Dynamips & Multiple NM-16ESWs

I’ve been playing with Dynamips/GNS3 quite a bit lately. My current topology has 16 routers, some serving as Frame Relay routers, other as SP backbone routers, and others as switches via the NM-16ESW module. I found it easier to logically separate the connections to routers from the connections to other switches by using two EtherSwitch modules. I used the first one (Fa1/0 – 15) to connect to routers, and the second one (Fa2/0 – 15) to connect to the other switches. For some reason, I could never get the switches to talk to each other.

Here’s my test case, assuming Switch1 and Switch2 are connected to each other using Fa2/15 on both ends. (The same config was used on each side).

Switch1# conf t
Switch1(config)# vlan 99
Switch1(config-vlan)# int fa2/15
Switch1(config-if)# sw ac vlan 99
Switch1(config-if)# int vlan99
Switch1(config-if)# ip add
Switch1(config-if)# no shut
Switch1(config-if)# end

Once spanning tree (802.1d) has taken its sweet time to come up…

Switch1# sh span vlan 99 brief

I tried to ping the other side, but got no response. Debugging showed me that the other side wasn’t even replying to ARP requests. The ‘encapsulation failed’ means the router has no entry in its adjacency table for the specified IP address.

Sending 1, 100-byte ICMP Echos to, timeout is 2 seconds:

*Mar  1 00:05:06.771: IP: tableid=0, s= (local), d= (Vlan99), routed via RIB
*Mar  1 00:05:06.775: IP: s= (local), d= (Vlan99), len 100, sending
*Mar  1 00:05:06.783: IP: s= (local), d= (Vlan99), len 100, encapsulation failed.

Switch1# sh adjacency detail
Protocol Interface                 Address

After much googling, it seems that Dynamips has a problem with more than one NM-16ESW module in a router. I modified my .net file so that all connections were on a single NM-16ESW module. I could then ping other devices right away. My adjacency table looked much better the second time around:

Switch1#sh adj de      
Protocol Interface                 Address
IP       Vlan99          
                                   0 packets, 0 bytes
                                   ARP        never     
                                   Epoch: 0
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