Questions tagged [vlan]

For questions about Virtual LANs (VLANs) which are used to divide what otherwise would be a single OSI layer-2 broadcast domain into multiple, logical OSI layer-2 broadcast domains.

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84 votes
4 answers
422k views

Why and how are Ethernet Vlans tagged?

I hear about VLAN tagging, but I don’t quite understand the concept. I know a trunk cannot accept untagged packets without configuring a native VLAN, and that access ports only accept untagged ...
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39 votes
13 answers
68k views

Difference between Subnet and VLAN?

You often hear the terms Subnet and VLAN used interchangeably. With the ubiquitous nature of IP these days, when are the two not considered roughly the same from a high-level, understanding that ...
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33 votes
1 answer
81k views

Using SNMP to retrieve the ARP and mac-address tables from a switch

I would like to get ARP tables from a switch to a syslog-ng server that has been set up on Ubuntu Server 12.04 LTS. I have read about SNMP and I know the server will act as a manager and the switch as ...
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  • 685
25 votes
5 answers
66k views

Why is 4096 the max value for VLANs

The Tag Protocol Identifier is set to 16 bits. In the CCNA certification it is said that it's not possible to have more than 4096 VLANs. 4096 in binary is 1000000000000000. Since 16 bits are allocated ...
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  • 353
24 votes
5 answers
29k views

Accidentally Removed Allowed VLANs from Cisco Switch Dot1Q Trunk

I am adding a new VLAN to an existing trunk port between two Cisco Catalyst switches (3750's). In the process of adding the new VLAN, it appears that I've removed the existing allowed VLANs on the ...
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  • 8,248
23 votes
9 answers
25k views

Why can't devices on different VLANs, but on the same subnet, communicate?

I have a question about switching. I have two devices connected to a switch with IP addresses 192.168.5.20 and 192.168.5.10. Both devices have the same prefix, /24. That means they are on the same ...
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  • 377
21 votes
7 answers
32k views

Introductory level explanation of VLANs

What's the basic use case(s) for VLANs? What are the basic design principles? I'm looking for something like a two paragraph executive summary style answer so I can determine if I need to learn ...
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21 votes
2 answers
70k views

vlan 0, 1, and 4095 are reserved: what are they reserved for?

This answer says vlans 0 and 4095 are reserved. Wikipedia agrees that vlans 0 and 4095 are reserved. This article says that Cisco reserves vlan 1 (in addition to vlans 0 and 4095). But I am having ...
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21 votes
6 answers
67k views

What, exactly, is required to make Airplay work across VLANs? [closed]

It seems like AirPlay works out-of-the-box, vaguely speaking, only within a LAN. It's not clear to me exactly why, but it looks like at least discovery relies on broadcasts. Wikipedia states that ...
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  • 884
20 votes
10 answers
5k views

Easily deploying new VLANs in a large environment

How do people handle the need to add VLANs to all switches on their network (or even a subset of devices)? We are adding around 6 new VLANs per week at the moment and as the network grows this is ...
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  • 1,675
19 votes
3 answers
82k views

Is the "default VLAN" simply the default native (untagged) VLAN on all interfaces that have no configuration?

... or does the "default VLAN" have some wider meaning? Also, can it / should it be changed? For instance, if a switch is going into part of a network that is only one VLAN and it's not VLAN 1, is ...
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17 votes
8 answers
18k views

When *not* to create a SVI for a L2 VLAN?

When creating VLANs for just L2 on a switch -- routing will be handled by a device within that VLAN such as a load-balancer -- it isn't necessary to create the vlan interface. As a matter of habit, I ...
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17 votes
3 answers
28k views

Adding a simple VLAN on a Juniper MX

after years away from JunOS, working with Foundry (now Brocade) routers, I find myself with a new Juniper MX10. All I want to do is create a simple VLAN such that two ports share one routing ...
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  • 171
15 votes
3 answers
19k views

Will duplicated MAC address give problems, if in different VLANs?

A small company has 2 Cisco Catalyst 3550, with 24 ports each, connecting their equipaments. After some trouble, we found that two NICs had the same MAC address. After the initial reaction of blaming ...
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15 votes
3 answers
7k views

Configure Cisco ASA in Transparent mode: Layer2 DMZ w/ Vlan translation

I am in the middle of a project to migrate some existing switched ethernet dot1q trunks behind an ASA firewall... these trunks have five vlans each (numbered 51 - 55). This is a simple drawing of the ...
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15 votes
1 answer
11k views

Should ARP requests be VLAN tagged?

I'm wondering if the ARP requests should be tagged when being sent out of a VLAN tagged interface? It seems logical to me that the ARP request should be tagged, since it needs to travel all the ...
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  • 298
14 votes
3 answers
48k views

How to Monitor Vlan Traffic?

We have migrated to a new ISP who provide us several services (Internet+VoIP+VoD) and have divided each service in one vlan 100,101,102 respectively. Now I'm looking for a solution to monitor each ...
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  • 2,018
13 votes
3 answers
35k views

Mac-flap due to roaming clients on wireless

We are running an Aerohove wireless network (controller-less).. I occasionally get mac-flaps within the VLAN tied to a SSID. I know this is coming from a roaming client.. The problem is that the ...
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  • 894
13 votes
3 answers
2k views

VLAN and subnet relationship

Many have pointed out that a VLAN can hold one or more subnets. If your default gateway for this subnet is on a router or a L3 switch then how could there be more than one default gateway in a VLAN ...
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  • 1,005
13 votes
3 answers
12k views

OSPF Stuck in Exstart Adjacency State

According to this article two routers will get stuck in the exstart adjacency state when their configured MTU sizes do not match. Does a VLAN tag affect this as well even if the routers involved are ...
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  • 311
12 votes
2 answers
11k views

VXLAN vs VLAN over layer 3

I read that one of the benefits of VXLAN over VLAN is that it can spawn across WAN and multiple layer 3 networks by creating overlay layer 2 networks. However AFAIN VLAN can also do the same using ...
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  • 123
12 votes
1 answer
35k views

Why should the native VLAN never be used?

Currently studying for a CCNA Security, I have been taught to never use the native VLAN for security purposes. This old discussion from Cisco forum states it very clearly: You should never use the ...
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12 votes
2 answers
7k views

Tracing TCN Messages in STP

We have about 20 vlans on a L2 network running Rapid PVST+ where the root bridge is a stack of Cisco's 3750 switches. I am a bit puzzled by the number of TCN notifications I receive on the switch.. ...
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  • 894
11 votes
11 answers
4k views

Locate a host on Network

What is the best method of locating a particular workstation on a VLAN ? I sometimes need to do this, if a workstation IP address, shows up on an ACL Deny torrent use High bandwidth usage (Top ...
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  • 467
11 votes
2 answers
3k views

IPv6 DHCP Server vs Router

I am setting up a testing environment for dualstack. I currently have a server (Windows 2008 R2), 2 switches and a router installed and configured. The server is the Primary Domain Controller and it ...
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  • 163
11 votes
2 answers
2k views

Handling VLAN based MPLS circuits with site specific internet access

I'm having difficulty wrapping my head around how to set this up and the MPLS vendor is being no help so I figured I would ask here. I have a 2 node MPLS each site having internet access on the same ...
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  • 111
10 votes
5 answers
6k views

VLANs over internet

How does VLANs work over the internet and span across multiple geographical locations? VLAN headers are removed when frames move to another switch or a router? How does it happen?
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10 votes
5 answers
4k views

WHY and HOW do routers and vlans break broadcast domains?

Everybody says routers (and vlans) break broadcast domains, but nobody goes into WHY that is, it seems. What's the router logic? Say I have three routers on my lan, with one main router and the ...
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  • 333
10 votes
2 answers
1k views

Can a switch that does NOT support VLAN capability be considered a managed switch if it has other features?

So the question is in the title. If a switch does not support Virtual Local Area Network capabilities can it still be considered a managed switch? What other features should it have to be considered ...
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  • 1,431
10 votes
4 answers
1k views

VLAN assignment philosophy

I have several Layer 3 switch/routers which are all connected via an OSPF routed network. Also attached to each switch are two other networks. I have to assign a VLAN to each of these networks, I ...
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  • 103
10 votes
4 answers
35k views

VRFs, VLANs and subnets: difference

I have a basic understanding of VRFs, VLANs and subnets. I understand that VLANs operate on L2, and subnets and VRFs (lite) on L3. What I don't understand, is why you would choose one over the other ...
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  • 205
10 votes
2 answers
2k views

Can a switch that does not support VLAN process traffic from a Trunk that is not the native VLAN

Recently encountered a setup where the engineer had a Multilayer Cisco switch with a trunk carrying VLAN 41 to an HP switch that did not support VLANs. What should the HP switch do with the 802.1q ...
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  • 469
10 votes
1 answer
541 views

Does L-ASA-SC-10= require a reboot?

We have some existing ASA-5555X in multiple context mode, and we are using one context per-vlan as a transparent layer2 firewall. Over time, we have been adding to this solution, and we're about to ...
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10 votes
3 answers
6k views

Setting up PIM Sparse mode

I am working on trying to setup our network for multicast traffic. In the coming months we will be installing new equipment that sends out multicast traffic. In the past we have only had 1 VLAN so we ...
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  • 453
9 votes
2 answers
2k views

Voice VLAN information

I am aware of the ability to show the access VLAN assigned to an interface on a cisco switch. How about showing specifically the voice VLAN an interface is assigned? #show run int fa1/47 interface ...
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  • 512
9 votes
1 answer
1k views

NAT VLANs with same IP Subnets

I have a VMware environment in which VMs are running a simulation suite. The software used has hardcoded IP addresses, about 10-15 VMs, and we are running multiple instances of this software each in ...
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  • 93
8 votes
5 answers
58k views

Switchport mode access VLAN & switchport mode trunk on one interface

Googling around to see if this setup on one port is correct (it doesn't look right to me), I'm not really seeing a straightforward answer. There is an interface with the following configuration: ! ...
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8 votes
3 answers
1k views

routing between vlans not working

I need to put a Cisco router behind an ISP router. (The ISP doesn't allow us to use their modem only.) I've called the ISP technical service and they've set up the connection with me. Their router ...
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  • 213
8 votes
2 answers
25k views

Which VLANs are allowed on this trunk?

I'm getting some mixed signals when configuring a trunk on a Cisco switch and I'd like to know which VLANs are actually being allowed on it. The way I want it to be is that only VLANs 1,10,20,30,99 ...
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8 votes
1 answer
2k views

SubInterface on a router

How many sub interfaces can be created on a cisco router to route between vlans on a connected switch?
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  • 83
8 votes
1 answer
1k views

What was the intended use for VLANs 1002, 1003, 1004, 1005? (fddi trcrf fddinet trbrf)

The output of show vlan brief includes VLANs 1002 - 1005: 1002 fddi-default active 1003 trcrf-default active 1004 fddinet-default active ...
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  • 127
8 votes
4 answers
41k views

Access Multiple VLANs from One Computer

I inherited a bit of mess and am preparing to redo our IP addressing scheme and also split our network into multiple VLANs. Possibly something like: 10.1.10.0/24 VLAN 10 VOIP 10.1.20.0/24 VLAN 20 ...
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  • 81
8 votes
2 answers
6k views

How is IEEE 802.1ad (aka VLAN Tagging, QinQ) valid, when the packets are too large?

Recently i've been dealing with MTU issues. And it all seems to stem from the fact that the ethernet adapter on newer computers default of a frame size of 1504 bytes: >netsh interface ipv4 show ...
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  • 185
8 votes
3 answers
12k views

Regarding Dot1X dynamic VLAN assignment

Situation: I am trying to get 802.1X working for me. I want RADIUS server to dynamically assign VLANs to ports based on RADIUS reply attribute for particular user. I have an HP E2620 switch and a ...
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  • 837
8 votes
2 answers
1k views

carrier's vlan tags overlap my own

I have a 3 site network: one is a data center colo and the other two are offices. The new Metro Ethernet service we just installed requires the colo site's traffic destined to the offices to be in ...
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  • 450
8 votes
1 answer
212 views

What is VLAN Hopping

I am starting on my journey into the world of Networking and have come across a term called VLAN hopping. The sentence I read was that if the Native VLAN on two switches connected to each other via ...
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8 votes
4 answers
8k views

How do I set up a VLAN between an ESXi VM and Physical Hardware? [closed]

First of all, I am a total noob. The extent of my VLAN experience is setting up a guest network on a wireless router. I have recently gotten a request to set up a public network requiring filtering ...
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  • 81
8 votes
1 answer
2k views

Native Vlan Cisco Catalyst and Layer 3 switch

I have run into an interesting issue setting the native VLAN between one of my Cisco Catalyst 2960 and my Layer 3 switch (third party). Here is the setup, the layer three switch is set up to run on ...
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  • 403
8 votes
1 answer
283 views

Upgrading our Network to Managed Switch with Internet Routing

I am a business owner with multiple BT Buisness Broadband lines. We have two sets of VOIP lines and two sets of PC's, each with there own switch board. The problem that we have is that our lines go ...
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8 votes
3 answers
597 views

Map the underlying structure of a VLAN

Is there any way to detect the underlying layout physical layout of a VLAN. For instance, to know whether two devices in the same VLAN are connected via the same wifi AP. To clarify, this would be ...
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