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ASR9000/XR Using MST-AG (MST Access Gateway), MST and VPLS



In this document the concept of MST (multiple spanning tree) access gateway and the difference to regular MST will be highlighted. Also it shows some design scenarios of when to chose for which particular implementation.

Spanning Tree

It is generally well understood that in a layer 2 network loops are disastrous. Spanning-Tree Protocol (STP) aids in the detection of loops and breaking that loop to prevent broadcast and unknown unicast packets for circuling around for forever and bringing down the network eventually.

STP operates by selecting a root bridge in the network, who will have all its ports in forwarding and the other switches determining their path cost towards the root switch.

The way the root switch is elected is by means of a priority and when all are equal the lowest switch mac address is selected to be the root. While in default settings a loop is prevented, for good network design it is important that the priorities are set correctly to define where the root switch (and its potential backup) are going to be located.


Root switch election

  1. The Root Bridge is usually determined by an administratively assigned Priority number
  2. If all switches have the same Priority, the switch with the lowest MAC address becomes the Root Bridge
  3. All switch ports begin in the Blocking state to prevent loops
  4. The Root Bridge, once elected, is the only bridge with all ports active (Forwarding state)
  5. Root Ports on other switches are placed in Forwarding and provide the lowest cost path back to the Root Bridge
  6. A port stays in a Blocking state if STP determines that there is another path to the Root Bridge with a lower (better) cost

The states that a port can be in are determined as follows:

  1. Blocking – Starts in this mode, and stays in Blocking if STA determines that there is a better path to the root bridge. Port only listens for BPDUs from other bridges (Max age 20 seconds)
  2. Listening – Enters this mode after the Root Bridge election, when BPDU updates are being used to find the lowest cost path to the Root. Attempts to learn other paths to root bridge, to ensure that a loop won’t be created if it begins Forwarding (15 second transition)
  3. Learning – Enters this mode after Listening. Port adds learned addresses to its table, still not allowed to send data (15 second transition)
  4. Forwarding – Enters this mode after Listening. Port is now able to send and receive data – Normal operation

Just as a summary overview the follow topology:


In this example we consider this picture after all BPDU's have been exchanged. Based on priority and mac address S1 is elected as root bridge.

S1 will move both his ports from blocking to forwarding.

Switch S2 will move his root facing port in forwarding mode, which will become the RP (root port).The Port towards S3 will also be in forwarding

because S3, although having a direct link to the root switch S1, the path cost is higher (100) then going via S2->S1 (38). For this reason

the port on S3 to S1 will be blocking.

S4's path to the root bridge is either via S2 or S3. The path to S2 is a higher cost then via S3, ehcne S4->S2 is blocking and the path

to S3 is chosen and that port on S3 will become the RP.

This picture assumes that there are no vlans and just plain ethernet, also this diagram shows a local area network.

What if we want to use multiple vlans or interconnecting this network via a carrier ethernet network to a remote LAN?

Multiple VLANs

In the scenario where you have multiple vlans, regular STP will block the link for all vlans. While this prevents the loop, it is maybe not that

efficient as one node/path is completely in standby mode. It might be nice to forward a few vlans over to switch 1 and the others to switch 2.

Effectively that means that Sw1 is then root for a vlan set and Sw2 for another vlan set.

Regular STP cannot do this, and the logical evolution of htat is MSTP (multiple spanning tree) which is hte more standardized version, and PVST(+) which is a cisco proprietary solution.

They effectively achieve the same PVST and MSTP, one of the key differences is that MSTP sends the bpdu's out untagged on the port, where

PVST sends the bpdu's inline with the vlan, hence are vlan tagged.

Connecting Layer domains via a carrier ethernet network

The most common way to connect 2 separate l2 segments or networks together is via VPLS.

With VPLS the edge nodes from the provider are aggregating the customers L2 traffic, and participate in the L2 spanning tree loop prevention as well as Pseudo Wires over an MPLS core to remote PE's (Provider Edge) to bring the traffic from one segment to the remote site.

The way VPLS works and the interaction with MST or MSTAG and what the differences are will be discussed below.


MST allows us then to run an STP instance per set of vlans that we can configure.

A sample configuration on the ASR9000 PE node looks as follows. Each set of vlans is defined under an instance.

We can adjust the priority per instance if needed, multiple vlans per instance are allowed also.

Sample MST configuration on the ASR9000

spanning-tree mst MYSTP_DOMAIN
name testme

! The name of the MST region is very important, it must be the same for all switches in this region.

! also the definitions of your MST instances need to be the same on all nodes.
revision 1
instance 0
  priority 4096
instance 1
  vlan-ids 100
  priority 4096
instance 2
  vlan-ids 101
  priority 4096
interface TenGigE0/3/0/6
! Interfaces that are enabled for MST. Note that these are the main interfaces
interface TenGigE0/3/0/7
interface Bundle-Ether100
interface GigabitEthernet0/0/0/27

To provide a more graphical example of how MST can be used with 9k's is shown here:


In this case we have a clear STP loop between the 2 9K PE devices interconnceted via a bundle ethernet.

Associated configuration for this example would be as follows:

STP portion

spanning-tree mst Example

name testme

revision 1
instance 0
  priority 4096
instance 1
  vlan-ids 100
  priority 4096
interface Bundle-Ether100
interface GigabitEthernet0/0/0/27

With this config we are providing the ability to the 9k PE to send bpdu's out to the other 9k and the access switch.

Also with this config, one link will be blocked and if we elect either 9k to be the root switch (say the one on the left), the link marked RED will be blocked.

This config however doesn't provide for any data configuration forwarding. For this we need to establish a separate bridge-domain whereby we pull in the right EFP's for forwarding the data traffic:

Data Plane portion

interface bundle-e100.100 l2transport

encapsulation dot1q 100

rewrite ingress tag pop 1 symmetric

!rewrite is optional depending on whether all EFP's are in the same vlan.

interface gigabitethernet 0/0/0/27.100 l2transport

encapsulation dot1q 100

rewrite ingress tag pop 1 symmetric

!rewrite is optional depending on whether all EFP's are in the same vlan.


bridge group EXAMPLE

bridge-domain FWD_1

interface g0/0/0/27.100

interface bundle-e100.100

You need to repeat this configuration for every vlan you want to forward. There is another article detailing more about vlan rewrites and the EFP concept in case you're interested. See the related documentation section for a reference.


VPLS is the concept of connecting multiple layer 2 domains over an MPLS network for instance. On the ASR9000, a bridge-group is used to pull in the attachment circuits (physical interfaces towards the lan segment) and Pseudo Wires (PW) to the remote PE's.

A sample configuration achieve vpls looks as follows, this provides the configuration for the data plane and assumes there are no loops in your L2 topology either at the customer access site or within your VPLS domain.

To prevent loops in the access network we need to leverage either MSTP, MSTAG or PVSTAG. We'll discuss MSTAG in the next section below.

bridge group VPLS
  bridge-domain vpls_1

! the bridge-group vs domain is just a configuration hierarchy, it doesn't serve any special functionality.
   interface GigabitEthernet0/0/0/0.100
   ! Phyiscal interfaces towards a subscriber switch
   neighbor pw-id 123
   ! for H-VPLS we can use PW's also as an attachment circuit
   vfi vpls_1_vfi_1
    neighbor pw-id 333
    ! definition of a pseudo wire underneath a Virtual Forwarding Instance
    neighbor pw-id 444

The IP address providing in the "neighbor" statement are the MPLS router ID's from the remote PE's. the PW-ID is an arbitrary number, unique, that defines the VC label.

Whether you put the PW's in the VFI or outside the VFI or across VFI's depends on your needs and whether you need SPLIT HORIZON (see below).

VPLS and L2 Loops

The following picture explains what might happen when we don't use any STP in a VPLS scenario.

In this case there is a loop, but the access switch doesn't know about it because both the 9k PE's, nor the switches

form a closed ring.


Even if we'd be running MSTP in this scenario there is no loop detected, since by default BPDU's are not forwarded over the pseudo wires.

A potential solution might be to run an L2 link between the 2 southern PE's so that there is a loop on the SOUTH segment and indeed one UP link will be blocked from the access switch to one of the 2 PE's as per regular (M)STP.

The problem is here however that a (broadcast/unknown unicast) packet arriving on the Left South PE's pseudo wire is now then sent to the access switch south AND over the interchassis link (not drawn in this picture) to the SOUTH PE on the right. There will be a loop again.

A proper solution for this model is the use of MST Access gateway which will be highlighted below.

Split Horizon

Normally in a bridge domain, broadcast and unknown unicast from Attachment circuits are replicated to all bridge ports.

Obviously packets are never sent to the AC or PW that the traffic was actually received on.

By default AC's can always forward packets to each other and to (all) Pseudo Wires.


So traffic from the AC "west" will be replicated over all PW's and the South-West AC.

When traffc arrives on a PW then by default packets are never sent out the PW that they are received on and to other Pseudo Wires in the same

VFI. All PW's in the same VFI share the same split horizon group. And traffic is not replicated within the same split horizon group.

When packets arrive on the Pseudo Wire they are NOT forwarded out of PW's in the same VFI.


You can also move Attachment Circuits into a split horizon group to prevent them from speaking with each other by means of the "split horizon" group command underneath the interface which is configured in the l2vpn bridge-domain.


bridge group TEST

bridge-domain SAMPLE

interface g0/1/0/0


interface g0/1/0/10

split horizon

interface te0/2/0/3


neighbor pw-id 100

In this case traffic cannot flow between 0/1/0/0 and 0/1/0/10.

MST and Split Horizon

For illustrational purposes consider this sample VPLS design.

In order for MST to work, we need an inter PE attachment circuit to exchange the BPDU's between the two PE nodes drawn in blue.

In this given example the PE_left (PE-1) is considered ROOT.


Imagine there is a broadcast coming in on PW-1. Because of split horizon the traffic is not replicated to the PW 2, 3, 4 and 5. But traffic will go down the AC-2 and also sent over to AC-1.

When traffic enters the PE_right (PE-2), it will not go down AC-3 because it is blocking since PE-1 is root, but it will enter the VFI and gets replicated to the PW's in the VFI there so PW 3, 4 and 5. This poses a big problem considering with a replication loop back to PE-3 and also to PE-1.

Omitting PW-5 solves part of the issue so that traffic is not replicated back to PE-1, but it might slow down convergence in case AC-2 is going offline and PE3/4 have not yet updated their mac tables yet.Traffic will still get back to PE-3 and PE-4.

The AC-1 is required for BPDU, and you might want to consider only creating an EFP for hte untagged traffic (BPDU's), but then you might have also forwarding issues in case the PW 1 and 2 are down and you want to send traffic over the AC 1 to PE-right so it can forward the traffic for us to the PE3 and 4.

Wouldn't it be nice if we can live without the AC-1 all together, still run spanning-tree to the CE and have optimum convergence?

Yes. Enter MSTAG.


In MSTAG we define the BPDU's on the PE nodes statically presenting them as 1 virtual bridge to the CE.

One link will be blocked, but there is no need for an inter chassis link anymore in this case.

What’s the main function of the MST access gateway?

  1. Send pre-canned BPDU into access network at hello timer
  2. Snoop the TCN from access network, flush its local MAC address table and trigger VPLS MAC withdrawal accordingly

Major Advantage – scale and local significant
Light MST implementation, for example, it doesn’t keep STP state machine, it doesn’t need to handle received BPDU (except TCN)

The MST is per port scope

Other Advantages
Doesn’t require inter-PE special PW, no single point of failure, no temp L2 loop.

Much robust than the “MST over special PW” solution
Standard based solution, inter-operable with 3rd vendors, work with any network topology
Self protection, even with user mis-configuration, it won’t cause L2 loops

MST convergence depends on the number of VLANs in the access ring and the MST implementation of the access switches. In any case, don't expect 50msec convergence time
With Cisco 3400 as access switch, the baseline convergence show sub second for link failure, sub 100msec for link recovery, 2-3 seconds for node failure




Note that in this configuration we use the interface with suffix .1 in the MSTAG configuration.

This means we need to define an EFP (Ethernet Flow Point) to capture the BPDU's and TCN packets. In fact, we're not even using the bpdu's received, as we perceive ourselves to be root on the 9k and send these precanned BPDU's out.

We will consume the TCN (topology change notification) and send these into the VPLS network as mac withdrawl messages.

interface gigabitEthernet 0/0/0.10.1 l2transport

encapsulation untagged

Aside from the MST configuration we still need to configure our bridge domains with the EFP's for the data forwarding and our Pseudo Wires to our remote PE's as described above.


MSTP/MSTAG scale for the ASR9000

1)      MSTP: There is a single protocol instance which can have the

standard 64 MST Instances (MSTIs) within it. These 64 MSTIs create 64

logical spanning tree topologies  within one MSTP region/ domain.

2)      MSTAG: You can create a separate protocol instance per physical

interface and each each protocol instance can be in a separate MSTP

region by itself and each one can in turn support 64 MST Instances

(MSTIs) within it.

In general MSTAG is more scalable (multiple regions with 64 MSTIs each)

but can only be used if the ASR9K is in the root (or backup root)

position for every MSTI. MSTP is the normal Cat 6K like version but you

can use all 64 MSTIs without any issues. Both of these can interoperate

with any IEEE standard MSTP implementation so should work with the N7K


Related Information

For more details on regular MST and some IOS interoperability considerations, check this reference:

ASR9000 MST interop with IOS/7600: VLAN pruning

Learn more about vlan rewrites and the concept of EFP's

ASR9000/XR Flexible VLAN matching, EVC, VLAN-Tag rewriting, IRB/BVI and defining L2 services

Xander Thuijs, CCIE #6775

Sr Tech Lead ASR9000

Cisco Employee

Thanks Max!

if you have one  9k or edge switch, then you you'd want to make a link bundle aggregating the 2 circuits from the access switch to the 9k/edge: no need for spanning tree as there is no loop and more agg bandwidth!



Hi Xander! Thanks for your response.

Maybe, I was not clear enough. Please, look at this topology and you will understand my question. I think, I still need spanning-tree because the access network is a L2 ring with access switches.

ScreenHunter_02 Jul. 04 16.39.jpg

Thanks again



Cisco Employee

Aha thanks Max for that clarification, this design won't benefit from MTSAG per-se here.

Either you will want to run full MST or G8032 is a perfect candidate for this ring design also!



Hi Xander!

Yes, I know that g.8032 would be perfect, but the access network does not support it, yet. Think about the topology above, with more than 100 access rings with more than 8 switches each, connected to the same ASR9K. That´s why I want to use MSTAG; to minimize the interaction between access rings, running independent MSTAG "instances" in each ring.

Regardless desing issues or best practices, I want to know if MSTAG is supported with only one Access Gateway.



Cisco Employee

I believed this to be working as a TCN/topo change would just flush the mac table of the BD. I confirmed this with our l2vpn developers if this is meant to be supported and it is.

so you're good to go with using mstag on 1 edge device with 2 legs onto a ring.


Thanks for this confirmation. We are going to do some tests very soon. One more thing... we know when a BPDU with TC bit on is received, the ASR9K flush the mac table of the BD and triggers an EoMPLS MAC withdraw. I´d like to know if a TC received on one leg of a ring, also triggers a TC out of the other leg of the ring.




Hi Alexander

I wanted to ask , if do you know how many MSTAG processes  supports ASR9K?


Cisco Employee

Hi Diego,

1)      MSTP: There is a single protocol instance which can have the
standard 64 MST Instances (MSTIs) within it. These 64 MSTIs create 64
logical spanning tree topologies  within one MSTP region/ domain.

2)      MSTAG: You can create a separate protocol instance per physical
interface and each each protocol instance can be in a separate MSTP region by
itself and each one can in turn support 64 MST Instances (MSTIs) within it.

In general MSTAG is more scalable (multiple regions with 64 MSTIs each)
but can only be used if the ASR9K is in the root (or backup root)
position for every MSTI. MSTP is the normal Cat 6K like version but you
can use all 64 MSTIs without any issues. Both of these can interoperate
with any IEEE standard MSTP implementation



Alexander Thanks for the info.

the query is oriented to many regions logical of MSTAG supports ASR9K?

The idea is to use the ASR9K as aggregator. As we access ring closing  against this router and would like to know how many I can set MSTAG regions in one ASR9k?


Interface gi0/1/0/1 and gi0/1/0/3 RING_1

Interface gi01/0/0 and gi0/1/0/2 RING_2

And each ring a region MSTAG. (The idea is to have 30 rings with 10 switches)



Hi to all,

i have the following topology (see pic) and i was wondering how my rep can unblock the port when L2 link beetwen ASR goes down.

Any idea?


Cisco Employee

In AG mode you don't need an L2 link between the two access gateways. Remember that AG assumes that both nodes are the same to the ring or access. Also there is no true STP or REP running between the two AG nodes, so this L2 link will constitute a loop that cant be resolved.

You are better of, IF NEEDED, to make a PW between the two AG nodes and even that is not required.



The l2 link is a Multi-point local-bridging to extend vlan from one node to all other (with nome i mean a 3800) so i need it.

My problem is when l2 link is down REP do not open its bòocked port

Cisco Employee

That wouldnt be correct Alessandro: the AG's don't exchange state information with each other and the only thing they do is REACT on TCN. Do do not originate a TCN. Upon reception of a tcn it will constitute a mac flush.

If the ring is broken, some devices take the left AG, and some take the right AG. Mac learning allows them to be bridged together if you have a PW between the AG nodes.



hi Alexander, great document!! I've only one question. If I'm using 2 ASR9K, and each one have an interface looking to the L2 Ring. I will use a EFP untagged to capture the BPDU's, but I need to establish a PW and bridge it to the untagged EFP's to make the MAC withdrawal work? Or if I don't have that PW, the ASR will send the MAC withdrawal over all PW that are bridged withs EFP's of the same physical interface?

Thanks in advance!

Cisco Employee

Thank you Pablo!

Oh well here is the deal, if the 9k is running MST or MSTAG and you define an untagged EFP, we would still consume the BPDU's and they are not transparently forwarded anymore eventhough you have an untagged EFP.

If you do NOT run MST/MSTAG and you define an untagged EFP, they are subject to transport as per VPWS configuration defined (don't multipoint these untagged efp's otherwise you'd confuse the hell out of STP).

The MAC withdrawel is an LDP message that is sent between the AGG nodes and is not dependant on the untagged EFP.

In MST/MSTAG mode, a TCN bpdu is captured by the 9k/AGG and converted into a MWD ldp message.

The TCN is not sent across in that case.


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