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Layer 2 Loop-Free U Access - VLAN Extension possible

Todd Bren

Cisco says in it's 642-874 study guides that L2 Loop-Free U design in access layer has these characterictics:

The following are characteristics of loop-free U access:

■ VLANs are contained in switch pairs (no extension outside of switch pairs).

■ No STP blocking; all uplinks are active.

■ Layer 2 service modules black hole traffic on uplink failure.

Why would VLAN extension be not supported in this topology? See attached picture where it seems it could be working.



Marwan ALshawi

hi Todd,

you maybe right to some extend, it dose work in the case all the up links are up and the access switches daisy chained

however it is not optimal design interims of layr 2 extension,redundancy  in case of a device or link failure

if you you have the inter switch link configured in layer 2 then this cloud be better but you need to remove the interswitch link between the access switches


I'm also reading 642-874 and the same question has arised in my head...

Could anybody explain in in detail? Why VLAN extension doesn't happen? What settings are used?

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