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Tuesday, January 1, 2013

Stacking between 2x HWIC-4ESW on Cisco ISR 1841

On a Cisco ISR 1841 there are two expansion slots for WIC, HWIC, AIM or VIC modules. The most popular module I use is the HWIC-4ESW which add 4x FastEthernet ports, from time to time there is a need for more FE ports which in this case we can use the second expansion slot and insert another HWIC-4ESW, but here there is a little tricky problem, due to software/hardware limitation both HWIC’s can’t communicate with each other else we use intra-chassis-stacking.

Intra-chassis-stacking allow connecting 2x HWIC-4ESW using interface configuration and external connection between two physical ports, hence from 8 ports we can actually use only 6.

Here is a short summary:

-          The external link can be done with crossover or straight cable (on HWIC only)

-          Only 2 HWIC or EtherSwitch network modules can be used in any platform

-          HWICs do not support Ether-channel

-          Only one SPAN port supported. SPAN port output also contains always a dot1q-tag (NM-ESW don’t have this limitation)

-          In order to achieve that two ESW share the same L2 domain, you have to configure intra-chassis-stacking with the stacking-partner command (shown below) and connect the corresponding ports externally

-          stacking-interfaces have to be configured as trunks

-          “show interface” for stacking-ports shows UP/DOWN while working correctly


interface FastEthernet0/0/3
 switchport mode trunk
 switchport stacking-partner interface FastEthernet0/1/3
interface FastEthernet0/1/3
 switchport mode trunk
 switchport stacking-partner interface FastEthernet0/0/3


Router#show interfaces fastEthernet 0/0/3
FastEthernet0/0/3 is up, line protocol is down
  Internal Stacking Link Active : Fa0/0/3 is stacked with Fa0/1/3
  Hardware is Fast Ethernet, address is 68ef.bdbc.69ed (bia 68ef.bdbc.69ed)
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Auto-duplex, Auto-speed
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:22:28, output never, output hang never
  Last clearing of "show interface" counters never

 Note that the line protocol is down but if the Internal Stacking Line is active it means that the stacking is working.


  1. helped

  2. Thanks for the info. I have two of these on my 1841 and was tearing my hair out trying to figure out why ports FastEthernet0/1/0 - 3 weren't giving out relayed DHCP addresses to my client. I was using this module as it is easier to get to. Finally plugged the client into FastEthernet0/0/0 and it started working. A sh int status showed both modules as being up/up. I even swapped the 2nd HWIC-4ESW out with a spare thinking it was U/S. Bloody Cisco.