[ednog] Using MPLS to replace layer 2 trunking

David Farmer farmer at umn.edu
Fri Jul 29 10:12:51 EDT 2005

On 29 Jul 2005 Vincent Celindro wrote:

> Just looking for comments, maybe seeing if anyone else is doing this. We
> have a couple of applications where it is a requirement to have devices
> reside on the same layer2 network, but are located off different routers.
> Essentially we would like vlan A to exist at both routing Sites 1&2.
> Traditionally this is done by trunking Vlan-A across routing sites/core.
> I would like to use MPLS specifically EoMPLS to achieve the same goal.
> I would like to not have to trunk VlanA across the infrastructure and
> the headaches associated with this, while gaining the other advantages
> associated with MPLS. As I understand this I can actually have VlanA
> local only to site1 create an EoMPLS tunnel and have all traffic exit
> at Site2 on VlanA or any other Vlan, and have the 2 vlans be in one
> Layer2 domain. This appears to be easily done, if I create the EoMPLS
> tunnel at both sites based upon a physical port or a sub interfaced
> port ( per vlan EoMPLS). 

We've been doing it for about a year, both layer 2 and layer 3 MPLS 
VPNs.  We prefer to use layer 3 where we can and use layer 2 where 
we must.  Which right now Layer 2 is used only for two or three legacy 
VLANs and cross core backhaul for Wireless to authentication boxes 
in a Data Center.  

> I'm guessing that most networks are like ours, where access layer
> switches are trunked to some aggregate device or directly to a 6509. So
> port based EoMPLS doesn't seem to be an option. I also don't think most
> of us use subinterfaces to connect access layer switches, trunking
> seems to be the standard. This also doesn't seem ideal, since you would
> have to have an EoMPLS tunnel per subinterface for a vlan, so I may
> need 10 tunnels just for trying to do this for a single vlan. 

The 6500/7600 only supports Port based EoMPLS or Pseudo-Wires, 
unless you have OSMs.  We are pushing (starting over year ago) for 
the next-gen PFC to support this, but that is still a little ways off.  There 
is a way to work around this though, a loop back cable between two 
GigE ports. one port is configured as a psudo wire Xconnect and the 
other as a trunk port.  Do this on both the 6509s in your diagram below 
and you have a virtualised trunk between them. (Yes it is a Hack!!!  But 
it works!)
> So looking for ideas, I'd like to end up with a single EoMPLS tunnel
> per vlan. The problem is how to force the traffic from Vlan-A into the
> tunnel? 

The only way to do this is to have a loop-back per VLAN or use OSMs. 
We doing a single loop-back and trunk limited to a small set of VLANs. 
Every Core Node has a virtual trunk back to a common Core Node. It 
has a full ste of trunks to all other Nodes. This is what we are doing for 
now and we are pusshing for a better solution in the next-gen PFCs, 
we are also interested in VPLS to do multi-point.  Today VPLS is only 
available if you have OSMs on the 6500/7600.
>                          _,--.   ___
>          Site 1         .'    >-'  |           Site 2
>        +--------+      |'          ....      +--------+
>        |        |    ,''              ]|     | 6509   |
>        | 6509   '''''' IP/MPLS CORE  \.------+ GIG/SFP|
>        | GIG/SFP|     \..              \     |        |
>        ++----+--+       |         .____'     +--+--`.-+
> trunk .'      |trunk    |   _-|   \      trunk |     `.  trunk
>      /        \         `---  `.__,|           /       `.
> +--.'--+   +---+--+                        +--+----+   +-`.---+
> +------+   +------+                        +-------+   +------+
>  Vlan-A       Vlan-A                         Vlan-A     Vlan-A
>     Access Layer switches                    Access Layer switches

Note: in the MPLS VPN Archetecture only PE routers are required, P 
or CE routers are optional and may be use if they help.  So the 
IP/MPLS Core you show may not be necessary, or the 6509s may be 
part of it.   

> Thanks,
> Vincent Celindro
> CCIE# 8630
> PGP FingerPrint A052 E1DE 6AB1 53A0 FC87  7196 5F0D 1781 4C1E A0CE
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David Farmer				Email:	farmer at umn.edu
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