Interface is part of multiple LANs

IT_Joe · ‎11-10-2023

From what I understand, an interface can be assigned one VLAN. There is only one subnet per VLAN, so how can an interface have multiple subnets?

To elaborate, here is the output:

ESWITCH#show vlan

VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi1/0/1, Gi1/0/2, Gi1/0/3
Gi1/0/4, Gi1/0/5, Gi1/0/6
Gi1/0/7, Gi1/0/8, Gi1/0/9
Gi1/0/10, Gi1/0/11, Gi1/0/12
Gi1/0/13, Gi1/0/14, Gi1/0/15
Gi1/0/16, Gi1/0/17, Gi1/0/18
Gi1/0/19, Gi1/0/20, Gi1/0/21
Gi1/0/22, Gi1/0/23, Gi1/0/24
Gi1/0/25, Gi1/0/26, Gi1/0/27
Gi1/0/28, Gi1/0/29, Gi1/0/30
Gi1/0/31, Gi1/0/32, Gi1/0/33
Gi1/0/34, Gi1/0/35, Gi1/0/36
Gi1/0/37, Gi1/0/38, Gi1/0/40
Gi1/0/42, Gi1/0/43, Gi1/0/44
Gi1/0/45, Gi1/0/46, Gi1/0/47
Gi1/1/2, Gi1/1/4, Te1/1/1
Te1/1/2, Gi2/0/1, Gi2/0/2
Gi2/0/3, Gi2/0/5, Gi2/0/7
Gi2/0/8, Gi2/0/9, Gi2/0/11
Gi2/0/13, Gi2/0/15, Gi2/0/17
Gi2/0/18, Gi2/0/19, Gi2/0/20
Gi2/0/21, Gi2/0/22, Gi2/0/23
Gi2/0/24, Gi2/0/25, Gi2/0/26
Gi2/0/27, Gi2/0/28, Gi2/0/29
Gi2/0/30, Gi2/0/31, Gi2/0/32
Gi2/0/33, Gi2/0/34, Gi2/0/36
Gi2/0/37, Gi2/0/38, Gi2/0/40
Gi2/0/42, Gi2/0/43, Gi2/0/46
Gi2/0/47, Gi2/0/48, Gi2/1/2
Gi2/1/4, Te2/1/1, Te2/1/2
Gi3/0/1, Gi3/0/2, Gi3/0/3
Gi3/0/4, Gi3/0/5, Gi3/0/6
Gi3/0/7, Gi3/0/8, Gi3/0/9
Gi3/0/10, Gi3/0/11, Gi3/0/12
Gi3/0/13, Gi3/0/14, Gi3/0/15
Gi3/0/16, Gi3/0/17, Gi3/0/18
Gi3/0/19, Gi3/0/20, Gi3/0/21
Gi3/0/22, Gi3/0/23, Gi3/0/24
Gi3/0/25, Gi3/0/26, Gi3/0/27
Gi3/0/28, Gi3/0/30, Gi3/0/31
Gi3/0/32, Gi3/0/34, Gi3/0/35
Gi3/0/36, Gi3/0/37, Gi3/0/38
Gi3/0/41, Gi3/0/42, Gi3/0/43
Gi3/1/2, Gi3/1/4, Te3/1/1
Te3/1/2, Gi4/0/1, Gi4/0/2
Gi4/0/3, Gi4/0/4, Gi4/0/5
Gi4/0/6, Gi4/0/7, Gi4/0/8
Gi4/0/9, Gi4/0/10, Gi4/0/11
Gi4/0/12, Gi4/0/13, Gi4/0/14
Gi4/0/15, Gi4/0/16, Gi4/0/17
Gi4/0/18, Gi4/0/19, Gi4/0/20
Gi4/0/21, Gi4/0/22, Gi4/0/23
Gi4/0/24, Gi4/0/25, Gi4/0/26
Gi4/0/27, Gi4/0/28, Gi4/0/29
Gi4/0/30, Gi4/0/31, Gi4/0/32
Gi4/0/34, Gi4/0/35, Gi4/0/36
Gi4/0/37, Gi4/0/38, Gi4/0/39
Gi4/0/40, Gi4/0/41, Gi4/0/42
Gi4/0/43, Gi4/0/44, Gi4/0/45
Gi4/0/46, Gi4/0/47, Gi4/0/48
20 VLAN0020 active Gi3/0/26, Gi3/0/29, Gi3/0/39
Gi3/0/40
23 guest-wifi active
40 CALL-CENTER active Gi1/0/39, Gi1/0/41, Gi2/0/4
Gi2/0/6, Gi2/0/10, Gi2/0/12
Gi2/0/14, Gi2/0/16, Gi2/0/35
Gi2/0/39, Gi2/0/41, Gi2/0/44
Gi2/0/45
50 VIDEO active Gi3/0/44, Gi3/0/45, Gi3/0/46
Gi3/0/47, Gi3/0/48
60 VLAN0060 active Gi3/0/33, Gi4/0/33
75 VOICE_75 active Gi1/0/1, Gi1/0/2, Gi1/0/3
Gi1/0/4, Gi1/0/5, Gi1/0/6
Gi1/0/7, Gi1/0/8, Gi1/0/9
Gi1/0/10, Gi1/0/11, Gi1/0/12
Gi1/0/13, Gi1/0/14, Gi1/0/15
Gi1/0/16, Gi1/0/17, Gi1/0/18
Gi1/0/19, Gi1/0/20, Gi1/0/21
Gi1/0/22, Gi1/0/24, Gi1/0/25
Gi1/0/26, Gi1/0/27, Gi1/0/28
Gi1/0/29, Gi1/0/30, Gi1/0/31
Gi1/0/32, Gi1/0/33, Gi1/0/34
Gi1/0/35, Gi1/0/36, Gi1/0/37
Gi1/0/38, Gi1/0/39, Gi1/0/40
Gi1/0/41, Gi1/0/42, Gi1/0/43
Gi1/0/44, Gi1/0/45, Gi1/0/46
Gi2/0/1, Gi2/0/2, Gi2/0/3
Gi2/0/4, Gi2/0/5, Gi2/0/6
Gi2/0/7, Gi2/0/8, Gi2/0/9
Gi2/0/10, Gi2/0/11, Gi2/0/12
Gi2/0/13, Gi2/0/14, Gi2/0/15
Gi2/0/16, Gi2/0/17, Gi2/0/18
Gi2/0/19, Gi2/0/20, Gi2/0/21
Gi2/0/22, Gi2/0/23, Gi2/0/24
Gi2/0/25, Gi2/0/26, Gi2/0/27
Gi2/0/28, Gi2/0/29, Gi2/0/30
Gi2/0/31, Gi2/0/32, Gi2/0/33
Gi2/0/34, Gi2/0/35, Gi2/0/36
Gi2/0/37, Gi2/0/38, Gi2/0/39
Gi2/0/40, Gi2/0/41, Gi2/0/42
Gi2/0/43, Gi2/0/44, Gi2/0/45
Gi2/0/46, Gi3/0/1, Gi3/0/2
Gi3/0/3, Gi3/0/4, Gi3/0/5
Gi3/0/6, Gi3/0/7, Gi3/0/8
Gi3/0/9, Gi3/0/10, Gi3/0/11
Gi3/0/12, Gi3/0/25, Gi3/0/29
Gi3/0/30, Gi3/0/37, Gi3/0/38
Gi3/0/39, Gi3/0/40, Gi3/0/41
Gi3/0/42, Gi3/0/43, Gi3/0/44
Gi3/0/45, Gi3/0/46, Gi3/0/47
Gi3/0/48, Gi4/0/1, Gi4/0/2
Gi4/0/3, Gi4/0/4, Gi4/0/5
Gi4/0/6, Gi4/0/7, Gi4/0/8
Gi4/0/9, Gi4/0/10, Gi4/0/11
Gi4/0/12, Gi4/0/13, Gi4/0/14
Gi4/0/15, Gi4/0/16, Gi4/0/17
Gi4/0/18, Gi4/0/19, Gi4/0/20
Gi4/0/21, Gi4/0/22, Gi4/0/23
Gi4/0/24, Gi4/0/25, Gi4/0/26
Gi4/0/27, Gi4/0/28, Gi4/0/29
Gi4/0/30, Gi4/0/31, Gi4/0/32
Gi4/0/33, Gi4/0/34, Gi4/0/35
Gi4/0/36, Gi4/0/37, Gi4/0/38
Gi4/0/39, Gi4/0/40, Gi4/0/41
Gi4/0/42, Gi4/0/43, Gi4/0/44
Gi4/0/45, Gi4/0/46
88 SERVERS active
100 VLAN0100 active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup

VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 0 0
20 enet 100020 1500 - - - - - 0 0
23 enet 100023 1500 - - - - - 0 0
40 enet 100040 1500 - - - - - 0 0
50 enet 100050 1500 - - - - - 0 0
60 enet 100060 1500 - - - - - 0 0
75 enet 100075 1500 - - - - - 0 0
88 enet 100088 1500 - - - - - 0 0
100 enet 100100 1500 - - - - - 0 0
1002 fddi 101002 1500 - - - - - 0 0
1003 tr 101003 1500 - - - - - 0 0
1004 fdnet 101004 1500 - - - ieee - 0 0
1005 trnet 101005 1500 - - - ibm - 0 0

Remote SPAN VLANs
------------------------------------------------------------------------------

Primary Secondary Type Ports
------- --------- ----------------- ------------------------------------------

For example, Gi1/0/41 is part of vlans 1, 40 and 75. It is not a trunk port.

Port Name Status Vlan Duplex Speed Type
Gi1/0/41 notconnect 40 auto auto 10/100/1000BaseTX

I understand how it's in notconnect, and I verified this by trying the Ethernet connection (I don't get internet).

How can an interface be part of multiple VLANs? Is an interface always part of the default VLAN?

Richard Burts · ‎11-10-2023

Your post states that F1/0/41 is in vlan 1. I do not see anything in your post that shows membership in vlan 1 for that port. It would be interesting to see the configuration of that port. I suspect that the configuration will show membership in a data vlan and then also membership in a voice vlan. This is done to support connections to ip phones which have a data vlan and a voice vlan.

HTH

Rick

Kasun Bandara · ‎11-10-2023

@IT_Joe there is deafferent ways to assign VLANs to interface.

1. access ports. access ports can have 1 or 2 VLANS. its data vlan and voice vlan.

2. trunk ports. trunk ports can allow more vlans and it will keep vlan tag in sending packets.

as per your output, port Gi1/0/41 is part of vlan 40 and 75. if your port is showing under any vlan in '[#show vlan' command, that means its not port of trunk. so your port should be using data and voice vlans. share the '#sh run int Gi1/0/41' to see existing interface config.

Please rate this and mark as solution/answer, if this resolved your issue
Good luck
KB

MHM Cisco World · ‎11-10-2023

How can an interface be part of multiple VLANs? the port that config as trunk and part of multiple VLAN NOT appear in show vlan
Is an interface always part of the default VLAN? Yes any port than not config as trunk or access vlan X by default is part of VLAN 1 that why VLAN1 can not delete.

Joseph W. Doherty · ‎11-11-2023

"There is only one subnet per VLAN, so how can an interface have multiple subnets?"

BTW, you can have multiple subnets per VLAN.

Might be done with a L3 interface having secondary IPs or via additional L3 interfaces connecting to the same VLAN (the latter generally via additional L3 devices but might also be done on same device via VRFs).

Richard Burts · ‎11-11-2023

I agree with Joseph about the question in the original post "There is only one subnet per VLAN, so how can an interface have multiple subnets?" The use of secondary addressing is the obvious and workable way to have multiple subnets per vlan. He raises another interesting possibility, additional L3 interfaces connecting to the same VLAN. My first thought was theoretically possible but would it work in reality. And I thought that if you had a single vlan, perhaps it might be vlan 100 and in the first port was a device with IP 192.168.1.1, and the second port in the vlan had IP 192.168.2.2, and the third port in the vlan had IP 192.168.3.3 would we have multiple subnets in the vlan? But would that really work? Then I had another thought: what if vlan 100 ran through 3 switches, perhaps switchA, switchB and switchC. What is switchA had interface vlan 100 with IP 192.168.1.2, if switchB had interface vlan 100 with IP 192.168.2.1, and switchC had interface vlan 100 with IP 192.168.3.1. In that case vlan 100 would have 3 subnets and each subnet would work.

HTH

Rick

Karsten Iwen · ‎11-11-2023

I think this concept of multiple Subnets on one VLAN can be explained even easier:

Let's think of a switch without any config. All ports are in VLAN1. Now, we connect two PCs with IPs in network 192.168.1.0/24. Can they communicate? Yes! Next, we add two PCs with IPs in network 192.168.2.0/24. They can also communicate with each other. But the first and last two PCs can't yet communicate. (Yes, I ignore Broadcasts here). For that, we add IP addresses to an SVI, primary in subnet one and secondary in subnet two. All PCs get a default Gateway pointing to the switch IP in its own network. And voila, we have a setup far from best practice but still often implemented.

Joseph W. Doherty · ‎11-11-2023

@Richard Burts wrote:

Then I had another thought: what is vlan 100 ran through 3 switches, perhaps switchA, switchB and switchC. What is switchA had interface vlan 100 with IP 192.168.1.2, if switchB had interface vlan 100 with IP 192.168.2.1, and switchC had interface vlan 100 with IP 192.168.3.1. In that case vlan 100 would have 3 subnets and each subnet would work.

Rick provides a fine example of what I had in mind by my "or via additional L3 interfaces connecting to the same VLAN (the latter generally via additional L3 devices . . ." I also had in mind other variations too.

@IT_Joe keep in mind a VLAN is just a L2 domain, it doesn't limit itself to a single L3 network although that's, by far, the most common case.

@Karsten Iwen's example is also great too.