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Beginner

Limitations of RSTP

Hi Team,

 

What are the limitations of RSTP ?  Is there vlan limitations for RSTP ?

 

Does this support more than 200 STP instances ?

10 REPLIES 10
VIP Advisor

Re: Limitations of RSTP

Look at the below guide and understand the Limitations :

 

https://www.cisco.com/c/en/us/td/docs/optical/15000r8_5_1/ethernet/454/guide/454a851_ethconf/454a851_configstprstp.pdf

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Beginner

Re: Limitations of RSTP

Hi Balaji,

 

Thank you for sharing document

 

I just want to what is the meaning of highlighted text below

"The ML Series supports per-VLAN rapid spanning tree (PVRST) and a maximum of 255 rapid
spanning-tree instances."

 

 

VIP Expert

Re: Limitations of RSTP

It means that the ML series (as most Cisco) switches support per-VLAN rapid-STP. (NB: Some vendors only support one rapid-STP for all the VLANs.) It further means that the ML series can support up to 255 VLANs running a rapid-STP. (I.e. if had more than that number of VLANs, you would need to disable rapid-STP on a number of VLANs in excess of 255.)
Beginner

Re: Limitations of RSTP

Thank you Mr. Joseph,

So in swiches like 6500, 2960x, 3850 and lastest C9000 series support more than 255 vlans in RSTP environment. I don't find those details in datasheet above mention swtiches
VIP Advisor

Re: Limitations of RSTP

All the model's nothing much changed so baseline remains the same.

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Beginner

Re: Limitations of RSTP

Hi Balaji,

 

I found some info regarding details in Cisco 6500 and Cisco 4500, where can i find these info for other switch modules.....3850 and 2960X

 

vlans234.PNG

 

VIP Expert

Re: Limitations of RSTP

I don't know. However, as 255 is a one byte counter, and that lots of STP instances load a switch, I wouldn't be surprised in many other Cisco switches have a similar limit.
Beginner

Re: Limitations of RSTP

According to Cisco TAC engineers STP limitation for 2960, 3850 and C9300 are 128
VIP Expert

Re: Limitations of RSTP

Well, they have access to information that's not public.

As I noted, 255 instances is a lot. Actually so is 128, so that's not surprising either.

When your get up toward such instance counts, MST makes much more sense, as supporting that many unique L2 topologies would be unusual.
Hall of Fame Expert

Re: Limitations of RSTP

Hello Prashanma,

the key to achieve scalability when the number of Vlans is greater then 128 is to move to spanning-tree mode mst as noted by Joseph.

With MST you can provision how the IEEE whole range of Vlans 1-4094.

(Please note that modern SVL uses Vlan 4094 on the SVL link) is mapped to few MST instances even before you create the Vlan.

Because MST consider two switches to be in the same MST region if they agree on the following parameters:

MST region name ( up to 32 characters is a string)

MST revision number

Vlans to MST mapping.

Actually only the IST instance 0 BPDUs are sent out with additional sections for each MST instance.

In the BPDUs an MD5 checksum of the Vlans to MST instances mapping is actually sent.

So MST requires a different approach: when you need to create a new Vlan you need to keep an unused Vlan from the subset of Vlans that are mapped to the MST instance that has the desired topology ( root bridge and so on).

So you first choice the topology the new Vlan should have and then you can pick a Vlan-id in the correct sub-set.

Changing the Vlans to MST mapping causes the MST region to be partitioned.

 

To be noted VTP version 3 has a feature that allows to carry the Vlans to MST mapping that is very interesting and handy.

 

This is likely the reason why modern access switches have not improved the number of RSTP instances

 

Hope to help

Giuseppe

 

 

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