Turning off Spanning-tree

Answered Question
Mar 17th, 2008

Question about turning off spanning.


We have a dual core (2 6513's) and are running layer 3 to top of rack switches (3560E's). In each rack there are 2 3560E's. 3560E#1 connects to 6513#1, and 3560E#2 connects to 6513#2. Both 3560E's are connected to each other for redundancy in case one of the links to the 6513's goes down, this link is a L2 trunk link. All connections are 1GB fiber. Is it safe to turn off PVST on both 3560E switches? Will this help with failover convergence?

Correct Answer by sundar.palaniappan about 8 years 11 months ago

Even by turning off PVST in your switches I highly doubt you would any see any faster convergence than the one ping drop you were seeing with STP enabled. Are you using the lowest value for HSRP hello and hold timers, which is 1 and 2 seconds. I assume you have setup HSRP to track the L3 interfaces. STP apparently isn't introducing any delay in your network and that's the reason why aren't seeing much delay in reconvergence. Hence, turning off PVST mightn't produce any more favorable outcome as far as reconvergence goes.

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Istvan_Rabai Mon, 03/17/2008 - 14:08

Hi Brandon,


I'm not sure if I understand your config well.


Could you please provide a drawing of your config with all the layer2 and layer3 links identified.

Also I would need to see the vlans in your config.


Thanks:

Istvan

mbroberson1 Mon, 03/17/2008 - 16:53

Istvan,


Thanks for your response. I have attached a small diagram at your request.


Thanks,

Brandon

Joseph W. Doherty Mon, 03/17/2008 - 16:45

Generally, if you have no intentional L2 loops in your topology, spanning tree isn't necessary. Removing it would also remove any spanning tree convergence but you'll still encounter L3 convergence.

sundar.palaniappan Mon, 03/17/2008 - 17:41

Your topology doesn't present any scope for STP loops as your core switches are separated from the rack switches via L3 connections. Therefore, you can turn off PVST on your switches. However, bear in mind any accidental addition of links that might introduce L2 loops in your setup can result in nightmare scenario. You might also want to consider the many features IOS has to offer like, RSTP/uplink fast/backbone fast/portfast etc, that helps STP reconverge faster.


Just want to add make sure the routing is setup to fully utilize the redundancy that exists in your network. Choose a routing protocol that converges faster. Depending on the routing protocol you choose there are configuration options that provide for fast reconvergence.



HTH


Sundar

mbroberson1 Mon, 03/17/2008 - 17:52

One of my reasons for posting this conversation was not only to see if turning off PVST was safe, but also in wondering if in disabling spanning-tree would that help with failover time if one of the links to the 6513's goes down. I am running HSRP between the to 3560E's (the vlan1 virtual ip address is the clients default-gateway) and when I test with a PC there is one ping loss. Although this will most likely not present a issue I was just wondering if there might be a way to prevent no ping packet loss?

Joseph W. Doherty Mon, 03/17/2008 - 17:59

May not guarantee no ping loss, but you can tune HSRP timers down from their defaults. (Also check if your 3560Es support HSRP with msec timers.)

mbroberson1 Mon, 03/17/2008 - 18:19

Joseph,


Thanks for your reply. Yes they do support msec timers. Tomorrow I'll start tuning from the HSRP defaults and let you know my findings.


Thanks,

Brandon

Correct Answer
sundar.palaniappan Mon, 03/17/2008 - 18:05

Even by turning off PVST in your switches I highly doubt you would any see any faster convergence than the one ping drop you were seeing with STP enabled. Are you using the lowest value for HSRP hello and hold timers, which is 1 and 2 seconds. I assume you have setup HSRP to track the L3 interfaces. STP apparently isn't introducing any delay in your network and that's the reason why aren't seeing much delay in reconvergence. Hence, turning off PVST mightn't produce any more favorable outcome as far as reconvergence goes.

mbroberson1 Mon, 03/17/2008 - 18:16

You are correct I have HSRP setup to track the L3 interfaces. Tomorrow I will try tuning the HSRP timers and post my findings. I really appreciate all you input.


Thanks,

Brandon

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