mac in two fdb's

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I have a combination of 460/440s in my network. We have a data(computer) vlan and a phone vlan.
Create vlan data tag 60
Create vlan phone tag 61
Conf vlan phone add port 4:21 tagged
Conf vlan data add port 4:21 untagged
configure lldp port 4:21 advertise system-capabilities
configure lldp port 4:21 advertise vendor-specific med capabilities
configure lldp port 4:21 advertise vendor-specific med power-via-mdi
configure lldp port 4:21 advertise vendor-specific dot1 vlan-name vlan Phone
configure lldp port 4:21 advertise vendor-specific med policy application voice vlan Phone dscp 46

When I plug in the phone it will get DHCP for a phone ip-address but the mac for that phone will show up in the fdb for both the data vlan and phone vlan.

Is this the expected result? I would assume that we don’t want all the data packets coming down to the phone.

Slot-1 # show fdb | include 4:21
c4:64:13:01:02:7d Data(0060) 0010 d m 4:21
c4:64:13:01:02:7d Phone(0061) 0043 d m 4:21

.1 # show iparp | include c4:64:13:01:02:7d
VR-Default c4:64:13:01:02:7d 2 NO Phone 61 55
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christopher madison

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Posted 3 years ago

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Brandon Clay, Escalation Support Engineer

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Hi Christopher,

This looks like it is normal behavior. I would expect the phone's MAC to show up in both VLANs, since it is acting as a switch to pass the PCs data.

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Paul Russo, Alum

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Hey Christopher

I agree it is normal for it to be in both VLANs to start.  When it first comes up and request DHCP it will do it over the untagged data VLAN.  Once it moves over to the voice VLAN it should release and renew its DHCP and will use the tag information that it has for the Voice VLAN and get the Voice VLAN IP address.

I would expect that after 5 min, the timeout for the fdb, and it sould only be in the voice VLAN.

Let us know if not.

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Johan Hendrikx

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this is how it works in our network.

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Nicolas Dreher

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I agree : it's completely normal to find the phone's MAC address in fdb for both VLAN as long as the phone's MAC is the only L2 interface directly connected to the switch port (the switch receives traffic for both VLANs coming from that MAC address).

But what you describe Paul, is the behavior of a phone without using LLDP MED. With LLDP MED active - that's the case for Christopher - the phone takes it's IP address directly in the "phone" VLAN subnet, what doesn't change the answer given hereabove.
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christopher madison

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When the phone boots it goes into the data vlan first and gets a data IP address. then after LLDP talks. it adds the phone vlan into the fdb. It also pulls a voice IP address. Eventually the IPARP table times out the Data IP address. however the data mac fdb entry continues to renew and never clears.
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Prashanth KG, Employee

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Hi Christopher, 
Thank you for the clarification. 

What happens if we manually clear the fdb on the port after the phone gets the IP address in the voice VLAN? Does it re-learn in data vlan after clearing?
If so, we need to check if there is a traffic coming into the switch port as an untagged packet with this source-mac-address. 

From the switch point of view, it will learn or refresh the mac-address in data VLAN whenever the untagged packet reaches the switch with the phone's mac address. 

Please let us know!