There is no interface in a specified vlan but i can connect to a device through this vlan

[Majid Jalinousi]

Hi

There was a Cisco Router 7606 with an interface Tunnel, tunnel source on this router was interface Gig 1/1 and tunnel destination was an IP address that when I was tracing route it i saw:

1 10.20.60.6 0 msec 0 msec 4 msec
2 10.100.30.2 8 msec *  36 msec

When I wanted to find destination of tunnel I reached to another 7606 router in the path,I connected to that router and i prompted command "show ip cef 10.100.30.2" and i saw:

10.100.30.2/32
  attached to Vlan30

then I prompted command "show run int vlan 30" and i saw:

interface Vlan30
 ip address 10.100.30.1 255.255.255.192
 ip ospf cost 100
end

But when i prompted command "show vlan" there was vlan 30 but no interface belong to vlan 30. When i prompted command "show vlan id 30" i saw 2 Trunk interfaces that this vlan was allowed on them, but  even when i SSHed to those devices there was no IP address 10.100.30.2.

How was it possible? That IP was reachable and when I SSHed it, responded me but I hadn't user and pass for connecting to it.

My question is, how is this device reachable when there is no interface belong to vlan 30?

Thanks in advance

[Norberto Salgado]

Hi,

if you output when you type "show vlan id 30" is because the VLAN exists. If didn't exist you should get something similar to the message below.

VLAN id 30 not found in current VLAN database

If VLAN 30 doesn't appear in the show vlan, it seems more a cosmetic error with the output of the command.

BR,

Norberto Salgado

[Majid Jalinousi]

Thanks for your respnose

But I didn't mean there is no vlan 30 in "show vlan", i meant there was no interface belong to vlan 30.

When there was no interface belong to vlan 30 how could I ping the device???

[Norberto Salgado · Accepted solution]

Hi Badi,

you've the SVI 30 associated to the VLAN and you're allowing this VLAN on the trunks. So if you're allowing it on the trunks you've interfaces on this VLAN. Do a show mac address table vlan 30, you will see there the devices that you're pinging and through which interfaces you know them.

BR,

Norberto Salgado

[Majid Jalinousi]

Thanks again

I did what you said and I confused completely.

I did a show mac-address-table vlan 30 and I saw the router has learned this mac address from interface po10. When I do sh cdp neighbor I found the router is connected through interface po10 to a cisco switch 3750.

When i SSHed to the switch and I did show mac-address-table I saw that switch has learned the specified mac-address through po10 that was connected to router.

This was kind of loop I think, was there a firewall between them?!

Again I SSHed the ip address 10.100.30.2 and this time I could connect to the device through my username and password, that was a 2811 router. how is it possible?!