Subject: stp and draft-ietf-ppvpn-vpls-ldp-01.txt

From: narasimhan g <[email-masked]>
Date: Mon, 12 Apr 2004 11:14:14 -0700 (PDT)

Hello,
 
I have a question regarding section 11 second last paragraph ( page 21 ).  It states
 
" In this model PE-rs may need to run STP protocol ... ".
 
My question is if the bridges in the ether access network are running
STP, is there a need for the PE-rs to run STP?  My reasoning is that
the PE-rs is just emulating a LAN so it need not run STP.
 
Would really appreciate any clarification/comments.
 
thanks,
 
narasim

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From: Ali Sajassi <[email-masked]>
Date: Mon, 12 Apr 2004 14:43:28 -0700

>the PE-rs is just emulating a LAN so it need not run STP.

If there exists several alternative paths through several different PE-rs 
for a given VPLS instance, then the PE-rs's need to participate in access 
network STP in order to decide which path (link or set of PWs) to block and 
which path to unblock for the VPLS traffic to get through over the core 
network. Otherwise, if they don't participate, a loop can exist among the 
PE-rs's. However, if the access topology is simplified as depicted in 
10.2.1, then it can be done without STP as described in that section.

-Ali

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From: narasimhan g <[email-masked]>
Date: Tue, 13 Apr 2004 13:31:20 -0700 (PDT)

Hi Ali,

Thanks for your response.
I have reproduced the figure in 10.2.1  below for easy reference.
I have added a link between MTU-s and PE2-rs so that it has three links.
I have the following points.

1.  Any loops among the PE-rs will be broken because of split horizon.
2.  Any loop at the MTU-s will be broken by the MTU-s running STP on
    the three PWs/links ( The three PE-rs's would just flood any BPDUs
    received from the MTU-s ).

Isn't this sufficient to break all loops?

So the PE-rs's need to run STP only if the MTU-s is not running STP.  Or
am I missing something obvious?

                                                          PE2-rs 
                                                          ------ 
                           Third PW/link                 /      \ 
                         /------------------------------|   --   | 
                        /                               |  /  \  | 
    CE-1               /                                |  \B /  | 
      \               /                                  \  --  / 
       \             /                                   /------ 
        \  MTU-s    /                     PE1-rs        /   | 
         \------   /                      ------       /    |    
         /      \ /                      /      \     /     |      
        |   --   |   Primary PW         |   --   |---/      | 
        |  /  \--|- - - - - - - - - - - |--/  \  |          |    
        |  \B /  |                      |  \B /  |          |   
         \  -- \/                        \  --  / ---\      |    
          ------\                         ------      \     |     
          /      \                                     \    | 
         /        \                                     \ ------  
        /          \                                     /      \      
       CE-2         \                                   |  --    | 
                     \     Secondary PW                 | /  \   | 
                      - - - - - - - - - - - - - - - - - |-\B /   | 
                                                         \ --   / 
                                                          ------   
                                                          PE3-rs 

thanks,

narasim

------------------------------------------------------
Ali Sajassi <[email-masked]> wrote:At 11:14 AM 4/12/2004 -0700, narasimhan g wrote:
Hello,

 
My question is if the bridges in the ether access network are running
STP, is there a need for the PE-rs to run STP?  My reasoning is that
the PE-rs is just emulating a LAN so it need not run STP.
If there exists several alternative paths through several different PE-rs for a given VPLS
instance, then the PE-rs's need to participate in access network STP in order to decide which path
(link or set of PWs) to block and which path to unblock for the VPLS traffic to get through over
the core network. Otherwise, if they don't participate, a loop can exist among the PE-rs's.
However, if the access topology is simplified as depicted in 10.2.1, then it can be done without
STP as described in that section.

-Ali

thanks,
 
narasim

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From: Ali Sajassi <[email-masked]>
Date: Tue, 13 Apr 2004 15:54:30 -0700

Hi Narasim,

Actually when I was talking about Ethernet access network, I had a more 
general topology with intermediate aggregate switches/bridges in mind than 
just a simple hub-and-spoke between u-PEs (MTU-s) and n-PEs (PE-rs). 
However, your picture is sufficient to discuss the point below. In other 
words, if we have intermediate bridges, then they would take the place of 
MTU-S in the figure below and the actual MTU-s's sit behind the 
intermediate switches. Now, if you are asking whether we can skip running 
STP on n-PEs in a general access network topology (but run the STP on all 
other bridges and u-PEs), then the answer is yes but at a price (and the 
price may or may not be important depending on SP criteria).

But before talking about its cost, let me just mention that there needs to 
be a full mesh of PWs among all the n-PEs belonging to that access network 
for relaying service provider BPDUs (e.g., a full mesh of PWs dedicated for 
SP BPDUs only). Furthermore, when the n-PEs receives SP BPDUs frames,  it 
needs to relay them over this set of full-meshed PWs.

Now, lets talk about the cost of not supporting STP on n-PEs. One of the 
constraint being imposed is that there cannot be any Ethernet links between 
the n-PEs belonging to a given access network. If there is such a link, 
then loop can occur among your n-PEs. In other words, if the customer 
traffic needs to be switched locally between two n-PEs of the same access 
network, then it has to go through the MPLS/IP network using the PWs (and 
thus over-loading your PWs for locally switched traffic). If n-PEs run STP, 
then this constraint can be removed. Also running STP on n-PE can result in 
a faster switch-over time in case of PW failure but this is secondary and 
it can be taken care of.

-Ali

At 01:31 PM 4/13/2004 -0700, narasimhan g wrote:
>for a given VPLS

---

From: narasimhan g <[email-masked]>
Date: Thu, 15 Apr 2004 16:04:31 -0700 (PDT)

Hi Ali,

> a faster switch-over time in case of PW failure but this is secondary and 

With respect to your response, I would like to know how split horizon and STP interoperate.

I have in the diagram below two PEs, PE1 and PE2, in the same ether access network.  They also
have a direct ethernet (Eth2) link between them.  Also PE1, PE2 and PE3 have the full-mesh
of pseudowires between them.  STP is running on ports Eth1, Eth2 and Eth3 and in the access
network.  But STP is not aware of the pseudowire PW1.

                            -------------------
                           /                   \   (Eth1)
                          /                      ----------PE1
                         /                     /           ||  \
                        .                      .           ||   \
                        .                      .           ||    \ ( PW2 )
                        .                      .           ||     \
                        .                      .           ||      \
                        .  Arbitrary ether     .     (Eth2)||(PW1)  PE3
                        .  access network      .           ||        /
                        .  running STP         .           ||       /
                        .                      .           ||      / ( PW3 )
                        .                      .           ||     /
                        .                      .           ||    /
                        .                       \  (Eth3)  ||  /
                        .                        ----------PE2
                        \                       /
                         \                     /
                          ---------------------

Traffic flowing from PE3 to PE1 over PW2 will be sent on Eth1 into the access network.
The access network could potentially send this traffic back on Eth3 link towards PE2.
( For example if Eth2 is blocked by STP ). Therefore PE2 would receive the same traffic
from PE3 ( over Pw3 ) and also from the access network via Eth3.  How do we avoid this?

thanks,

narasim

---

From: Ali Sajassi <[email-masked]>
Date: Thu, 15 Apr 2004 16:30:52 -0700

Hi narasim,

>network.  But STP is not aware of the pseudowire PW1.

When you run STP on PE1 and PE2, it needs to cover all the port (both 
logical and physical) or else loop can occur. So in your example, lets 
assume that we have 10 PEs (PE1 through PE10) and PE1, and PE2 belong to 
the same access network (e.g., same Ethernet island) as depicted above. In 
this case, a separate PW need to exist to carry provider's BPDUs for that 
island (and it is only for provider BPDUs and nothing more). In this 
scenarios all the core-facing PWs of the PE1 can be model as one logical 
port and all the PWs of the PE2 as another logical port. Furthermore, the 
SP BPDU PW would be analogous to BPDU stream of a Ethernet port and each 
customer VPLS (each set of PWs) would be analogous to customer VLAN over 
that logical Ethernet port. Then the STP can protect the PWs over its 
logical port. It should be noted that the STP is only run among the PEs of 
the same island (intra island) and not between islands - this is the key to 
scalability.

If what I am saying is not very clear (which can very well be the case), 
then i would refer you to the materials presented in 802.1 group on this 
subject over last couple of years.

-Ali