Re: VPLS model for L2VPN Framework document

Ali Sajassi wrote:
> 
>>
>>
>> So this is where I may (or may not) offer a different opinion from 
>> Norm and/or Ali.  I continue to be of the opinion that the "emulated 
>> LAN" (e.g. VPLS) should look like a LAN to the bridge function, which 
>> implies (to me anyway) that
>>  a) if that VPLS is provisioned as a trunk port to the bridge, then 
>> there are multiple VLANs per VPLS (not one VPLS/vlan) - this is after 
>> all how a bridge views a non-emulated trunk port
> 
> 
> Considering a VPLS as an "emulated LAN" with respect to the bridge, is 
> certainly another model. However, the issue of handling inter-island and 
> inter-provider loop prevention as well as scalability of expanding # of 
> VLANs across different islands and providers become much more 
> challenging (e.g., running STP between different islands and different 
> providers for tens of thousands of customers).
> In the model that considers a VPLS as emulated VLANs, the L3 connection 
> of the PE to the core is considered as a single trunk port and thus 
> split-horizon mechanism serves as a loop prevention mechanism for the 
> emulated VLANs.
> 

This "emulated LAN == LAN" is the traditional model used by emulated 
LANs over frame relay and emulated LANs over ATM.

"Emulated LAN = VLAN" is a model supported by "emulated LAN = LAN" 
because many LAN ports (access ports) have only a single VLAN.  But what 
it doesn't do is introduce separate control and data planes.

Maybe its just me, but bridged networks have limits.  They always have. 
  They weren't designed or intended to provide limitless worldwide 
connectivity.  They're designed for simplicity, flexibility, and and 
loop-free connectivity.  Global scaling was not one of the criteria. 
Thats why routers came along.  Geez, you work for Cisco, you have to 
appreciate that.



> 
>>  b) if that VPLS is provisioned as an access port to the bridge, then 
>> there is only one VLAN per VPLS (though as with tag stacking, one 
>> "carrier" VLAN may not equal one "customer" VLAN).
> 
> 
> A PW can certainly be considered as a trunk or access port

Assuming that PW is used as an P2P emulated LAN, sure.

> 
>> To me, this keeps the terminology and model incredibly consistent to 
>> the  existing briding models.
> 
> 
> If you consider, the L3 connectivity of a PE to the core as a trunk port 
> with each VPLS instance as a VLAN over this trunk port, then the 
> operation of this model would be consistent as well with the existing 
> bridging model.
> 

Well, no.  On a briding port, the control and data plane are tied 
togehe.  A VLAN VPLS can fail without the control VPLS failing, or vice 
versa.  Thats pretty inconsistent with existing models.