Why interface VLAN doesn't come up if there is no access port configured with VLAN=i/f VLAN and only trunks ports are configured

[ashish7aditya]

Hello Experts,

I have noticed that interface VLAN doesn't come up till the time I configure one of the switch port as access with VLAN = i/f VLAN & connect one end device like PC through it.

I have two switched connected as below:

switch#1 --------- switch#2---PC#1

I have configured interface VLAN#2 on switch#1 and #2 and have allowed VLAN#1-2 over the trunk link between two switches. I see interface VLAN doesn't come up (even I have changed the native VLAN from 1 to 2 on each switch).

Now moment I connect PC#1 on one of the access port(under VLAN#2) of switch#2, interface VLAN of switch#2 comes UP, however i/f VLAN for switch#1 is still down.

Why interface VLAN comes UP only after I generate a traffic from any local access port of that switch (under same VLAN = i/f VLAN). Why even after connecting PC#1, i/f VLAN on switch#1 was still down and no communication happens between PC#1 and switch#1?

Best Regards!

[Vasilii Mikhailovskii]

Hello.

I'm not sure what do you see on your devices.

The issue may come from VTP prune (so, disable it).

The best way to check L2 is:

 - show ip int vl ...
 - show span vl ...
 - show vlan id ..
 - show int trunk

Please provide the outputs from both switches.

[ashish7aditya]

Hello Vasilii/ Other Experts,

Good day.

Actually, I was just performing one practice exercise on packet tracer(diagram attached).

I have connected one switch(SW#16 in the attached diagram) from HWIC card(switch i/f of Router11) and have configured two interface VLANS. i.e. VLAN#2(IP 10.1.1.1/8) & interface VLAN#3(IP 11.1.1.1/8) on this router#11and I have configured switch#16 with interface VLAN#2(IP add DHCP) and further I have connected one PC from access port(VLAN#3) of switch#16. Again IP assignment for this PC is set as DHCP.

Router#10 was set as DHCP server and RIPv2 on both the routers was enabled along with DHCP helper IP on Router11 for each interface.

The PCs which are connected from Gi0/1 & Gi0/2 of router11 receives IPs from DHCP server, however, interface VLAN of switch 16 and PC41(connected from switch#16) doesn't receive IPs.

Then I realised that although I have defined VLAN#2 & VLAN#3 on HWIC(R11) but both the VLANs are operationally and admin down. I tried no shutdown under each i/f VLAN but still they don't come up.

I know if I am working on any of the native switch example 2960, moment I create VLAN#2 and then I change the i/f VLAN to VLAN#2, it's status comes UP. But with HWIC card in the router, it doesn't come up.

any Idea why ? and where I am wrong or missing any point?

I think maybe because we can't create VLANs on the router(2911) and thus, it need some external chatty device to come up and for this, moment I connect any PC through that switch under access VLAN = i/f VLAN, i/f VLAN comes up.

Best Regards!

[mcardinal]

This is a bug as far as I can tell - I have been working on this almost all day for a school assignment. Here is why I say it's a bug:

If I create a VLAN and assign an IP, but don't assign a switchport to the VLAN, it shows as down:

Switch(config)#int vlan 40
Switch(config-if)#ip address 10.10.40.2 255.255.255.0
Switch(config-if)#end
Switch#show ip int bri | I Vlan40
Vlan40 10.10.40.2 YES manual down down

Then, if I write the changes to memory and assign the VLAN to a switchport, it comes up:

Switch#wri me
Building configuration...
Compressed configuration from 7383 bytes to 3601 bytes[OK]
[OK]
Switch#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#int gi1/0/17
Switch(config-if)#switchp mod acc
Switch(config-if)#switchp acc vlan 40
% Access VLAN does not exist. Creating vlan 40
Switch(config-if)#
%LINK-5-CHANGED: Interface Vlan40, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan40, changed state to up

Switch(config-if)#end
Switch#
%SYS-5-CONFIG_I: Configured from console by console

Switch#show ip int bri | I Vlan40
Vlan40 10.10.40.2 YES manual up up
Switch#

Then, if I reload the switch without saving the changes, so the new VLAN is no longer assigned to a switchport, the VLAN is still up after the reload.

Switch#reload
System configuration has been modified. Save? [yes/no]:no
Proceed with reload? [confirm]
Switch# show int status
Port Name Status Vlan Duplex Speed Type
Gig1/0/17 notconnect 1 auto auto 10/100BaseTX <--- changes were not saved, this port is back to default VLAN 1

Switch#show ip int bri | I Vlan40
Vlan40 10.10.40.2 YES NVRAM up up <----- VLAN is still up up after reload with no switchport assigned to it
Switch#

At least I now know that I am not insane and it's a problem with Packet Tracer. Hope this helps someone from wasting hours like I did!

Matthew

[Joseph W. Doherty]

"At least I now know that I am not insane and it's a problem with Packet Tracer."

Possibly two things you're unaware of with PT.

First, it often only offers a subset of the features found on the devices it provides.

Second, sometimes the features it does provide don't match the real world device results.

If these issues are a problem, in the Cisco portfolio, CML, for the most part, has neither issue.

[vishalbhandari]

@ashish7aditya This behavior happens because an interface VLAN (SVI) will only come up if there is at least one active access port in the same VLAN that is in an up/up state.

In your setup, both Switch#1 and Switch#2 have interface VLAN 2 configured, but SVIs remain down because no active ports exist in VLAN 2 initially. When you connect PC#1 to an access port in VLAN 2 on Switch#2, the switch detects an active port, bringing VLAN 2's SVI up. However, Switch#1 still has no active ports in VLAN 2, so its interface VLAN remains down.

Even though VLAN 2 is allowed on the trunk, a switch considers the VLAN as active only when it has at least one active (connected) access port in that VLAN. The reason SVI comes up only after traffic is generated is due to STP (Spanning Tree Protocol) or MAC address learning delay, which requires actual traffic to be detected before the VLAN is considered active.

To fix this on Switch#1, connect an end device (like another PC or a loopback cable) to an access port in VLAN 2, ensuring at least one active port exists. This will bring the SVI up, allowing communication between PC#1 and Switch#1.

[mcardinal]

Greetings, 

Thanks for your reply. It appears that you missed what I detailed in my post. I am able to bring up the VLAN by assigning a port to that VLAN, however, if I reload the switch without writing the config, the SVI is still up, with no port assigned to it. This is a packet tracer bug.