Using two HWIC-4ESW in a Cisco 1841

Recently, there was a need for additional FastEthernet ports on a Cisco 1841. The router had already one  HWIC-4ESW attached, but all ports were occupied.
Fortunately, there was a second HWIC-4ESW at stock which was inserted immediately. Two additional ports was configured quickly to a vlan already in use on the other HWIC.

But then the problem occured:
The “new” ports on HWIC #2 couldn’t neither communicate with the VLAN-Interface on the router nor with other clients in the same VLAN connected to ports at HWIC #1.

In MAC-Address-Table, all was looking okay –> The “new” MAC-Addresses were seen in the corresponding VLAN.
But after pinging from the router, there were only “incompletes” in the ARP-Cache.

After further investigating, i came to the result, that there is a common VLAN-Database shared by both HWICs.
It seems that there is no direct communication between the attached HWIC-4ESW. Perhaps due to hardware- or design limitations.

My first try to fix the problem was a crossover cable between fa0/0/3 and fa0/1/3 which was resulting in a loop.
So, that was not an accurate solution ;-)
I found an answer and the limitations at Cisco Website (Cisco EtherSwitch Modules Comparison).

Here’s a short summary:

  • only 2 HWIC or EtherSwitch network modules can be in any platform
  • HWICs do not support Etherchannel
  • only one SPAN port supported. SPAN port output also contains always a dot1q-tag (NM-ESW don’t have this limitation)
  • to achive 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


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

! ESW #2
interface FastEthernet0/1/3
 switchport mode trunk
 switchport stacking-partner interface FastEthernet0/0/3


Though the “stacking-partner” command solved my problem, it’s annoying to lose 2 external ports for communication between modules.
In future, i would rather use a 2960-8TCL and connect it to the switch than attaching a second HWIC-4ESW.
Indeed there is an additional point of failure, but i personally sense this as the cleaner (and also cheaper) solution.

5 comments to Using two HWIC-4ESW in a Cisco 1841

  • Good stuff. I’m sure that’s going to save me some time since we have lots of deployments of the HWIC-4ESWs, and this is bound to come up. The stacking-partner thing is a real crock, though, and, like you said, using it loses 2 ports; since the HWICs cost (nearly) as much as a low-density switch, it seems the 2960 is the way to go.

  • Matija

    Perhaps you could bridge between vlans, like
    int vlan 1
    bridge group 1
    int vlan 11
    bridge group 1

    Then configure ports on HWIC in slot 1 to be in vlan 1 and hwic in slot 0 to be in vlan 11.

    Now you also need BVI interface to put ip address:
    int bvi 1
    ip address x.x.x.x - this will be gateway for vlans 1 and 11

    This is software bridging so you lose performance and cpu, however you save ports on ESW cards.

  • admin

    Nice idea.
    Does it work ?

  • Chris

    Mabye you can answer a question for me?

    I’ve got a 2811 with 1x HWIC-4ESW in it. This is used on the ‘inside’ to provide dual connectivity to cisco 3750 stack (2 switches).

    i need to add another HWIC-4ESW to the same 2811 to be used on the ‘outside’. Is this possible? I don’t really want to use VLAN’s as i’d like them to be “totally” seperate sets of ports. Will this work?

  • chuck

    Hi, I’ve just installed a 2nd HWIC to a 3825, following the stacking steps. The first device I added to the new card works fine. I added a second device (same VRF, different access VLAN) and I’ve got the exact problem you’ve described (Mac in table but incomplete arp entry). Any ideas? Thanks.

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