Additional trunking between switches

[UJ-Admin]

Hey folks,

Have three 2910 switches. Switch A I'll call the Core switch. Switch B & C are meant to be server switches. I (currently) have no additional cards in the back, nor any SFP's purchased. I have created a trunk w/two ethernets going from core to Switch A (ports 1/2 to 23/24), and a second trunk going from core to Switch B (ports 3/4 to 23/24 on last switch).

vLan 100 = 192.168.100.x

On the core, I have vlan 100, which is tagged to each above-mentioned trunk. I have port one (1)  through 12 on each switch (B & C) un-tagged to the 100 vlan.

I am looking for redundancy with my VMware server, so I have two ethernets from the host server (One active one standby) that I would put first to switch B, and the second to switch C, port 1 on each switch. I have a few stand-alone servers also on the switch, ports 5 & 6 (etc) all part of the vlan 100 IP scheme.

Now, provided I gave enough info, is this as redundant as I would get, or should I be doing something more (IE some sort of trunk between switch B & C or something else)?

Thanks,

James

[EricAtHP]

For the switches you have, you are doing really well and that is the best you can do. You still have a single point of failure in your core switch but at least a loss of switch b or c won't cause a loss of connectivity.

In the future, you could look at a differnt switch that supports a stacking technology. For example, the 2920's support a stacking module that makes up to 4 physical switches one logical switch and then you could do link aggregation between your servers and the stack of switches. That would give you redundancy and greater bandwidth.

[UJ-Admin]

Eric,

Thanks for the note back.  Actually, these are the 2920's (J9726A), I just checked.  So, if that is the case, then what would I need to make that next jump to have teh switches stacked, and will traffic at that point pass directly between the two switches w/o having to go to the core or no?

Thanks again for the quick response.

James

[EricAtHP]

Hi James,

To stack your 2920's, you would need two stacking modules (2x J9733A) and 1or 2 stacking cable(s). There are 3 options:

J9734A - HP 2920 0.5m Stacking Cable

J9735A - HP 2920 1.0m Stacking Cable

J9736A - HP 2920 3.0m Stacking Cable

When you install the modules and connect them with a cable, they will automatically create on logical stack. You should backup your configs first as one of the switches will become the master and its config will be used. You will have to reconfigure all of the ports for the second switch. The switch port numbers will also change. When they are not stacked ports are numbered 1-24 but after stacking the ports will be numbered 1/1-1/24 and then the second physcial switch ports would be numbered 2/1-2/24.

Once they are stacked, again they become a single logical switch. So you could have a 4 port trunk (link-aggregation) to your core switch that looked like this:

Core    2920 Stack

1          1/23

2          1/24

3          2/23

4          2/24

And it would be a single trunk (i.e. trk1) on both sides.

Then you could create additional trunks for your servers. For example trk2 could be 1/1 and 2/1 on the 2920 stack and connect to your servers. You could create them as LACP trunks if the servers support it. I like LACP because the trunk will only come up and operate as you expect if it is configured and cabled correctly. But a static trunk would perform just as well as an LACP trunk.

Finally, traffic will only go through the stacking link if that is the shortest path. In the example I just described above, traffic should not ever go through the stacking link because there are connections to everything (core and servers) on both 2920s. But if for example, ports 1/23 and 1/24 were to go down, then traffic from the server attached to port 1/1 could go accross the stacking link and up to the core on 2/23 and 2/24.

Also, if you do have servers that only have one network connection and one server is connected to port 1/10 and another to port 2/10, then traffic between the servers would go accross the stacking link. If you do have several of single connected servers, then you could add a second stacking cable and increase the interswitch bandwidth from roughly 22Gbps to 44Gbps.

Hope this helps,

Eric

[UJ-Admin]

Eric,

My apologies for not responding sooner.  That was the answer I was looking for.  I do have one other question semi-related.

If I do a NIC team (Broadcom on Dell Poweredge server), can I run each NIC to separate switches or, do I need to have the switches stacked first?

Thanks again,

James

[EricAtHP]

You need stacking to do distributed link-aggregation, aka mlag. After the switches are stacked, industry standard link-aggregation can be configured on the servers. I recommend LACP as it verifies th link before making it active.

[UJ-Admin]

Perfect.

Thanks,

James