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

 Configuration:

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

 Verification:

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 <OUTPUT OMMITTED>

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