Spanning vlans across access switches in distribution block.... please help

Unanswered Question
Feb 3rd, 2009

Hi All

Can someone please explain why Cisco states that in a Campus Hierarchical modle if Vlans are spanned across Access switches in a distribution block, then the Distrubution to distribution link should be Layer 2. Is this really necesary or just a recommendation, and if so why? Can't this link be a L3 link when spanning vlans across Access switches in distribution block, as I understand the benefit of having a L3 distribution to distribution link so that SPT is avoided.

Please help

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lamav Tue, 02/03/2009 - 10:17

Sid:

SHORT ANSWER:

If you choose to span your vlans across access switches, then yes, your distribution layer switches will have to have L2 trunks configured between them to pass vlan traffic. The trunk is said to provide "L2 adjacency."

LONG ANSWER:

Remember, a vlan is a L2 construct. A device that is part of a particular vlan should be able to conduct L2 communications with another device in the same vlan. So, if you have a host in vlan 10 that hangs off switch 1 and you have another host in the same vlan that hangs off switch 2, switch 1 and switch 2 will have to have a L2 trunk configured between them to allow vlan traffic to pass. OR, if each switch is dual-homed to a distribution layer switch, the L2 uplinks between access layer switches and the distribution layer switch can provide that L2 adjacency.

Moreover, you can have a routed access layer, which typically means that a vlan will be confined to that access switch, HOWEVER< there is a flavor of routed access layer that allows for the placement of a L2 trunk between ACCESS switches to allow the vlan to span across access switches. The uplink to the distribution layer, however, will be a L3 connection.

HTH

Victor

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