Let's clarify Spanning Tree Protocol, because it can be confusing.
I’m going to make this as painless as possible in the short beginning of this post, but details will follow for the rest that are curious in nature.
There are THREE!
802.1D - old, D stands for DAMNIT! DAMNIT nobody switched it to Rapid-PVST. You have one STP topology instance for all vlans with this version. Its old, let's move on.
802.1W: new, W stands for WIN! I don’t need a maintenance window to switch it from 802.1D, WIN WIN! You will have a separate STP topology instance for every vlan.
802.1S: newest, S stands for SUPERIOR, it is superior over the other two. But, in reality most networks that we are used to won’t need this, so S can also be confused with STUPID. However, if you are a service provider and/or your equipment can’t handle a large amount of SPT instances, etc. MSTP is for you.
What is the difference in these three versions? 802.1DAMNIT will make individuals that are going to Facebook wait about 50 seconds. But 802.1WIN, you can update your status on Twitter in about a few hundred ms. The above times depend on where the timers/hellos are at in their transition. No worries overall though, the two versions are backwards compatible, so go ahead and start migrating your site.
The fancy add-on features for 802.1DAMNIT, like Backbone Fast, Uplink Fast and Portfast, you can forget about ensuring these are configured correctly because 802.1WIN has it built in and handles it for you very ninja like.
The last thing I’ll cover in the quick review section is port roles. 802.1DAMNIT has five port roles to confuse you on purpose. Disabled, listening, learning, blocking and forwarding. 802.1WIN decided life shall be simpler, as we really only care about when people can get to Facebook, they are discarding, learning and forwarding.
802.1S is MSTP, which stands for multiple spanning-tree instances protocol. This article is already snow balling into the size of something that would smash my kid in the winter, so I'm not going to focus too much on this one. The main reason for developing 802.1SUPERIOR is because it is unmanageable for a device to handle 4094 SPT instances. You can customize a range of vlans into SPT instances, saving your resources. Compared to 802.1WIN, you have no choice, you have a different SPT instance running for each vlan. A lot of the niddy griddy details are the same and is backwards compatible with 802.1D and 802.1W. Overall, this is another article.
***DARE to CONTINUE***
Details for the curious:
802.1D - The overall 50 seconds comes from the following port transitions.
- 20 seconds need to expire first to change the original port state role. This is the result of the max age time of 20 seconds, it must time out.
- 15 seconds, it will sit and listen
- 15 seconds again, it will start to learn what it is listening too
Backbone Fast is a good add-on feature. To transition a port from blocking to forwarding will now take only 30 seconds. This is a Cisco proprietary feature, the reason it reduces it from 50 seconds to 30 seconds, is because it is eliminating the maxage timer of 20 seconds. This is due to the designated port receiving an inferior BPDU than the one it originally has stored, instead of maxage of 20 seconds, it skips this now and moves directly to the listening state.
Uplink Fast, please use the keyword “Uplink” as a remembering tool, to only be used on Uplinks to the root. This feature is also Cisco proprietary. This can reduce the failover time from a failed uplink of 30 seconds to 1 second. The 30 seconds is from the listening and learning states. Without this feature, you do have the option of adjusting the timers for the listening and learning states to a minimum of 7 seconds, total of 14 seconds at a minimum for recovery.
It is not recommended to configure this on switches without the topology knowledge of an alternative/backup root link. Therefore, in a lot of scenario's, this would be configured at the access layer that has 2 or more uplinks to the root. The reason this fails over so quickly is due to creating the feature creating an uplink group. Instead of going through listening and learning, once the root port fails it immediately elects the next port with the lowest cost from the group and makes it the root port.
802.1W - The major benefit of Rapid PVST is how it uses BPDU frames. When a port comes online or the topology changes, the switches exchange BPDU's using proposal/agreement flags. This is why it transitions so fast for ports. These flags are defined in 802.1D (TC and TCA), but RSTP now uses six bits of the flag byte.
These BPDU’s in 802.1W are now sent in EVERY hello, even if it does not receive any from the root. Comparing to 802.1D, which would only relay BPDU’s (when it receives a BPDU from root). With that, STP says that it has lost connectivity to its direct neighbor if it misses three BPDU’s in a row. Therefore, with hello’s at 2 seconds by default, it will take at a maximum of 6 seconds to recover from a failover. Remember, this is an event of detecting a failure in the topology, not a physical link failure, which is extremely faster.
These flags are used as part of a negotiation to determine which ports are going to be designated, non-designated or root port roles. When transitioning ports, the proposal agreements are extremely fast, they don’t use timers, they set the bits accordingly in the BPDU byte to communicate the change and synchronize.
Comparing to Uplink Fast failures, RSTP has its own built in technology, but better. 802.1D uses a dummy multicast generator to help speed up and clear the CAM table when there is a change in the uplink. RSTP uses its own features and clears it immediately due to receiving a BPDU with the TC flag bit set. This process skips a step in 802.1D, like having to wait to notify the root of a change. Now TC is propagating the TCN directly from the switch. This feature is native in RSTP.
The other cool thing about RSTP is EdgePorts, which is basically portfast, kind of. But the cool thing is anytime there is a change on an edgeport, it doesn’t create a TC (topology changes) now. Unless it receives a BPDU, then it will return to a normal STP port. Edge ports also don’t receive TC’s as well.
I would continue, but spanning tree is confusing and the longer I go on, the more confusing it gets. I hope this helps overall.
