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Re: 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 15
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