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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83
votes
4answers
418k 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 ...
37
votes
13answers
64k 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 ...
33
votes
1answer
75k 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 ...
13
votes
3answers
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 ...
18
votes
3answers
76k 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 ...
15
votes
3answers
6k 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 ...
6
votes
2answers
621 views

VLAN-Subnets 1:1?

I often hear people saying that each VLAN is a subnet (or is associated with a subnet) but is that really true? Assume I have 10 big offices. I will have a subnet for each office but only 2 VLANs ...
14
votes
3answers
46k 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 ...
2
votes
1answer
2k views

What is the purpose of native VLAN on a trunk link?

Most VLAN articles I've read recommend avoid using VLAN1 for ports with untagged traffic (aka access ports). I've heard enough of this nonsense. Except for the reserved values of 0x000 and 0xFFF, a ...
5
votes
2answers
4k views

Does a switch MAC address table sync across all switches in the same VLAN?

Ok, I have read up about this 16K limit for the MAC address table, or better known as the CAM table, on certain switches. Lets say I have 14 different edge switches with that 16K limit, and I have ...
8
votes
2answers
5k 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 ...
7
votes
2answers
7k views

Dell PowerConnect 5524 VLAN routing

I have a Dell PowerConnect 5524 that I'm trying to configure. What I want is simple routing between VLANs and the user guide is less than helpful. On page 220, it says (or implies) that all you need ...
5
votes
2answers
542 views

How does link aggregation work in reality?

I'm concerned that I'm misunderstanding how aggregators work in the scenario below: I have a project where I need 40 Gbps links between switches. My plan was to have my three switches use all four of ...
2
votes
2answers
6k views

Routing Between 2 different vlans but same subnet

I have a basic doubt understanding vlan communication. Scenario: Let's say a single switch and has been configured with 2 vlans (vlan 100 and vlan 200). Vlan 100 connects to lets say 192.168.100.0 ...
24
votes
5answers
62k 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 ...
19
votes
6answers
62k 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 ...
23
votes
8answers
19k 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 ...
21
votes
7answers
31k 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 ...
10
votes
4answers
30k 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 ...
5
votes
2answers
4k views

The best way to understand Vlans?

Every time i try reading about the configuration of a device and adding it to the network, i always face a problem. The problem is basically am not able to understand Vlan's, and how for example some ...
12
votes
3answers
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 ...
11
votes
11answers
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 ...
10
votes
2answers
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 ...
4
votes
1answer
660 views

Juniper EX4200 how to select s-vlan from c-vlan on dot1q-tunneling access port?

How do I tell Juniper EX4200 to push specific S-VLAN based on C-VLAN on q-in-q (dot1q-tunneling) port? For instance I want to instruct EX4200 to perform Q-in-Q using these rules: 1) If a single-tag ...
13
votes
3answers
33k 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 ...
8
votes
1answer
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 ...
7
votes
4answers
22k views

Access-Lists and VLAN and understanding traffic flow

I have a question that I am having a very hard time wrapping my mind around, and I think you might be able to square me away. When applying an access list to a VLAN interface (VLAN 32) in a L3 switch,...
4
votes
4answers
5k views

VLANS vs. subnets for network security and segmentation

What are the security implications when using subnets as opposed to VLANs for segmenting an enterprise network? We have strict security requirements surrounding the data we handle and need to ensure ...
3
votes
1answer
801 views

Multiple Subnets in a VLAN

Is it advisable to have multiple subnets with in a VLAN? If at all ports assigned with differently subnetted addresses are used in a VLAN, will they be able to communicate? Assume the different ...
3
votes
2answers
4k views

VLAN membership - Tagged and Untagged

I have never used VLANs before, so I'd like confirmation that it will work as I think. We have three Netgear GS724T and a fortinet router. If I configure a port to be Untagged on VLAN 1 with a PVID ...
2
votes
1answer
64 views

Multiple routers with one long range antenna

Wifi routers have a limit on the number of clients that can connect, Assuming i have many routers and i connect them together via wireless bridge, then i connect one single long-range antenna that ...
2
votes
1answer
3k views

Relation between Ethernet and 802.11 headers

I would like to ask about the relation between 802.11 and Ethernet L2 Link layer. Are they dependent of each other? Or can they be seen as independent L2 link layers (with only similarities in ...
8
votes
1answer
1k 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 ...
5
votes
3answers
60k views

How can hosts on two different VLANs communicate?

It is clear that hosts on two different VLANs can't directly communicate on a single switch, but how do they communicate to each other at all? Do they use a router? How does it work?
4
votes
1answer
10k views

Native VLAN Mismatch between two access ports on cisco switches

Ok, I have a cable between two cisco switches. The port on switch A is configured as switchport access vlan 2 and the port on switch B is configured as switchport access vlan 3. For the sake of the ...
4
votes
1answer
239 views

Moving to vlans from a basic network

I've currently got a network setup like this. Layer 1 2 cable modems -> Peplink Load Balancer 1 -> 192.168.4.0/24 for wifi and clients Layer 2 1 connection from the Peplink + Fiber -> Peplink Load ...
4
votes
2answers
2k views

Some question about the relationship between VLAN , DHCP and subnet

I am new here and very interested in computer networks. From a book, I have some questions, for which I need help. question1: Can there be only one working DHCP server in one VLAN? question2: Can ...
4
votes
1answer
2k views

What happens with tagged traffic on access port of Cisco switch?

I am broading up more this unanswered question: VLAN tagging and access port Let's assume I am trying to accomplish Double Tagging Attack. From my understanding, prerequisites of this attack is to be ...
4
votes
6answers
34k views

Connecting two VLANs together on a single switch

I'm doing some experiments and I want to know if it is possible to connect two VLANs on a single switch by using just a cable between one group of ports and the other. My goal is to simulate (using ...
3
votes
2answers
14k views

Remove interface VLAN 1 in cisco 2960 switch

From what I understand, there are protocols like VTP and CDP that no matter what will use VLAN 1. CDP and VTP are layer 2 meanwhile interface VLAN is layer 3. So, I wonder why I cannot delete ...
3
votes
2answers
16k views

What is VLAN1 interface in routers?

Yesterday I had a look at my router configuration, and I saw that my office's local LAN gateway's IP address is written on the router's VLAN1 interface. What is a VLAN1 interface in a router? Is it ...
3
votes
3answers
6k views

Cisco 3750 configuration for static routes per VLAN

I have a router that has 4 VLANs configured, with an interface on each VLAN, the router serves as a gateway and provides internet access. VLAN | Interface | Function --------------------------...
3
votes
1answer
835 views

Routing Between VLANs on Cisco 1921

I have setup a new VLAN for VoIP phones that are being installed. I would like to segment their traffic to Subnet 10.20.77.0/24. I added an IP address to the DHCP server of 10.20.77.11 to help serve ...
2
votes
1answer
159 views

Upgrading Cisco 819 to 809, can I connect devices to 2 routed ports and be on same subnet?

The company I work for has multiple remote "IoT" style sites connected back to the datacenter via a Cisco 819 cellular router. We are in the process of upgrading these to the newer Cisco IR809 routers ...
2
votes
3answers
4k views

Why is VLAN tagging required?

Again a basic question. Switches communicate information to each other regarding VLANs using VTP, so each switch is aware of the VLANs. It's MAC address table is as one of the follows. Now, why is ...
2
votes
1answer
315 views

HP V1910 - Why is tagged traffic going to VLAN1?

HP(3Com) V1910 switch with 2 vlans: VLAN1 (native) and VLAN191 The Trunk Port is configured like this: Why are frames tagged for '191' also being forwarded to VLAN1? Any ideas. Thanks
2
votes
2answers
122 views

Wireless LAN Controller

I have a Cisco 2500 series WLC, and a 1700 series LWAP. In this case, what provides the AP the DHCP IP address, my router or the WLC?? There is already a PoE port in the WLC, yet when I connect my ...
2
votes
1answer
5k views

How to configure DHCP pools for 2 VLANs/subnets sharing a switch

I've set up DHCP pools on a layer 3 switch that goes out to 13 layer 2 switches. Most of which have their own subnet and VLANs, and everything works fine when that's the case. But when there are two ...
2
votes
2answers
4k views

Aruba 2530 - IGMP Enabled, Querier on still no filtering

I have a in port 1 - 5 channels of multicast. On port 2 i have a computer. Both are in VLAN 10 untagged and a more bigger network tagged on port 3. I've enabled IGMP with: config, vlan 10, ip igmp, ...
2
votes
2answers
975 views

Designing OSPF mesh network

We have bought two new switches that will be used as the core in our network. They will be directly connect with some LACP link. The aggregation layer consists of 5 L3 switches that will have one ...