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44 votes

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

One of the things VLAN's do is take a physical switch and break them up into multiple smaller "virtual" switches. Meaning this Physical depiction of One switch and Two VLANs: Is identical in ...
Eddie's user avatar
  • 15k
30 votes

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

The whole point of Virtual LAN, is to create separate Layer 2 LANs on a single physical device. It is like building an armored and sonic-proof wall in a room to create 2 rooms. The people in each ...
JFL's user avatar
  • 19.7k
26 votes
Accepted

WHY and HOW do routers and vlans break broadcast domains?

Let's talk about it using this topology of three networks (red / orange / blue): A Router's primary function is to facilitate communication between IP networks. Which means if A wants to speak to D ...
Eddie's user avatar
  • 15k
25 votes

VLANs over internet

VLANs are a layer 2 construct, so they only exist on your local network and end at your router. The Internet has no concept of VLANs.
Ron Trunk's user avatar
  • 67.6k
21 votes
Accepted

Should ARP requests be VLAN tagged?

Whether the frame is ARP or not is irrelevent. Anything sent out a trunk port is tagged with the VLAN the frame is a member of: ARP is sent by the client, the client typically has no knowledge of ...
Eddie's user avatar
  • 15k
18 votes
Accepted

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

Yes. A managed switch is a switch you can configure in some way or other. Whether it supports VLANs or not is not the question. Even a switch (or a hub for that matter) that only provides status ...
Zac67's user avatar
  • 84.8k
17 votes
Accepted

VXLAN vs VLAN over layer 3

Yes, from the packet switching point-of-view, VXLAN is just a matter of sticking some encapsulation on top of an L2 frame: something that other protocols do as well. The real difference it makes is ...
mere3ortal's user avatar
  • 2,416
16 votes
Accepted

Why should the native VLAN never be used?

You can and most likely will need to use a native VLAN on your trunk ports, at least on Cisco switches, other vendors do it differently. But what you have to remember that the security risk is more to ...
SleepyMan's user avatar
  • 2,016
13 votes

VLANs over internet

VLAN trunks as we know them from campus or enterprise networks cannot be extended "natively" across the Internet or any other routed network (see Ron Trunk's answer). Encapsulation/Tunneling ...
Marc 'netztier' Luethi's user avatar
12 votes
Accepted

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

Those are "translation" VLANs for bridging ethernet and FDDI / Token-ring. You'll only find this hardcoded nonsense on older IOS switches. (It took Cisco eons after they stopped making FDDI ...
Ricky's user avatar
  • 32.2k
11 votes
Accepted

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

A switch not supporting 802.1Q tags should drop tagged frames. However, many simple switches don't comply to 802.1Q at all and they forward tagged frames just like untagged ones - for the most part ...
Zac67's user avatar
  • 84.8k
11 votes

routing between vlans not working

You don't have NAT enabled (you don't have ip nat outside on Vl 20). Without NAT, the ISP doesn't know how to route back to your 192.9.200.0 network. If you NAT to the VLan 20 address, the ISP ...
Ron Trunk's user avatar
  • 67.6k
10 votes
Accepted

How can hosts on two different VLANs communicate?

Two separate VLANs must communicate through a layer-3 device, like a router. Devices on a VLAN communicate with each other using layer-2. Layer-3 must be used to communicate between separate layer-2 ...
Ron Maupin's user avatar
  • 99.8k
10 votes
Accepted

VRFs, VLANs and subnets: difference

Each fills a different purpose, and all three may be part of an overall solution. Let's start with the oldest concept first. Subnets are the IP world's way of determining what devices are "...
Peter Green's user avatar
  • 13.3k
10 votes
Accepted

What's use of Port Isolation vs traditional VLANs?

Port isolation -also called private VLAN (thanks @Stuggi)- is a very useful feature for switches that connect end users. In a typical network you will have many end-users computers grouped together ...
JFL's user avatar
  • 19.7k
10 votes
Accepted

How not to route on a Cisco router

While Ron and Ron's answers actually enable routing but prevent traffic, their suggestions still pack both host groups into the same routing instance. There's another way to think about this: a) ...
Marc 'netztier' Luethi's user avatar
10 votes
Accepted

MAC Address learning process

It depends on the manufacturer. For Cisco switches: Switch port configured as access and frames received tagged on different VLAN Frame is dropped Switch port configured as trunk and frames ...
Ron Trunk's user avatar
  • 67.6k
10 votes
Accepted

VLANs over internet

Normally VLANs are only used within a site or at least within a network that is controlled by a single entity. It is possible, to carry encapsulated Ethernet traffic (potentially including VLAN ...
Peter Green's user avatar
  • 13.3k
9 votes

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

Complementary to the existing answers, which cover the question from a design and theory point of view ... Instead of asking "why don't they communicate?", let's ask "what happens when they try to ...
jonathanjo's user avatar
  • 16.3k
9 votes
Accepted

Stretching VLANS across a network infrastructure

That is actually overly broad. There is nothing technically wrong with VLANs being used over a large network area as long as the network is designed properly. In modern networks, you can use more ...
FrameHowitzer's user avatar
8 votes

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

Vlan 0 does have an use. When a client requires CoS, but doesn't know the vlan, it tags its traffic with VLAN 0, which is recognized by the switch to mean the default untagged vlan of that port, ...
flow in's user avatar
  • 81
8 votes
Accepted

How does link aggregation work in reality?

What really happens is that any one flow only uses one of the links. Different flows are assigned to different links based on a hashing algorithm, so, in aggregate, you get the full bandwidth of the ...
Ron Maupin's user avatar
  • 99.8k
8 votes
Accepted

The best way to understand Vlans?

A broadcast domain / L2 segment / VLAN has several weaknesses and is prone to a number of attack vectors, most importantly: any host can directly talk to any other host broadcasts from any host reach ...
Zac67's user avatar
  • 84.8k
8 votes

VLAN Bandwidth Issues?

VLANs themselves don't advertise. The services running on the VLANs might. Simply extending existing production VLANs across the VPLS link might propagate "garbage" (esp. unwanted broadcasts), so best ...
Zac67's user avatar
  • 84.8k
8 votes
Accepted

NAT VLANs with same IP Subnets

With a bit of VRF-lite and VRF-aware-NAT and the help of the Cat-3850's routing capability, here's some config snippets that should work, or at least get you halfway there - all based on the diagram ...
Marc 'netztier' Luethi's user avatar
8 votes

Posible to connect VLAN switch through dumb switch?

An unmanaged switch will only have one VLAN. Some unmanaged switches will drop tagged frames as damaged, others will strip the tag, and some will simply pass the frames unchanged. Unless you try it, ...
Ron Maupin's user avatar
  • 99.8k
8 votes
Accepted

What is VLAN Hopping

VLAN hopping is when a client (unauthorized node) can add VLAN tags to its frames to enter VLANs that it's not supposed to be in or when it can generate MVRP/GVRP frames to make its port a member of ...
Zac67's user avatar
  • 84.8k
8 votes
Accepted

Switching interface VLAN ID Mid-Production

I was wondering if there was a way to switch out the VLAN ID from VLAN1 to VLAN10 without impacting the connectivity of the end users? No. First, you must remove the addressing from VLAN 1 before ...
Ron Maupin's user avatar
  • 99.8k
7 votes

Why do we use VLAN?

Besides the unknown unicast flooding that you allude to, broadcasts are necessary to the operation of ethernet. Some protocols require broadcasts. For instance, hosts use ARP requests (broadcasts) to ...
Ron Maupin's user avatar
  • 99.8k

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