Connecting unmanaged switch to managed switch

[paranengmoose]

Hi, I am trying to understand how my customer's setup is working properly.

I told them it would not work, but they proceeded to buy the unmanaged switch and it is now working.

So embarrassing.

Here is a brief diagram of the setup.

Phone ---- Unmanaged switch ---- managed switch

On the interface of managed switch, I have access vlan 100 and voice vlan 5 configured.

In my understanding, the managed switch should drop if the frame is not tagged with vlan 100 or vlan 5.

But when I connected the phone to the unmanaged switch, it got all information from CUCM and can make outbound and inbound calls.

The models of unmanaged switch and managed switch are POE-SW800-P4 and Cisco Catalyst 2960X.

Please advise.

Thanks

[Carlos Villagran]

Hi!

Between the unmanaged SW and the Managed SW do you have a trunk configured?

When you say 'The interface of managed switch' you mean SVIs?

When Phone boots it sends a broadcast to DHCP listening servers which are in its same broadcast domain unless helper address is configured in a node, if this is the case, probably there is something listening in the same broadcast domain as the Phone and of course replying or redirecting to the DHCP server, however more info is needed since there are many scenarios.

Best regards!

JC

[paranengmoose]

Hi Carlos,

it is the physical interface, which is gigabitethernet 0/x.

Trunking was disabled on the interface with the command switchport mode access.

Thanks

[Carlos Villagran]

Hi!

Phones are getting into the access vlan 100 and getting IP from that VLAN, since the phone have no way to know which vlan is the voice VLAN (due to CDP/LLDP negotiation stuff, Phones are connected in the Unmanaged SW) so yeah, if your Call Manager is able to provide DHCP service to VLAN 100 that is probably what is happening. 

Hope it helps, best regards!

JC

[Joseph W. Doherty]

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Posting

In my understanding, the managed switch should drop if the frame is not tagged with vlan 100 or vlan 5.

Well, your managed switch might allow for native, i.e. untagged, frames too.  If fact, if an access port is only set to access vlan 100, vlan 100 frames, by default, would be sent out untagged.  Only the voice vlan 5 packets would be expected to be tagged.

Although your port might be configured for voice vlan 5, it's possible your phone is not tagging its frames and is running over vlan 100.  If so, that would easily explain why the unmanaged switch doesn't cause a problem.

It's also possible, your unmanaged switch passes vlan 5 tagged frames, as is, especially if the frames are not maximum size.

[devils_advocate]

An unmanaged switch has no concept of Vlans, it simply sends frames untagged out all its interfaces, including those to other switches.

The switchport you have is expecting two types of frames, one Untagged and one tagged. The Access Vlan is 100 so any frames it receives which are untagged, it will assume they are part of this Vlan. The voice vlan is 5 so its expecting tagged frames in this vlan but as the traffic is transiting an unmanaged switch, I don't believe it is going to receive any tagged frames.

Has the phone picked up an IP address in the voice vlan or in vlan 100?