PVSTP to RSTP

[Andy White]

Hello,

Our switches use the default PVSTP and due to the rapid convergence time I'm considering RSTP or even PVRSTP.  I was wondering you use guys in the field normally use and are happy with, why wouldn't I use RSTP?

Due to all root ports being re-calculated I guess I should change this out of business hours?

Thanks

[Peter Paluch]

Hi Andy,

Personally, I see no reason not to use RSTP. In fact, new versions of the 802.1D standard do not specify STP at all, and instead talk about RSTP only.

One thing to be very careful about: it is necessary that all access ports be configured as PortFast ports. In RSTP parlance, this means they are considered edge ports and won't be affected by topology changes in the network, particularly by the Proposal/Agreement process. Without configuring the ports towards end hosts as PortFast, a Proposal/Agreement wave may result in their temporary blocking but because hosts do not send Agreements, these ports would be blocked for 30 seconds before reaching the Forwarding state again. Just turning on the RSTP without making sure that access ports are configured as edge ports may actually worsen the perceived convergence time in your network. Quite a few network administrators learned this the hard way.

Ideally, your switches should be configured using spanning-tree portfast default global configuration command that causes all current and future access ports will be considered as PortFast ports.

Depending on how many VLANs you run, you could actually also benefit from running MSTP. However, MSTP is more complex to understand and initially set up, but rewards you in consuming less resources, and is the standardized way to go in multi-vendor VLAN-aware environments.

You should definitely request a maintenance window for STP upgrade. There will be network outages.

Best regards,

Peter

[Andy White]

Thanks this is very informative.  My root switch is a stack of 2 3750s which I'm replacing with 2 3750Gs soon so I guess I could make it RSTP then and set portfast globally?

I'm reading MSTP at the moment as part of my CCNP and hmm I think I need to get my head round that first

I guess I could also consider RPVST+ too, but I don't think I will benefit from this.

[Arumugam Muthaiah]

Hi Andy,

• RSTP is Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of a bridged network. The drawback is that these mechanisms are proprietary and need additional configuration.
  

• Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1D standard more than a
  revolution.
  

• RPVST+ (also known as PVRST+) Is basically when you mix the two above. You have an instance of rapid STP running per VLAN.
  

You can use RSTP for the rapid convergence and it is advisable do the change during off business hours.

Refer:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

Regards,

Aru

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[Joseph W. Doherty]

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Posting

In my current employer's environment, our team supports about 6,000 Cisco devices, most of which are switches (although half or more are L3 switches).  Our current standard actually calls for RSTP and portfast [and bpduguard] on each edge port.  (Personally, I configure the global portfast [and global bpduguard] command that Peter describes.)

We see little difference in PVST vs. RSTP operationally, but then individual port portfast (and backbone and uplink fasts) was also configured for PVST.  Additionally, when there's intentional redundancy, it's either Etherchannel or L3.  So, unsure our usage confirms RSTP is better, but this sort of answers your first question, in the sense we're not unhappy with it.

In answer to your second question, yes this is the sort of change you should make during a maintenance window.  If you're doing large scale migrations, also keep in mind you can have both STPs in the same topology; i.e. they will inter-operate.

BTW, if you do go with global setting for portfast (and bpduguard), on some 3xxx switches, I've noticed the IOS will insert individual portfast command on applicable ports.  I've also found, you can removes these individual port portfast command if you want to reduce the "clutter" in the config; i.e. portfast is still active on the port.

Note: when I mention PVST, ours is the default Cisco variant - which is actually PVST+ (?)

[Andy White]

Thanks so much again another informative real world reply.

I will default my switches to RSTP.  I also use BPDUGuard and Root Guard on my edge ports and want to introduce DHCP snooping (ealier post).  Most ob my trunks I have upgraded to Etherchannels as I hate wasted bandwidth just sitting there.

It is nice to have a temlpate build like this as a standard.

Just one thing is RSTP enabled just on the Root or all?

Thanks

[Praveen AR]

Hello Andy

RSTP need to be enabled on all switches.

[Peter Paluch]

Hello Andy,

It does not make sense running both BPDU Guard and BPDU Root Guard on a single port. The BPDU Guard will force the port into err-disabled state when a BPDU is received. The Root Guard won't ever have a chance to have its turn

Best regards,

Peter

[Andy White]

When you put it like that is makes absolute sense. 

Thanks again.