[c-nsp] xconnect on ASR9k - Type 5 PW, what happens with VLAN tag?

[Marcin Kurek]

Hello,

Recently I ran into some problems while testing xconnect between ASR9k 
and I realized that I'm lacking some basic knowledge. I read a bit of 
cisco docs, but unfortunately it didn't help enough.

I'm using fairly simple topology in the lab:

The topology is fairly simple:

cisco 4948 ------ ASR9006 ------- MPLS cloud (ASR9k) ------- ASR9001 
------- cisco 4948

All I want to do is to test connectivity between SVI configured on both 
4948 switches with different packet sizes and encapsulation.

My configuration is as follows:

ASR9006:
interface TenGigE0/0/1/5.2001 l2transport
  encapsulation dot1q 2001
!
l2vpn
  pw-class EoMPLS
   encapsulation mpls
   !
  !
  xconnect group TEST
   p2p A
    interface TenGigE0/0/1/5.2001
    neighbor ipv4 2.2.2.2 pw-id 2001
     pw-class EoMPLS
    !
   !

ASR9001:
interface Bundle-Ether10.2001 l2transport
  encapsulation dot1q 2001
!
l2vpn
  pw-class EoMPLS
   encapsulation mpls
   !
  !
  xconnect group TEST
   p2p A
    interface Bundle-Ether10.2001
    neighbor ipv4 1.1.1.1 pw-id 2001
     pw-class EoMPLS
    !

On both 4948s system MTU is set to 1552 bytes. Links inside MPLS cloud 
are configured with MTU 9000.
I didn't change the MTU of TenGig interfaces of ASRs towards 4948, so 
it's default 1514.

RP/0/RSP1/CPU0:ASR9006#sh l2vpn xconnect detail
Group TEST, XC A, state is up; Interworking none
   AC: TenGigE0/0/1/5.2001, state is up
     Type VLAN; Num Ranges: 1
     VLAN ranges: [2001, 2001]
     MTU 1504; XC ID 0x1080007; interworking none
     Statistics:
       packets: received 402, sent 23797
       bytes: received 361925, sent 1909567
       drops: illegal VLAN 0, illegal length 0
   PW: neighbor 2.2.2.2, PW ID 2001, state is up ( established )
     PW class PW_CLASS, XC ID 0xc0000003
     Encapsulation MPLS, protocol LDP
     Source address 1.1.1.1
     PW type Ethernet, control word disabled, interworking none
     PW backup disable delay 0 sec
     Sequencing not set

I have two questions.
1. PW type Ethernet means Type 5 PW (port-based PW). Cisco docs says:
"The ingress PE transports frames received on a main interface or after 
the subinterface tags have been removed when the packet is received on a 
subinterface. There is no requirement to send a tagged frame over a type 
5 PW, and no dummy tag is added by the EVC-based platforms".
So in this scenario, is the dot1q tag popped by ingress PE even though I 
didn't use "rewrite ingress tag pop 1"?
What is the difference between these two configs assuming Type 5 PW?

interface TenGigE0/0/1/5.2001 l2transport
  encapsulation dot1q 2001

interface TenGigE0/0/1/5.2001 l2transport
  encapsulation dot1q 2001
  rewrite ingress tag pop 1

What factors determine PW type?

2. How to interpret Access Circuit MTU in the above output? I noticed 
that it changes depending on subinterface configuration (whether I use 
rewrite command or not) and MTU.
When I don't configure "rewrite ingress tag pop 1" it is 1504. If I do, 
it drops to 1500.
I understand this as a way to tell the other side "hey, the maximum 
payload size I can accept is 1504 bytes".
But this wouldn't make sense if the tag was stripped by ingress PE, 
becasue egress PE would have to slap the tag back in, so 1504 + 4 + 14 = 
1522 which exceeds interface MTU...

Can anybody guide me in the right direction please?

Thanks,
Marcin