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MSTP Help Please

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C Delaney
Advisor

MSTP Help Please

Dear All,

I'm changing my current MSTP configuration and need some help please.

Currently I have 3 2824 switches linked in a triangle with 2 Gb trunks. I have 3 VLANs (1, 2 and 3) and NO routing so have configured 3 instances - one for each VLAN.
Switch 1 is root for instance 1, switch 2 is root for instance 2 and switch 3 is root for instance 3. All good so far.

I plan to add another switch into the mix in order to get a mesh topology (and get more edge ports which is my main problem). This means that each switch will have 3 trunks rather than 2. Can anyone suggest a sensible way to configure MSTP for this new topology please?

Many thanks in advance.

Chris
15 REPLIES
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

basically I don't think you need a "full mesh network", where all switches have connections to all other switches.

Take into account what logical connections will be made accross your network.
most sites have many clients to some servers (+internet)
If the fourth switch only is to connect more clients, look at where the corresponding servers are connected (on the same vlan).
attach the new switch to this one and one other for redundancy.
You don't need adjustment for MSTP.

But then again think if this may be overkill if the servers have no redundant connections to two different switches.

=== for future ===
You could think about designating two switches as "core" and two other as "access".
where the core are stp-root.
one mstp instance can be root for two vlans or one core can run two instances.
when no furter network expansion is expected, connect your servers to the core.

do you have an "external" router between vlan's?
C Delaney
Advisor

Re: MSTP Help Please

I've enclosed a (very crude - so apologies) diagram of what I currently have and what I want to implement. The network is split into the three VLANs; one for iSCSI traffic, one for switch management and one for VMWare VMotion traffic. I want them kept completely separate so there is no routing whatsoever and there are no connections to anything outside.

I basically need more edge ports in the DR iSCSI VLAN and so thought that a mesh would probably be the best thing. Switches 1 and 2 need to talk to each other most of the time and similarly switches 3 and 4 need to talk to each other most of the time. The reason for this is that the SAN appliances (in each room) act as single storage groups (production storage and DR storage) and so there is a large amount of traffic going between them.

Obviously there's traffic going between the two storage groups but it's mainly replication.

So do I need to do anything else to the MSTP configuration? If so then what do you think would be the best idea?

Many thanks.

Chris
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

for every logical link you only pass max. two switches.

for each MSTP instance some ports furthes from the root will be blocked.
look at attachment
with sw2 as root sw3 will block two ports and the link sw4-sw1 will block one port.
match this with your data-flows for each vlan.

I suggest making only sw1 and sw2 root's
C Delaney
Advisor

Re: MSTP Help Please

Pieter,

Thanks for this. So, given that most of the traffic is iSCSI with occasional bursts of high-volume VMotion traffic do you think that it would be best to have:
- Instance 1, iSCSI VLAN, root switch 1.
- Instance 2, VMotion and management VLANs, root switch 2.

How does that look?

If I lost both switches 1 and 2 would 3 and 4 still be able to cope and not block any of the VLANs? Presumably they'll realise that switches 1 and 2 are missing and that only the one link remains between them?

Cheers.

Chris
Pieter 't Hart
Honored Contributor
Solution

Re: MSTP Help Please

when using MSTP, all switches in your network must use the same MSTP-configuration.
only you designate some switch as root for normal operations so when adding/removing switches from your network, allways the same switch gets to be root.

if both sw1 and 2 are down, no dedicated root is active and sw3 and 4 have to negotioate who will be root, but eventually a new root will be selected.
so no roblem there.
C Delaney
Advisor

Re: MSTP Help Please

Thanks Pieter that's brilliant.

Chris
C Delaney
Advisor

Re: MSTP Help Please

Sorry Pieter, I have another quick question. If I have the iSCSI VLAN in instance 1 with Switch 1 as the root then won't the link between Switches 3 and 4 be disabled? I need that link to remain open as the two DR SAN appliances will be talking to each other across it.

What's the best thing to do in this situation?

Cheers.

Chris
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

No, in your scetch both DR-SAN appliances are connected to both sw3 (as well as sw4)
so they can talk directly through sw3 and don't need to cross the link between sw3 and 4.

look at attached scetch of possible dataflows
C Delaney
Advisor

Re: MSTP Help Please

True. However, each SAN appliance has 3 NICs which I planned to connect up as per the enclosed diagram. Two to one switch and one to another.

That won't affect performance will it?

Cheers.

Chris
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

Chris,
what do you want to achieve with this?

If you use every NIC for a separate vlan then there is no difference in what we discussed before.

if you want to team /trunk two NIc's that's connected to the same switch, then the situation is different.
If those devices communicate intensively try to keep them on the same switch.
It's no help for redundancy to put them on different switches.

If the communicate not so intensively then you want to use the link between sw3 and sw4 that also is a 2G trunk so both appliances are connected at 2G (allthough extra switch in between).

you can move the STP root to sw3 or sw4.
But that would cause a problem in the lower side of the scetch as I assume you have the same 3 NIC's in the production devices.

another option is use different vlans for production-SAN and DR-san.
those vlan's are only active on two of the four switches (1/2 or 3/4).
then you need to configure some routing to communicate between san's.
C Delaney
Advisor

Re: MSTP Help Please

Hi Pieter,

Basically I want to have full redundancy so if any of the switches or links (or multiple links) fails then everything will still work. Including if an entire site fails - so if the production side goes then the DR continues to function and similarly if the DR side goes the production continues to function.

If, for example, I connected all of the DR SAN appliances' NICs to Switch3 and Switch3 then died it would mean a loss of connectivity. If I spread the appliances' NIC connections across the two switches (3 and 4) then there will still be connectivity even if one of the switches fails.

Would it be easier if I just bought a new 48-port switch and kept the triangle topology?!

Many thanks.

Chris
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

No I think you best install a "core" layer.
look at this scetch based on cisco campus design.

the switches at a site are not locally interconnected all trafic is through the core.
but there is full redundancy.
C Delaney
Advisor

Re: MSTP Help Please

What about if I make the links between Switches 1 & 2 and Switches 3 & 4 3Gb rather than 2Gb - will that make any difference to the calculation of the spanning tree?

Cheers.

Chris
C Delaney
Advisor

Re: MSTP Help Please

Hi Pieter,

Sorry to keep bothering you with this - do you think that my previous suggestion might be a potential solution or will it not make any difference?

Many thanks.

Chris
Pieter 't Hart
Honored Contributor

Re: MSTP Help Please

Hi Chris,
I can understand you want to try keeping it at 4 switches.
I don't think changing from 2G to 3G channel will make much difference.
A directer impact would be modifying the cost of the link between 1 and 4 (for this vlan), so that link will block and 3-4 will open.
but beware data may have to passs a lot of switches then (2-1-3-4).