Stacking and LAGs

Unanswered Question
Sep 21st, 2010

Hi, need a little straightening out in my head...

How does stacking effect LAGs?

If a switch can only support 8 ports per LAG, yet you need more ports to be part of the LAG, the answer, I am told, is to stack the switches...

A bit confused...

If I have a 2-switch stack, does that mean each switch can support 8 ports, thereby allowing 16 ports on the LAG?

:-(

I have this problem too.
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Peter Paluch Tue, 09/21/2010 - 12:48

Hello,

I am afraid that stacking does not allow you to increase the number of ports in a single channel. According to the Configuration Guide for 3750 series, even with stacked switches, an EtherChannel may contain only up to 8 active links (with LACP, there can be at most 8 active and 8 standby links in a single group, a total of 16 ports but only 8 of them are carrying data). These are both hardware limitations and limitations of the EtherChannel management protocols (LACP and PAgP).

In my opinion, needing more than 8 links in a single EtherChannel is worth considering whether a different link layer technology may be better suited to cope with your bandwidth requirements. For example, instead of combining 9 or more GigE ports into a single EtherChannel, why not consider moving to 10GigE?

Best regards,

Peter

ex-engineer Tue, 09/21/2010 - 13:53

Hi, thanks.

We can go to 10Ge.

The purpose of me posting this question, though, was for instructional purposes. I want to know why stacking is recommended as a solution.

Stil dont see why...

Peter Paluch Tue, 09/21/2010 - 14:00

Hello,

Stacking itself allows you to make several switches behave as a single, compact unit, operating over a single, shared MAC address table, sharing the control plane, and performing inter-switch communication over a dedicated, high-speed interface (the StackWise interconnect). For all purposes, the switch stack behaves as a single switch, ranging from configuration in a single please (a single configuration file controls all switches in the stack), behaving as a simple bridge in STP, EtherChannel, inter-VLAN routing and so on...

The switch stack combined with EtherChannel allows you to spread the ports among several physical switches in a single stack. Even though the ports are on different stacked switches, to the opposite party, they appear as if they were located on a single switch. In addition, if a particular switch in a stack goes down, the remaining switch will continue transporting data and the EtherChannel will still work (although in a reduced throughput until the first switch comes back up and adds the ports back to the EtherChannel).

Best regards,

Peter

Giuseppe Larosa Tue, 09/21/2010 - 14:00

Hello Joe,

>> I want to know why stacking is recommended as a solution.

for better resiliency and fault tolerance by taking member links of the bundle from different member switches of the stack you can ensure that no single device or single link failure event will make the bundle totally not operational.

The surviving switches have to keep using the same LACP system id even in case the failed switch had the master role. If so there is consistency and the LAG can survive to any single device or link failure.

if you put all the LAG links in a single device the bundle will be down if that device fails

Edit:

Peter I didn't see you were answering

Bye

Hope to help

Giuseppe

ex-engineer Wed, 09/22/2010 - 15:23

Thanks, guys.

Actually, I understand and already know everything you have said about stacking.

The problem is that I was misunderstanding what the person's solution was. Its hard to describe here, but suffice it to say that I was mistaken about what the person was telling me.

Thanks

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