More STP topics

Ferret999

Okay I have gone and confused myself here with the different types of spanning-tree.

Firstly there is CST or 802.1Q spanning-tree which runs over the Native VLAN, is this right? Can it run with ISL as it's trunking protocol? Also does it only support STP or does it also support RSTP?

Next there is PVST which requires ISL as a trunking protocol so it will not work with 802.1Q. I take it this runs normal STP and cannot run RSTP. Am I right on this?

Next there is PVST+ which also works with ISL. However can PVST+ work with 802.1Q as it's trunking protocol? Also I take it this also runs STP but can it run RSTP?

Next as I understand it there is MST which runs with RSTP but not STP. Is this right? Also which of the two trunking protocols does it support?

Thanks again

dtlokee

Ferret999 wrote:

Okay I have gone and confused myself here with the different types of spanning-tree.
Firstly, by the 802.1Q standard it only supports a single instance of spanning tree for all VLANs, but for Cisco this wasn't good enough so they adapted PVST to PVST+ (notice the + ) which allowed a STP domain to run an instance of spanning tree up to the maximum supported for the switch hardware (on the 2950 it's 64)

Then we moved out of the Bronze Age and into the middle ages of STP, per-VLAN rapid spanning tree which is based on the 802.1w standard. This allowed the spanning tree network to more quickly transition all the interfaces into forwarding or discarding state. It eliminated the standard listening and learning delay. This method still had a drawback in that it runs a separate instance of spanning tree for each VLAN, thereby limiting the number of VLANS you can create.

Enter MST. MST implies that RSTP is in use, you don't configure it, it is done automatically when you se the mode to MST. The benefit to MST is you can group together multiple VLANs into one instance thereby reducing the number of instances and allowing you to create more VLANs. The typical reason behind having multiple instances is to elect separate root bridges to better utilize your redundant links. Most stp regions only have 2 or 3 switches that could act as the root, so you really don't need more than 2-3 instances of spanning tree.

Take a typical switch block with 6 access layer switches connected to 2 distribution layer switches which are connected to the core. It only makes sense to have the distribution layer switches act as the root, and since there are only 2 you really only need 2 instances of spanning tree.

I've rambled long enough

EdTheLad

Ferret999 wrote:

Firstly there is CST or 802.1Q spanning-tree which runs over the Native VLAN, is this right? Can it run with ISL as it's trunking protocol? Also does it only support STP or does it also support RSTP?

CST doesn't run on the native vlan, it is not aware of any vlan,as far as Cisco is concerned CST runs on Vlan 1.BDPUs generated on 802.1q trunks are destined for mac address 0100.0ccc.ccd(ieee address) only for vlan 1, bpdus for other vlans have a destination mac address 0100.0ccc.cccd which is a cisco proprietary address.Vlan 1 and the native vlan are two different concepts, vlan 1 is used to communicate with a nonCisco device i.e. a device running CST.

Ferret999 wrote:

Next there is PVST which requires ISL as a trunking protocol so it will not work with 802.1Q. I take it this runs normal STP and cannot run RSTP. Am I right on this?

PVST means multiple instances of spanning-tree are supported i.e. a spanning-tree domain per vlan, so a port can be blocking,forwarding etc per vlan.PVST is supported on ISL trunks only, RSTP+ runs on dot1q also.Cisco have two versions of pvst and two versions of rstp which are ISL(pvst,rstp) dot1q(pvst+,rstp+)

Ferret999 wrote:

Next there is PVST+ which also works with ISL. However can PVST+ work with 802.1Q as it's trunking protocol? Also I take it this also runs STP but can it run RSTP?

PVST+ works with qot1q not ISL.

Ferret999 wrote:

Next as I understand it there is MST which runs with RSTP but not STP. Is this right? Also which of the two trunking protocols does it support?

MST runs internally with rapid spanning tree but the IST at the boundary will run with CST.It can work over dot1q trunks and ISL trunks.Internally it will use rapid spanning-tree.

I think the main think you need to do is differeniate between the encapsulation and the STP.

EdTheLad

dtlokee wrote:

The typical reason behind having multiple instances is to elect separate root bridges to better utilize your redundant links. Most stp regions only have 2 or 3 switches that could act as the root, so you really don't need more than 2-3 instances of spanning tree.

The main reason for MST is to reduce the number of BPDUs getting flooded throughout the network.BPDU processing is very cpu intensive and can easily melt a network.You need to be very careful at the mst boundary that you prune vlans off trunks.

Netstudent

So in this cisco BPDU, the destination address is the IEEE 0100.0ccc.cccd, then I guess the source MAC is the address of the sending switch.

ITdude

Derek, you could say that you have "spanned" many generations.

dtlokee

EdTheLad wrote:

dtlokee wrote:

The typical reason behind having multiple instances is to elect separate root bridges to better utilize your redundant links. Most stp regions only have 2 or 3 switches that could act as the root, so you really don't need more than 2-3 instances of spanning tree.

The main reason for MST is to reduce the number of BPDUs getting flooded throughout the network.BPDU processing is very cpu intensive and can easily melt a network.You need to be very careful at the mst boundary that you prune vlans off trunks.

If the goal is to reduce the number of BPDUs, you could use 802.1D, but that would eliminate our ability to designate multiple root switches, and would not allow us to utilize the redundant links on the network. MST is a balance of both. I haven't had issues with switches runnign 64-128 VLANs and the CPU time (using 3560, 3750, 4500, and 6500) but I understand your point. I generally find the need to implement MST in companies that have exceeded the number of spanning tree instances of the hardware. MST is complex to implement and maintain, I generally work in an environment where I provide turnkey solutions and the long term management is done by somone else, if they don't know how to manage MST, it's a tough sell.

As long as the root bridges are in the MST region I have not had issues where they are connected to PVST switches, unless the PVST switch connected 2 MST regions, that was bad.

EdTheLad

I haven't had issues with switches runnign 64-128 VLANs and the CPU time (using 3560, 3750, 4500, and 6500) but I understand your point.

I wouldn't expect you to have an issue as 128 vlans prob isnt enough to cause an issue. My customer has a huge layer 2 network consisting of thousands of vlans.If the vlans are pruned off the trunks,bdpu's stop being sent, blocked ports become forwarding and the network falls over.
On a 6500 when the bpdu rate starts kicking above 250-300 bpdus/sec it starts getting hairy.
Anyway i'll stand by my original comment and say mst was designed to make a network more scalable with regards to bpdu propagation, but as with pvst you have the ability to load-balance.

dtlokee

It's both. MST allows you to reduce the number of BPDUs while allowing you to designate different root bridges for each instance, providing load balancing.

Ferret999

Thanks for all the help guys