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TOPIC: VLAN interface and subinterface

VLAN interface and subinterface 10 years 8 months ago #13816

  • bigal
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Hey everyone,
can somebody carify what is the difference in the Cisco switches between a VLAN interface and a VLAN subinterface.

Thanks
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Re: VLAN interface and subinterface 10 years 8 months ago #13818

  • havohej
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hi.

Vlan subinterfaces??
if talking about a multilayer switch device, you can transform a layer 2 switching interface into a layer 3 only router interface, and give it also an ip address, or create subinterfaces each with its own ip address or subnet, within the same physycal interface, so the entire physical port is a trunk port, maybe connected to another switch swith some vlans.

Vlan Interfaces are the traditional software interfaces created in the switch.

Thats what I understand

But this question is quite odd, I tried to create a vlan subinterface in my switch, and the command for example:

SW4507(config)#int vlan 77.?
% Unrecognized command
SW4507(config)#int vlan 77.

so it does nos recognize the switch, dont know if with new ios versions, you can create this ones?? guessing this one, I dont think so.
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Vlan 10 years 8 months ago #13872

  • gepon
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Hey guys, let be right about something. I think you can only configure subinterfaces on a router that has a trunk link with a switch. A switch is a layer two device so it does not have an ip address.so when you set you switcport to trunk mode, then you can do intervlan routing.
the command to create sub-interfaces can only be applied at the router interface.

#interface fa0/0
-if)#no ip address
-if)#no shutdown
-if)interterface fa0/0.1 (were 1 stands for the vlan created)
sbif)encap dot1q or isl vlan #
)ip address /// msk///

So i think we can't create subinterfaces on a switch.
Bsc.Management, CCNA,
SERVICE TO HUMANITY IS THE BEST WORK OF LIFE.
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Re: VLAN interface and subinterface 10 years 8 months ago #13894

  • bigal
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Here's what made me doubt... here is the simple configuration...I know that the switch is pretty old and the IOS version is old too 12.05 for a Catalyst 3500 XL.

Switch#vlan database
Switch(vlan)#vlan 900
VLAN 900 added:
Name: VLAN0900
Switch(vlan)#exit
APPLY completed.
Exiting....

Switch#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#int vlan 900
Switch(config-subif)#?
Interface configuration commands:
arp Set arp type (arpa, probe, snap) or timeout
bandwidth Set bandwidth informational parameter
carrier-delay Specify delay for interface transitions
cdp CDP interface subcommands
default Set a command to its defaults
delay Specify interface throughput delay
description Interface specific description
exit Exit from interface configuration mode
ip Interface Internet Protocol config commands
management make this the management interface
mtu Set the interface Maximum Transmission Unit (MTU)
no Negate a command or set its defaults
ntp Configure NTP
service-policy Configure QoS Service Policy
shutdown Shutdown the selected interface
standby Hot standby interface subcommands
timeout Define timeout values for this interface

Those are the commands in the sub interface VLAN...
I have never seen VLAN subinterface them in newer IOS version, maybe it has been corrected in newer IOS version. Does someone know if that's the case?
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Re: VLAN interface and subinterface 10 years 8 months ago #13903

  • Chris
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bigal,

I can confirm the presence of the "config-subif" prompt when configuring a vlan interface on the 3500XL series. We happen to have the same switches for our Cisco lab and they too run the 12.0(5) IOS software.

However, I should note that the above prompt is only visible when entering vlan interface 2 and above. Entering vlan 1 interface gives us the expected "config-if" prompt.

I went ahead and created several vlan interfaces and since a "show run" revealed the following:

[code:1]interface VLAN1
no ip directed-broadcast
no ip route-cache
!
interface VLAN2
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN3
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN6
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN23
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN90
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN100
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN300
no ip directed-broadcast
no ip route-cache
shutdown
!
interface VLAN900
no ip directed-broadcast
no ip route-cache
shutdown[/code:1]

I'd say its safe to assume that the so called 'VLAN sub interfaces' are treated as normal interfaces, even though the prompt suggests elsewise.

If anyone else has any further information, I'd be interested hearing them.

Cheers,
Chris Partsenidis.
Founder & Editor-in-Chief
www.Firewall.cx
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Re: VLAN interface and subinterface 10 years 8 months ago #13927

  • bigal
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So Chris, just to be perfectly clear...
a VLAN sub-interface in 12.0(5) is the same thing as a VLAN interface in the newer versions of the IOS!
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