Re: VC type 4 and VC type 5

Hi Gaurav,

The key to both questions is to understand the VC types and Vlan rewrite
operation. Allow me to explain.

1) VLAN tags have local significance on PE-CE links. Hence, if 2 sides of
the MPLS core are in different vlans, the PE routers automatically does a
"vlan rewrite" operation for packets received from Psuedowire (PW) and going
out to Ethernet (Access Circuit / AC), so their respective CE routers
properly interpret them. Now, what happens to vlan tags on packets coming
from CE routers and going out to PW is a different topic. You need to
understand how PW was setup to begin with, and the VC-types negotiated for
different Access circuits.

2) VC-type is identified and negotiated when a PW is setup b/n 2 PE routers.
You did this by configuring the "xconnect .." command on the PE side
interfaces. The 2 PE routers must agree on the VC-type before the PW can
come up. The PE side interface can be broadly in 3 different modes.

(a) Port mode (xconnect under Main interface, VC-type 5)
(b) Vlan mode (xconnect under a Sub interface, as you have done; VC-type 4)
(c) Tunnel mode (xconnect.. or mpls l2transport.. on SVI interface on PE,
VC-type 4 or 5)

*Now, onto your Questions:*

(1) ".. But is original label from CE preserved ?"

This depends on what VC-type / mode the 2 PE routers negotiated. In your
case, the PW is operating in Vlan-mode (because each PE has xconnect under a
vlan sub-if). For packets coming from CE routers, the *vlan tag is preserved
and transported across the PW* to the other PE router. For packets going out
to CE routers, each PE router is automatically doing a "vlan rewrite" before
packets are sent out. For example: PE1 would get an MPLS encapsulated packet
with Vlan 140 from PE2, but would rewrite the Vlan to 201 before sending out
the CE1.

(2) "How is it changed / re-written at PE2 ? Kindly explain .."

Same as above explanation. PE2 would MPLS packet with Vlan-201 in the
Ethernet Header it encapsulated, and would rewrite to Vlan-140 before
sending the packets out to CE2.

*Side-note:*

PE routers which have their PWs operating in Port-mode do not care about
whether or not Vlan tags are present, and would transparently pass the
frames across the MPLS cloud. The Tunnel mode is one where the PE router
decides to add another Vlan-tag (called S-Tag) to distinguish the services
given to a customer, or to distinguish traffic belonging to diff customers
within the MPLS cloud.

Feel free to test the different combinations. I hope that helped a bit.
Naveen.
#24532

On Fri, Mar 4, 2011 at 9:58 AM, GAURAV MADAN <gauravmadan1177_at_gmail.com>wrote:

> Pending confusion is : ( please look at scenario below )
>
> CE1 e0/0.201 ---------------- e0/0.201 PE --------------- CLOUD
> --------------- PE e1/0.140 ----------------e1/0.140 CE2
>
>
> PE1
> ====
> int e0/0.201
> encap dot1q 201
> xconnect 10.1.1.1 33 pw-class TEST
> !
>
> PE2
> =====
> int e1/0.140
> encap dot1q 140
> xconnect 20.1.1.1 33 pw-class TEST
> !
>
> We are connecting CE1(201 vlan) to CE2 (140 vlan )
>
> Q 1 : When vlan tagged pkt of 201 reaches PE1 .. Is this dot1q label
> preserved / striped in above config .. There are 2 labels added i
> know ( VC label and IGP/MPLS label ) .. But is original label from CE
> preserved ?
>
> Q 2 : How is it changed / re-written at PE2 ? Kindly explain
>
> I think things will start making sense if these 2 questions are answered .
>
> Thanks in advance
> Gaurav Madan
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Fri, Mar 4, 2011 at 2:19 AM, ccie19804_at_gmail.com <ccie19804_at_gmail.com>
> wrote:
> > Look in Ciscopress Layer 2 VPN Architectures, it has good detailed
> explanation on these topics.
> >
> > Swap
> > #19804
> >
> >
> > Sent from my iPhone
> > Plaese ingore seplling mistkaes
> > ___________________________
> >
> > On 3 Mar 2011, at 16:53, GAURAV MADAN <gauravmadan1177_at_gmail.com> wrote:
> >
> >> Friends ,
> >>
> >> Is there a good write-up on this ? Any blog / white paper etc ..
> >>
> >> I am finding it hard to understand by reading how vlan tags / VC ids
> >> are travelling across .. EVC concept added to it are making things
> >> even more confusing ..
> >> Please point to good doc if you know of one ?
> >>
> >> Thanks
> >> Gaurav Madan
> >>
> >>
> >> Blogs and organic groups at http://www.ccie.net
> >>
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>
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Received on Fri Mar 04 2011 - 13:46:26 ART