, If the switch already has the VLAN 301 on that, I readed that the traffic from that VLAN will not be accepted by the trunk, since DTP is already using the same VLAN for mapping purposes. So, is there any control  for that mappings? I mean, if I just add a new VLAN, this vlan is the same used for the mapping (vlan 301 in our example), will this vlan pass traffic to the other switch thru the same trunk?

5 - If I have one side configure manually to be an ISL trunk and the other one to use the DTP, of course it wil form a trunk using ISL, but how does the mappings work on the side of the ISL manually configured (since it has no DTP on it)?


thanks in advance!

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ronaldobf Sun, 12/06/2009 - 14:36
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Guys!


Forget it!


I just mixed 2 different topics! A complete misunderstanding.. I gess I was sleeping hehe...


DTP does not make any mappings... Please, disregard that topic and if possible, close it.


I just readed that the mappings I was talking about was the "allowed vlan" and related commands.

Giuseppe Larosa Mon, 12/07/2009 - 10:13
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Hello Ronaldo,

be aware that a later revision of ISL removed the limitation to standard vlans and now on modern IOS versions it should be possible to have a vlan like 3001 on an ISL trunk. ISL big header has a 15 bits field for vlan-id.

of course 802.1Q trunks are largely preferred now for their advantages: smaller overhead, stackability ( Q in Q) standards based


and yes DTP deals with negotiating the trunking status of the link.


Hope to help

Giuseppe

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