The damned subif bug of the day
août 16th, 2008
Strange behavior in the pipe! Welcome!
I was working on a bug PoC glad to an old fashion 2611 router when the problem begins: Packets can’t reach their destination from and to an ethernet sub-interface with the physical interface UP and without IP address.
I do ping 192.168.0.23 which is linked to ethernet0/0 by a cross-over cable.
CDP can be exchanged as it’s layer 2 and use of the physical interface.
I’ve tested too with the 2611 linked to a 2500 router and to a 2900 switch.
Same behavior : I can’t ping from and to the sub-interface (192.168.0.100).
And of course, it works like a charm when I setup it with the physical interface.
Do you know a mystic trick to do this working except use other hardware or use dynamips linked to the host computer interface ?
Anyway, I really need to use the subif else my bug PoC should not be viable 
Show commands output:
hacking2600#sh ip int brief
*Mar 1 00:07:43.848: %SYS-5-CONFIG_I: Configured from console by console
Interface IP-Address OK? Method Status Prot
ocol
Ethernet0/0 unassigned YES manual up up
Ethernet0/0.1 192.168.0.100 YES manual up up
sh run
interface Ethernet0/0
no ip address
full-duplex
!
interface Ethernet0/0.1
encapsulation dot1Q 1 native
ip address 192.168.0.100 255.255.0.0
!
When I try ping, debug ip packet said “encapsulation failed”.
There is nothing special in wireshark: The PC responds to the ICMP requests but looks like the router drop the stuff.
Tried with 2 different versions of ios.
Don’t get fucked by the the network mask.
Debug ip packet details output :
*Mar 1 00:44:38.907: IP: tableid=0, s=192.168.0.100 (local), d=192.168.0.23 (E
thernet0/0.1), routed via RIB
*Mar 1 00:44:38.907: IP: s=192.168.0.100 (local), d=192.168.0.23 (Ethernet0/0.
1), len 100, sending
*Mar 1 00:44:38.907: ICMP type=8, code=0
*Mar 1 00:44:38.907: IP: s=192.168.0.100 (local), d=192.168.0.23 (Ethernet0/0.
1), len 100, encapsulation failed
*Mar 1 00:44:38.911: ICMP type=8, code=0.
Success rate is 0 percent (0/5)
hacking2600#
*Mar 1 00:44:44.112: IP: s=192.168.0.100 (local), d=224.0.0.5 (Ethernet0/0.1),
len 64, sending broad/multicast, proto=89
Entry Filed under: ios
