cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
14976
Views
15
Helpful
4
Replies

About Nexus 7000 command "spanning-tree pseudo-information"

I read the configuration guide but I don't get it.

What are "spanning-tree pseudo-information" do and sub command

vlan vlan-range designated priority value and vlan vlan-range root priority value

Could you explain how to use these command and what effect ?

Thank you.

4 Replies 4

rahurao
Level 1
Level 1

Hi,


The spanning-tree pseudo-information command is used to configure a hybrid vPC and non-vPC peer switch topology. It is used to change the designated bridge ID so that it meets the STP VLAN-based load-balancing criteria and then change the root bridge ID priority to a value that is better than the best bridge priority. You then enable the peer switch.

You need to know that if you previously configured global spanning tree parameters and you subsequently configure spanning tree pseudo information parameters, be aware that the pseudo information parameters take precedence over the global parameters.

This example shows how to configure a hybrid vPC peer switch topology:

switch# configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

switch(config)# spanning-tree pseudo-information

switch(config-pseudo)# vlan 1 designated priority 8192

switch(config-pseudo)# vlan 1 root priority 4096

switch(config-pseudo)# exit

switch(config)# vpc domain 5

switch(config-vpc-domain)# peer-switch


Please check the configuration guide for the same:

http://www.cisco.com/en/US/docs/switches/datacenter/sw/4_2/nx-os/interfaces/configuration/guide/if_vPC.html

HTH

Rahul

Hello Rahul

this configuration from official doc is not still clear for me.

Let's assume hybrid vPC topology like on the picture

RED and GREEN dashed lines are vPC VLANS of Servers 1 & 2 (assume they are vlans 1 & 2), while BROWN is non-vPC VLAN for ASA (vlan 3),

we whant to use mensioned fieature and have both AGG-1 & AGG-2 as root-bridges for the vlans.

What configuration will we apply in this case?

The spanning-tree pseudo-information command was originally developed for Virtual PC (vPC) and vPC+ designs in order to allow users to create a hybrid vPC and non-vPC peer switch topology. In order to accomplish this, two different BPDU priorities are sent by the switch. Though this command was created in order to work in vPC environments, it fits well in the scenario that is described in the previous section.

When you globally enable this command, there are then two different STP priorities: a lower value (or, better priority) when the switch is connected to the FP (FP core ports up/ready), and a higher value (or, worst priority) that is used in the BPDUs that are sent by the switch after it reloads.

These CLI commands are used in order to configure the FP switch to send the two BPDU priorities:

switch(config)#spanning-tree vlan x priority 8192

switch(config)#spanning-tree pseudo-information

switch(config-pseudo)#vlan x root priority 4096

The value that is set by pseudo-information command is the priority that is used by the FP switch when it is connected to the FP network, so it must be a lower value than the information that is set by the CLI spanning-tree vlan x priority command.

jbekk
Level 1
Level 1

Hi,

 

When you enable vPC you are basically grabbing two Nexus switches and saying.... "I need you two to pretend to be the same switch from a Layer 1 & 2 perspective so I can connect things redundantly to both of you using port-channel".

 

You have many options available to dictate how far these two switches should go in order to pretend that they are the same device. For some networks, the complexity of enabling all the vPC features is simply overhead that isn't necessary... but for other environments we must provide the best-of-the-best in regards to uptime and flexibility.

 

STP is a Layer 2 blocking protocol... and the vPC switches are pretending to be the same box at Layer 2. Therefore they should pretend to be the same STP root bridge if we're going to connect things to both switches using link aggregation... right? Absolutely! But this isn't on by default... this is what the vPC peer-switch feature provides; two switches pretending to be the same STP root bridge.

 

Then comes the next complication - What about in networks where we mix VPC connected and non-VPC connected devices? Should both switches be the same STP root bridge now? Probably not... especially if you want granular control over where STP blocks occur. This is what the pseudo-information feature provides.

 

TL;DR version:

- VPC "peer-switch" allows two switches to pretend to be the same STP root bridge

- STP "pseudo-information" allows the switches to have different STP root bridge settings for non-vPC ports and vPC enabled ports on the switch

 

For pseudo-information:

1. The "root" setting is the STP root bridge used for VPC ports.

2. The "designated" settings the the STP root bridge to use for non-VPC ports

 

As far as I can tell the global STP settings (i.e. default spanning-tree) should remain configured as a last resort in case VPC features break or are disabled/etc... or if the STP pseudo-information configuration is missing.

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community:

Innovations in Cisco Full Stack Observability - A new webinar from Cisco