Simple LAN Design Question

Unanswered Question
Jun 4th, 2008

Hi All,

I am trying to design a small LAN using 3 Extreme Summit switches as access switches, all L2 only, 1 Cisco 6509 as core/distribution, and 1 Cisco 3825 router for WAN access. There are 3 VLANs required, 1 on each Extreme switch. What is the best way to connect everything?

I have been considering creating trunk ports on the 6509 to each Extreme switch. Then enabling routing on the 6509. Does it make sense to trunk even though I only have 1 VLAN going across the trunk? Is there an alternative?

I have this problem too.
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Joseph W. Doherty Wed, 06/04/2008 - 08:39

If you only have one VLAN on each edge switch, you don't need to trunk or define any VLANs on edge switch, you only need to assign the 6500 port they connect to within a VLAN. However, if you think you may want to have multiple VLANs on the edge switches in the future, defining the link between the edge switch and the 6500 as a trunk might ease future configuration and/or avoid some service interruption when you want to have the additional edge VLANs.

popokitgoo Wed, 06/04/2008 - 12:56

Thanks for the explanation! As a result, the 6500 will do the VLAN tagging, correct?

Jon Marshall Wed, 06/04/2008 - 12:59

If you enable the links between the Extreme switches and the 6500 to be 802.1q trunks then both the 6500 and the Extreme switches will tag vlans when packets are transmitted across the link.


robertblasey Wed, 06/04/2008 - 23:04

... if asked 'should I trunk between switches' I'll never understand why people refrain from trunking just because the dont need it at the moment. One aspect often overlooked is the ability to transport Layer2 QoS designation inside the .1q tags. Another alternative is the enabling of DTP (dont know if Extreme has this feature). If used wisely - considering the LAN-security implications - its a great feature.

We buy and sell Cisco equipment with a bunch of world class features - and dont use much of 'em. Most of these features are even tought at the CCNA level.


Joseph W. Doherty Thu, 06/05/2008 - 03:57

An excellent point about L2 CoS and trunks! Although many engineers aren't fully aware of all the available features (not easy since there are so many), of those that do (or know many), some seem to believe if you can, also means you should. You also correctly note "If used wisely". I wanted to reemphasis that point because besides the issue of DTP likely not supported on Extreme (Cisco proprietary?) or perhaps w/o VTP one switch sending tagged frames to another which it doesn't understand, extending VLANs may open up other network issues. So again, "used wisely" is important.


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