We know that in a fully redundant Layer 2 topology, STP will find a Root Bridge, Find the shortest path to the Root Bridge, and block all other ports as shown in the below diagram.
In my previous articles about VTP I mentioned that frames are sent over all trunk ports and when utilising VLANs on switches to segment a network you need to use trunk ports to transport frames from each VLAN between network devices. But how do you configure a trunk port?
The main functions of VTP V3 are exactly the same as with VTPv1 and V2, however VTPv3 has some additional features and configuration requirements.
- VTPv3 is backward compatible with switches running V1 and V2.
- Allows on the VTP Primary Server to make VLAN changes.
- Supports all 4094 VLANs.
- Supports propagating MST configuration changes
- Provides feature specific primary server configuration.
- Allows the use of PVLANs (Private VLANs)
- Per Port VTP configuration is supported
VTP or Virtual Trunking Protocol, is a Cisco proprietary protocol that is used to share VLAN information between switches. In a small topology such as our little 3 switch topology or it a small office network where you might have 3 or 4 switches, configuring VLANs manually on each switch might not be such a big deal. But think about a large scale campus network or Datacenter network where you have hundreds or thousands of VLANs and having to manually configure those VLANs or make changes on each device. This is why Cisco invented VTP.
Imagine if you will, your boss comes to you and says "Great news, we're getting all new equipment so now we can replace our old Catalyst 3500XL series core switch and 2900XL series access switches from the 1990's to some brand new Catalyst 9300 series switches" You might think all your Christmas's have come at once if you walk into a job using equipment that old and get some new stuff, either that or you could be clinically insane for taking a job like that?
MSTP or 802.1s, is an industry standard version of STP that incorporates RSTP timing with the ability to load balance VLANs across instances of spanning tree. If you recall, 802.1D runs a single instance of spanning tree for all VLANs within a network called the Common Spanning Tree or CST. MST takes this concept and expands on it by allowing you to assign VLANs to an Instance of spanning tree and allowing multiple instances to be configured, hence the name Multiple Spanning Tree. This also means that you can have a root switch for each instance of MST as well.
Spanning tree protects our layer 2 network from broadcast storms and helps to create a loop free layer 2 topology. STP takes advantage of additional features that have been added over the years help prevent unnecessary changes to the network or to help prevent unintentional or sometimes malicious activities from occurring on our networks. Some of these features include
- UDLD (Unidirectional Link Detection)
By default STP works straight out of the box. You buy a switch and plug it in and boom STP works. Well, 802.1D (PVST if using Cisco Switches) works by default. While PVST is great, it's slow so we would configure it for PVRSTP or MST. Again, these work straight away and by default will choose which ports are forwarding and which are blocking. But what if we want to manually choose one link over another? To do this, we first need to understand what we want to change and how the port election process works.
Root Port selection
I've talked a little about STP link failures in previous articles, but let's take a deeper dive into what STP does during the various STP failure scenario's. There are two types of link failures that can occur in a Layer 2 STP topology.
- Direct Link Failure: This is when a physical link fails and the port state changes to down.
- Indirect Link Failure: This is when there is not a physical failure, but there is no data flow over the link.
PVRSTP+ or Per VLAN Rapid Spanning Tree Plus is a Cisco Proprietary extension of the Rapid Spanning Tree protocol that, like PVSTP runs a single RSPT instance per VLAN configured on the switch. 802.1W RSTP is an improvement on 802.1D STP in that it allows for much faster transition between port states with modifications to the port states, and also with the addition of some port roles and types.
802.1W Port States, Roles and Types
802.1W RSTP reduces the number of port states from 802.1D to 3.