RSTP MST region interconnected via non RSTP participating switches, possible?

sullivan.p · ‎09-13-2012

Hi All,

Apologies if this is a bit of a dumb question, but I'm having trouble finding an answer after much searching.

If I have an MST region, that is inter-connected by other switches not participating in the MST region, will the MST BPDU's still make it from one side to the other allowing me to form my MST region?

The MST region is inter-connected by switches providing 802.1Q tunneling (point-to-point QinQ via a port-channel so I'm not worried about loops here) in order to connect up designated VLANs on either side. Only having just started touching on 802.1w as part of CCNP study, I'm not clear how the RSTP BPDU's are propagated using 802.1w, whether they are multicasted and therefore capable of traversing the link or whether each switch in the region must be directly connected to each other?

Cheers,

Patrick.

krahmani323 · ‎09-13-2012

Hello Patrick,

In this specific case the MST BPDU can be tunneled through the q-in-q switch.

The 3560/3750 configuration guide explains how the PDU tunneling is being performed to forward the multicast at the other side =>

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/12.2_55_se/configuration/guide/swtunnel.html#wp1006657

The switch supports tunneling of CDP, STP, including multiple STP (MSTP), and VTP. Protocol tunneling is disabled by default but can be enabled for the individual protocols on IEEE 802.1Q tunnel ports or access ports.

To verify show l2protocol ....

If not configured the command under the q-in-q tunneling interfaces is l2protocol-tunnel [cdp | stp | vtp] : Enable protocol tunneling for the desired protocol. If no keyword is entered, tunneling is enabled for all three Layer 2 protocols.

Best regards.

Karim

Peter Paluch · ‎09-13-2012

Hello Patrick,

You have posted this question twice, the second thread is here:

https://supportforums.cisco.com/message/3733409?tstart=0#3733409

I will try to answer here.

If I have an MST region, that is inter-connected by other switches not  participating in the MST region, will the MST BPDU's still make it from  one side to the other allowing me to form my MST region?

Assuming that there is no explicit tunnel between the two MST regions that traverses the RSTP region, then the answer is no - the MST BPDUs will not pass through the RSTP region. If MSTP detects that a port is connected to a RSTP switch, it will revert to RSTP operation, using MSTI 0 (IST) as the reference for interoperation with non-MSTP world. This way, a proper MSTP implementation will never speak with MSTP BPDUs to a RSTP/STP switch. Even if it did, though, the RSTP/STP switches would not flood the MSTP BPDUs. In the best case, RSTP/STP switches would try to process them as common RSTP/STP BPDUs (which is possible because the beginning fields of MSTP BPDUs are compatible with RSTP/STP BPDUs and are derived from MSTI 0), in a worse case, they would ignore them altogether.

The MST region is inter-connected by switches providing 802.1Q tunneling  (point-to-point QinQ via a port-channel so I'm not worried about loops  here) in order to connect up designated VLANs on either side.

Tunneling makes things more complicated. Cisco's implementation of Q-in-Q tunneling does not tunnel BPDUs by default. The Catalyst 3560 Configuration Guide at

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/15.0_1_se/configuration/guide/swtunnel.html#wp1006070

specifically states: "When a port is configured as an IEEE 802.1Q tunnel port, spanning-tree bridge protocol data unit (BPDU) filtering is automatically enabled on the interface." This means that, by default, any BPDUs received from the customer are dropped and not tunneled across. If you specifically require that the BPDUs received on the 802.1Q tunnel port are tunneled to the other site, you must specifically configure the so-called Layer 2 Protocol Tunneling for STP. For more information, see:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/15.0_1_se/configuration/guide/swtunnel.html#wp1005050

I'm not clear how the RSTP BPDU's are propagated using 802.1w, whether  they are multicasted and therefore capable of traversing the link or  whether each switch in the region must be directly connected to each  other?

Let's consider the non-tunneled scenario here. MSTP BPDUs are sent to the same multicast MAC address as STP/RSTP BPDUs, therefore they are not flooded - just like STP/RSTP BPDUs are not flooded. Their propagation will be terminated at the nearest switch that speaks some version of STP. Once more, the MSTP BPDUs will not traverse the RSTP region but it does not mean that the two MSTP regions will be totally oblivious of each other. The MSTI 0 is responsible for interacting with outside world. Both regions would speak with MSTI 0 to the RSTP region, and through the RSTP region, to each other. This is the basic rule of how MSTP provides interoperability with other regions or older STP versions.

MSTP interaction with other regions or non-MSTP regions is not difficult but somewhat specific, so this needs a great deal of thinking. Please feel welcome to ask further about anything you would like to have clarified.

Best regards,

Peter

sullivan.p · ‎09-13-2012

Great, thanks for the detailed answers that clears things up a lot.

And, thanks for the heads up Peter - I've deleted the other thread, not sure how that happened.

So, basically if I want to be able to setup the topology as a single MSTI region through the QinQ tunnel that our provider supplies us, I need to ask them to disable BPDU Filtering by enabling Layer 2 STP protocol tunneling on their devices?

Again, much appreciated.

Peter Paluch · ‎09-13-2012

Hello Patrick,

So, basically if I want to be able to setup the topology as a single  MSTI region through the QinQ tunnel that our provider supplies us, I  need to ask them to disable BPDU Filtering by enabling Layer 2 STP  protocol tunneling on their devices?

Yes. Specifically, ask them to make sure your MST BPDUs are transparently tunneled across your provider's network.

Also make sure that all switches on both your locations use identical MSTP region configuration (region name, revision, VLAN-to-instance mapping). Otherwise, your network would again be split into two or more MSTP regions.

Best regards,

Peter

Mohamed Sobair · ‎09-13-2012

Hello Patrick,

Technically yes, its possible to form an MST region between non-directly connected Switches using 802.1q tunneling.

The RSTP BPDUs are multicasted and the Switches can be indirectly connected in this case.

The Dot1q Tunneling technique uses two tags, One Tag (Outer TAG) is the Provider tag who is tunneling your Vlans from one side to another, the second tag is the (Inner Tag) is the client tag which carried over the tunnel.

You Can just consider the Provider here as a Cable Provider, it doesnt have visibility to your Inner Vlans/Tags and BPDUs for those Vlans doesnt even get participated with the Provider. Once the Vlans gets Tunneled, the RSTP BPDUs should be able to form an MST region with the other side. The Provider doesnt even need to particpate with you in STP.

HTH

Mohamed