Re: VC type 4 and VC type 5

nice explanation Naveen !

On Sat, Mar 5, 2011 at 12:46 AM, Naveen <navin.ms_at_gmail.com> wrote:

> 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
> > >>
> > >>
> _______________________________________________________________________
> > >> Subscription information may be found at:
> > >> http://www.groupstudy.com/list/CCIELab.html
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Sat Mar 05 2011 - 01:46:12 ART