multiple switch modules on ISR

Unanswered Question
Sep 12th, 2008


Would you please verify when it is necessary to stack the switch modules on an ISR router?

1. HWIC + HWIC (4esw, 9esw...) - stacking must be configured with the "switchport stacking-partner" command. This is described in the documentation.

2. HWIC + NM (4esw or 9esw... + NM-16ESW-PWR or NME-16ES-1G...) - ????

3. NM + NM (NM-16ESW-PWR... + NM-16ESW...) - ???

Thanks in advance,


I have this problem too.
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Giuseppe Larosa Fri, 09/12/2008 - 06:06

Hello Mladen,

it looks like that when you have multiple switch modules in ISR you need to stack if you want to use them together.

the reason is that otherwise the inter module communication would use forwarding resources of the ISR that are left unused with external stacking

Intrachassis Stacking

Multiple switch modules may be installed simultaneously by connecting the Gigabit Ethernet (GE) ports of the EtherSwitch network module. This connection sustains a line-rate traffic similar to the switch fabric found in Cisco Catalyst switches and forms a single VLAN consisting of all ports in multiple EtherSwitch network modules. The stacking port must be configured for multiple switch modules to operate correctly in the same chassis.

•MAC address entries learned via intrachassis stacking are not displayed.

•Link status of intrachassis stacked ports are filtered.


Hope to help


mladentsvetkov Fri, 09/12/2008 - 06:11

Hello, Giuseppe

Thanks for the response, but you have covered only the case when GE etherswitch module is used.

Is there documentation, that explains how to cover the cases I have explained in the first post?



Giuseppe Larosa Fri, 09/12/2008 - 06:39

Hello Mladen,

thanks for your kind remarks.

my idea is that whatever is the type of modules either HWIC or NM you need to use stacking if the modules are "switch" modules


Requirements for Installing Two Ethernet Switch Network Modules in a Single Chassis

A maximum of two Ethernet switch network modules can be installed in a single chassis. If two Ethernet switch network modules of any type are installed in the same chassis, the following configuration requirements must be met:

•Both Ethernet switch network modules must have an optional Gigabit Ethernet expansion board installed.

•An Ethernet crossover cable is connected to the two Ethernet switch network modules using the optional Gigabit Ethernet expansion board ports.

•Intrachassis stacking for the the optional Gigabit Ethernet expansion board ports is configured. For information about intrachassis stacking configuration, see the 16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 series feature document.

Without this configuration and connection, duplications will occur in the VLAN databases, and unexpected packet handling may occur.

If you mean a normal NM NM-2FE2W or similar this is a "router" module so it doesn't need to be stacked to a switch module and will use the router backplane for forwarding.

The underlying idea is to avoid to use router backplane to make two "switch" modules to communicate between them, leaving the router backplane just for L3 forwarding.

Hope to help


mladentsvetkov Fri, 09/12/2008 - 06:49

Hello Giuseppe,

I have already read this extract from the documentation.

The question is, what happens when you have HWIC-4ESW (or HWIC-9ESW) and NM-16ESW-PWR (or NM-16ESW) on the same ISR. Which rule applies? The one with the "Gigabit Ethernet" or the one explained in the HWICXXX documentation (

Thanks in advance,



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