Subject: Reference model for H-VPLS node

From: Olen Stokes <[email-masked]>
Date: Wed, 12 Dec 2007 06:12:42 -0800

There was significant discussion at the last IETF about the correct
reference model to be used when discussing OAM for a H-VPLS node.  For the
H-VPLS network in Figure 2 of draft-stokes-l2vpn-cfm-vccv-oam-00.txt, the
reference model described in that draft is shown below in Figure A:

         |--- PW-1 ----|  |--- PW-13 ----|  |---- PW-3 ----|
Emulated |             |  |              |  |              |
LAN      |             |  |              |  |              |
....................................................................
.        |             |  |              |  |              |       .
.|-------|-----|  |----|--|-----|  |-----|--|----|  |------|------|.
.| ----------- |  | ----------- |  | ----------- |  | ----------- |.
.|     VPLS    |  |     VPLS    |  |     VPLS    |  |    VPLS     |.
.|  Forwarder  |  |  Forwarder  |  |  Forwarder  |  |  Forwarder  |.
.| ------|---- |  | -----|----- |  | -----|----- |  | -----|----- |.
.|.............|..|................|................................
 |       |     |  |      |      |  |      |      |  |      |      |
 |       |     |  |      |      |  |      |      |  |------|------|
 | ------|---- |  | -----|----- |  | -----|----- |  | -----|----- |
 | |         | |  | |         | |  | |         | |  | |         | |
 | | Bridge  | |  | | Bridge  | |  | |  Bridge | |  | |  Bridge | |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  |      | |  | |         | |  | |         | |  | |      |  | |
 | ---|------- |  | ----------- |  | ----------- |  | -------|--- |
 |----|--------|  |-------------|  |-------------|  |--------|----|
      |                                                      |
      |                                                      |
     CE-1                                                  CE-3

     MTU1-s            PE1-rs           PE3-rs           MTU3-s

     Figure A: H-VPLS node using only VPLS Forwarder connections

An alternative model based on
http://www3.ietf.org/proceedings/04nov/slides/l2vpn-9.pdf is shown below in
Figure B:

                          |--- PW-13 ----|
                Emulated  |              |
                 LAN      |              |
                ....................................
                .         |              |         .
                . |-------|-----|  |-----|-------| .
                . | ----------- |  | ----------- | .
                . |     VPLS    |  |     VPLS    | .
                . |  Forwarder  |  |  Forwarder  | .
                . | -----|----- |  | -----|----- | .
                ..|................|................
                  |      |      |  |      |      |
 |-------------|  |      |      |  |      |      |  |-------------|
 | ----------- |  | -----|----- |  | -----|----- |  | ----------- |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  Bridge | |  | | Bridge  | |  | |  Bridge | |  | |  Bridge | |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  |   *  | |  | |    *    | |  | |    *    | |  | |  *   |  | |
 | ---|---|--- |  | -----|----- |  | -----|----- |  | ---|---|--- |
 |----|---|----|  |------|------|  |------|------|  |----|---|----|
      |   |              |                |              |   |
      |   |---- PW-1 ----|                |----- PW-3 ---|   |
     CE-1                                                  CE-3

     MTU1-s            PE1-rs           PE3-rs           MTU3-s

Note: "*" = emulated bridge port

    Figure B: H-VPLS node using emulated bridge port connections

Please note that Figure B is my interpretation of Ali's charts from his
November, 2004 presentation.

There are definite implications based on the differences of the two models.
The VPLS forwarder only model in Figure A treats a H-VPLS network with PW
access the same as a simple full mesh VPLS network.  The bridge model sees a
single emulated LAN interface to the (H)VPLS network and therefore is not
concerned with the structure of that (H)VPLS network.  

The emulated bridge port model treats a H-VPLS network with PW access the
same as a H-VPLS network with Ethernet access.  In effect, it "separates"
the network into a full mesh VPLS emulated LAN and individual access
networks that can be PW or Ethernet.  This "separation" is a result of the
presence of the bridge module between these connections.

One difference from an OAM perspective is that the emulated bridge port
model allows 802.1ag MPs to be associated with PW access connections such as
PW-1 in Figure B above. 

There may be implications to other protocols such as spanning tree.  In the
emulated bridge port model, PW-1 is seen by the bridge module and could be
considered separately should the bridge module in PE1-rs participate in
spanning tree.  However, in the VPLS forwarder only model, only the single
emulated LAN would be included in spanning tree. 

As I said during the L2 VPN meeting, I believe that choosing the correct
reference model is a decision to be made by the working group.  Therefore,
please use the mailing list to state your views on the correct reference
model to be used for a H-VPLS node.

Cheers,
Olen