encapsulation dot1q native keyword and inter-vlan routing problem

Hello
I have a 3com 3300 3c16780A switch configured with 4 vlans
connected to a cisco 1750 on a trunk port. All vlan ports are untagged
except the trunk port which belongs to multiple vlans.

On the cisco 1750 I setup 4 subinterfaces on fast0/0.1 fast0/0.2 etc..
with ip addresses and 802.1Q encapsulation using the command
"encapsulation dot1q <vlanID>".

The problem is that I'm not able to ping from a machine on the native
vlan to its default gateway. All other vlans work - I'm able to ping
the default gateway from a pc as well as another machine on another
vlan on the switch. For example when the default native vlan is vlan1,
routing happens between vlan 2 and vlan3, but on vlan1 - no ping
succeeds even to it logical subif ip address (setup as default gateway
for that vlan/subnet).

This is because vlan 1 is marked native - the new option starting with
ios 12.1 - as shown in "show vlans" command

But if I switch to subif fast0/0.3 and make it native using
"encapsulation dot1q 3 native", My machines on vlan 3 cannot ping
their default gateway, while vlan1 starts working !!

Whats with this native keyword? Are my frames going untagged on native
vlan which is creating a problem?

Rgrds

 

The default VLAN for Cisco is vlan 1 and it is the default native vlan
and native vlan is UNTAGGED.

So do not use vlan 1 for user VLANS and you should be okay...

 

Hmm.. I thought it would somehow work even though vlan 1 is untagged,
because sending trunk port (on switch) would not tag it and router
trunk port expects vlan1 to be untagged while rest would be tagged
.... i.e when the router sees an untagged frame it lumps it in vlan
1.... ?!!
So the solution is never to use vlan 1 in practice when using 802.1Q
and trunk ports .... ?

Rgrds

 

Actually, the practice is not to use the same VLAN for users and
management.