IPv4 Header Game

I found a website that creates simple games, and I used it to make one [IPv4 Header game] to help with identification of fields in the IPv4 header.

Might follow this up with others for other headers /  frame contents etc.

Multiple Spanning Tree and Cisco Per-VLAN Spanning Tree interactions

MST and PVST+ interoperability

This confused me for quite some time, but turns out to be relatively simple, so I thought I would write a quick post about it.

The case of MST interoperating with CST and RSTP is straightforward, since both type of spanning tree will have a single instance (IST in case of the MST process) with a single root etc. These can be used to interact and determine root bridge for the entire network (an extended single spanning-tree instance).

PVST+ interaction is more complex, since each VLAN has its own instance, each with potentially a different root bridge and spanning tree topology (which is kind of the point of the technology!) and determining port roles for boundary ports (i.e. the ports interconnecting the MST and PVST region) that is consistent for all VLANs is much more difficult.

First of all, VLAN 1's BPDUs are used to represent the entire PVST+ region, and IST (MST instance 0) repesents the MST region side using PVST Simulation.

PVST Simulation

MST uses PVST+ BPDUs to speak to all PVST+ instances, each containing the same IST information. This allows PVST+ to make a consistent choice about a port's role and state. IST also needs to be sure that VLAN 1's BPDUs represent the state for all VLANs in the PVST+ region.

The port roles in MST - PVST+ boundary ports are: Designated, Root, and non-designated.

MST boundary Designated Port

An MST boundary port will become designated if BPDUs for VLAN 1 are superior to received PVST+ VLAN1 BDPUs.

Also, to maintain PVST+ simulation consistency, all received BPDUs (i.e. for all VLANs) on an MST boundary DP must be inferior.

MST boundary Root Port

Keeping in mind that an MST region can be modeled as a single switch, it follows that for an MST boundary port to become a Root Port toward the CIST root bridge it must be receiving the superior VLAN1 BPDU of ANY MST region boundary port.

Also, to maintain PVST+ simulation consistency, all received BPDUs for VLANs other than VLAN1 on an MST boundary RP must be identical or superior to those of VLAN1.

PVST Simulation Inconsistency

An inconsistency arises if the root bridge region for non-VLAN 1 instances is different to that of VLAN 1, which are indicated to the switch by the consistency criteria above.

If the PVST Simulation consistency criteria are not met, then the port will be placed in a blocked state (designated PVST Simulation Inconsistent or Root Inconsistent) until the criteria are met.

In the diagram, the MST region is root for VLAN1 (on switch DLS1), and is therefore trying to become root for all VLANs on its boundary ports. However, PVST+ has been configured to consider ALS1 as root bridge for VLANs 10 and 20, and ALS2 for VLANs 30 and 40. In this case, they are sending superior BPDUs for these VLANs to the MST boundary ports, which are then protecting the network by placing those ports into blocking state until the inconsistency is resolved.

An example of an error message on the console of DLS1 (a 3750) is shown below:

%SPANTREE-2-PVSTSIM_FAIL: Superior PVST BPDU received on VLAN 10 port Fa0/1, claiming root 4106:001b.0ddc.e580. Invoking root guard to block the port.

 This can be resolved in one of two ways:

1. Change the VLAN 1 root bridge to either of the PVST+ bridges.
2. Change the priority of VLANs 10 - 40 to be higher (inferior) to VLAN 1 on both the MST and PVST+ switches.

Spanning tree and superior BPDUs

SPANNING TREE SIMPLICITY

The bewilderment surrounding the Spanning Tree Protocol and root ports and designated ports (well it bewildered me anyway!) can be immensely simplified by one idea:
It's all about SUPERIOR BPDUs.

Superior BPDUs

So first of all, what is a superior BPDU? It's one that 'wins' i.e. is the LOWEST in the following ranking. If any one is a TIE, then the next lowest down is used to break that tie:

1. Root Bridge ID (RBID)
2. Root Path Cost (RPC)
3. Sending Bridge ID (SBID)
4. Sending Port ID (SPID)
5. Receiving Port ID - only used is very rare cases and is not carried in the BPDU, it is assigned locally.

All the information in 1-4 above is carried (along with the timers) in every BPDU that is sent by every switch running STP.
So how does this help? It explains almost everything about the STP process and convergence, and helps, in my mind, to very succinctly define root port and designated port!

Convergence steps

To recap on the three fundamental steps that need to occur for STP convergence:

1) Elect a root bridge 2) Determine root ports 3) Determine designated ports

Elect a root bridge

Electing a root bridge is determined by the lowest RBID (i.e the superior one) in any BPDU circulating the network. It is determined to be a SUPERIOR BPDU because it has the lowest value in the first superiority criteria. Since the superior RBID is placed into all forwarded BPDUs during the election, thereafter EVERY BDPU WILL HAVE THE SAME RBID. So you can discount it!

Determine root ports

Determining the root port (RP) for any switch is done on the basis of lowest 'resulting' path cost (i.e. RPC in the BPDU + receiving port cost) to the root bridge, which is the SECOND SUPERIORITY CRITERIA. It makes sense that there can only be one lowest cost path to the RB from any other switch, and therefore that there can only be one RP per switch.

Now we already know that RBID is going to be the same in every BPDU, so what's next? Root Path Cost.

And the RP, therefore can be very simply defined as the ONLY port on the switch RECEIVING the SUPERIOR BPDU. There can only one port, because there can only be one superior BPDU. If RPC is a tie, then go to the next criteria, and so on. You also know that BPDUs are not sent out of RPs, because there would be no point. Why? Because you already know that the most superior BPDU on the segment ARRIVED on that port, and yours is sure to be ignored as inferior. Also the BPDU stored on a RP is always the superior one of any sent on the segment.

Determine designated ports

Similarly, the designated port (DP) is the only port on the SEGMENT that is SENDING the SUPERIOR BPDU. RPCs in the sent and received BPDUs are simply compared against each other, without modification. How does it know? Because it doesn't hear any that are superior. If it does, it knows it's not the DP, and stops sending them!  Again, because there can only be one superior BPDU on the segment, only one port can be sending it.

This means that ports that are not disabled and, although not connected to another switch, are participating in STP are also designated ports; hence they do not get put into blocking state.

A port that uses 'portfast' setting is a special case since it does not send BPDUs and therefore cannot really be considered a DP, but it is immediately placed into Forwarding state.