[j-nsp] l2circuit between ASR9k and MX80

[Marcin Kurek]

Hi Saku,

Thanks for your explanation, I found it helpful.

> What dummy tag? Is there [ SVLAN CVLAN ] stack coming to the CSCO, or just
> [ SVLAN ]?
>
> If just SVLAN, then this should happen
>
> a) 3101 comes to CSCO from LAN
> b) CSCO pops 3101
> c) CSCO detects its VLAN EoMPLS and pushes 3101 back
> d) Frame sent to MPLS WAN
> e) JNPR gets it, does not nothing, sends out to LAN
>
> And reverse
>
> a) 3101 comes to JNPR from LAN
> b) JNPR does nothing
> c) CSCO gets it
> d) CSCO rewrite 3101 to 3101 due to VLAN mode detected
>
>
> If there were CVLAN in in addition to SVLAN, it would be left untouched, and
> would be seen after SVLAN.
>
> Adding 'output-vlan-map swap' on JNPR would allow the config to work with
> mismatching SVLANs in either end.

No, I was testing very basic scenario with SVLAN only coming to cisco.

Okay, so I need to take a step back and ask for a clarification about VC 
Type 4 and VC Type 5 if you don't mind.

My understanding is that on ASR9k the VLAN tag manipulation is 
completely independent of negotiated PW type.
We can have two cases here:
a) Type 5 - port mode
b) Type 4 - VLAN mode

In either case, we can do whathever we want with the tags by means of 
"rewrite ingress" commands.

According to the standard, when using VC Type 5 frames transported over 
the PW _MAY_ have VLAN tags attached.
On the contrary, when using VC Type 4 frames _MUST_ have a VLAN tag 
attached.

If we have VLAN EoMPLS like in our example and we have stripped the 
SVLAN tag on ingress, something need to be pushed to the VLAN tag stack, 
right?

Quote from cisco docs:
"In order to address this possibility, the EVC platforms insert a dummy 
VLAN tag 0 on top of the frame for type 4 PWs"

"However, if you use a type 4 PW between an EVC platform and a non-EVC 
platform, this might lead to interoperability problems. The non-EVC 
platform does not consider the top VLAN tag as the dummy VLAN tag and 
instead forwards the frame with the dummy VLAN tag 0 as the outer tag"

So what the heck is the dummy tag? When you mentioned that CSCO detects 
VLAN EoMPLS and pushes 3101 back, did you mean the "dummy tag" as cisco 
calls it?

Regarding VC Type 4 and Type 5 discussion, there was another thread on 
cisco-nsp a few days ago.
Waris Sagheer wrote:

"Little bit history, VC type 4 came into being since some hardware did 
not have the capability to manipulate the tag at egress hence the 
concept of dummy tag. VC type 4 is pretty much becoming non existent and 
is being supported on legacy platforms only. VC type 5 will be the 
default signaling moving forward where users will have the flexibility 
to add/remove tags based on the EVC configuration.

I found it interesting, although I still don't understand what problems 
does VC Type 4 solve.
If some hardware couldn't manipulate the tag at egress (but was capable 
of manipulation at ingress), how does the dummy tag help?
Using our example, how they handled situation where they had [ SVLAN 
CVLAN ] tag stack and the SVLAN was different at both sides?

Many thanks,
Marcin