RSTP versus MSTP for a MLS Network?

[Rodrigo Curado]

Hi,

I've been reading and testing several setups and getting my mind clear on

a couple of issues regarding the adoption of RSTP versus MSTP for

a Multi-layered Switched Network, so I hope that someone can help me out with

some of my points/questions.

I've attached a diagram of the setup I'm using at the lab.

Let me explain what I've deployed there:

* I'm using Cat6509 for switches A through F, and let's name them Core

* I've 802.1Q trunks between those switches according to the diagram

* I'm running OSPF between switches A through F and redistributing some

connected and static prefixes, like the SVI 3 /24

* The R(emote) equipment symbolizes routers or switches connected to the Core

switches. The idea is to have trunk interfaces, access interfaces and routed

interfaces

At the end of the day this is what I'm looking for:

a) having a Layer 3 infrastructure between the Core to support as many

services as possible at that layer (VPN/MPLS and non-VPN/MPLS based) [Vlan

3,300..3XX]

b) having a Layer 2 infrastructure for special services waiting to be ported to

a Layer 3 setup [Vlan 2,200,201]

c) having the Layer 2 Vlans aggregated on each Core switch

"terminated" on a Router with a trunk (PPPoE services) [Vlan 100,101]

With this kind of setup should you recommend, RSTP or MSTP?

I'm having this problems:

1) RSTP limits on the number of STPs I can run, I haven't found yet

a documented limit. For the Cat3750 it's 128, what about Cat6509?

2) I don't need to have STP on Vlan 300..3XX, but I rather have STP enabled on

all the others, don't know for sure about the PPPoE Vlans cause I'm using

mac-learning disable

3) From what I've read I need to map Vlans 300..3XX to a different mst instance

each, is that correct? I've tested different scenarios in lab and without that

singular mapping I had some issues like having some SVI's down due to port

blocking

4) Due to problem 3) I will hit the 16/64 mst instance limit in the near

future, so MSTP also looks like having some scalability issues with my setup.

Many thanks,

Rodrigo

[aghaznavi]

The benefits of PVST:

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop.

The benefits of PVST+:

Per VLAN Spanning Tree Plus (PVST+) provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification and is not supported on non-Cisco devices.

The benefits of MST:

- Combines the best of PVST(+) and 802.1Q.

- Load balancing can be achieved.

- The CPU is spared by only computing specific instances.

- Flexible mapping of VLANs into instances

Also, MST will provide all the functions provided by RSTP as 802.1s. MST is an amendment to 802.1Q. It extends the IEEE 802.1w rapid spanning tree (RST) algorithm to multiple spanning trees. Additional functions includes rapid convergence and load balancing in a VLAN environment.

More information on MST can be found below:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

The following website gives a synopsis of how RSTP and MSTP work:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008007e71a.html#wp1082480