Questions tagged [switch]

For questions about network switches, the term commonly refers to a multi-port network bridge that processes and routes data at the data link layer (layer 2) of the OSI model.

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30 votes
4 answers
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How does a switch learn a switch table?

Say the switch table is empty. If computer A sends a frame destined to computer B, the switch will broadcast asking who has the mac address of B. What if C suddenly sends a frame to A? What is the ...
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  • 825
20 votes
4 answers
15k views

Why is the CAM table in a switch called CAM table and not MAC table even though it holds MAC addresses?

I know the CAM table in a switch holds MAC addresses and the ports that are associated with the respective MAC addresses. There are no such thing as CAM addresses from my knowledge, so why is it ...
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  • 1,441
33 votes
1 answer
82k 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
3 votes
1 answer
263 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 ...
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40 votes
4 answers
39k views

Bits per second vs. Packets per second

Lately I have been checking datasheets of several switch models from diferent vendors. For a given switch, vendors publish a couple of figures that I think are a measure of the capacity/performance of ...
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11 votes
4 answers
22k views

Two hosts of different subnets connected to switch

Two host of different subnets, let it be A - 10.1.2.1/24 and B - 10.1.3.1/24 be connect to the same switch S. Can they communicate? So as far I understand: A sends a packets with IP of B S receives ...
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  • 113
12 votes
1 answer
4k views

Mac / CAM entries from non-Cisco switches with Q-BRIDGE-MIB

How do I do community string Indexing for other switches like D link DES 3526 and Brocade Fastiron SX800? I am running a java program to poll a specific vlan on a Brocade sx 800 l3 switch. The ...
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19 votes
2 answers
25k views

Ethernet autonegotiation differences between (10M | 100M | 1G) Ethernet

I am studying for CCNA and on Wendell Odom's book is said that(regarding autonegotiation): When autonegotiation fails on one node, to choose (half/full-duplex) we must use the rule: If you have a 10/...
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46 votes
6 answers
104k views

Difference between control plane, data plane and management plane?

I think I understand these concepts, but I'm a little rusty. Can someone give a concise, easy-to-understand explanation of these concepts? The planes are logical concepts, aren't they? Is this a Cisco ...
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  • 950
14 votes
3 answers
49k views

How does a switch learn a MAC address not in its lookup table?

I have a relatively dumb question. Suppose the Switch just started, and it received a frame that contains a destination MAC address for a network device not in its MAC addresses table. What happens ...
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8 votes
3 answers
6k views

Does a switch understand packets? If yes, what is the frame terminology for?

I was a little confused between packet and frame. The confusion was based on, if a switch is a layer 2 device, how does it reads a packet from a layer 3 device, and where is the 'frame' thing which is ...
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  • 83
14 votes
3 answers
3k views

Is there such thing as a network bridge?

Is there such a thing as a network bridge or is it always meant as a switch implementing a bridging function? Whilst bridges are described as devices that create separate collision domains in network ...
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12 votes
2 answers
39k views

What purpose is the Uplink port in layman's terms and do I need it?

I have an old Linksys switch (EZXS55W) and it has worked fine for many years now, but I tried to hook something up today to the last open port and noticed the device didn't work when I did this. When ...
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  • 223
10 votes
3 answers
41k views

Spanning tree Default Priority Values

This question is a repost of the same question in The Cisco Learning Network; however, the answers are unique to Stack Exchange. I have always accepted that the defualt priority value of STP (802.1d) ...
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  • 1,358
10 votes
3 answers
1k views

What is a "Layer 3 Switch"?

What is a "Layer 3 Switch"? I essentially understand the OSI model, well maybe I don't and maybe that is why I don't understand. Its just that I don't see how/when/if a "managed switch" becomes a "...
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3 votes
1 answer
5k 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 ...
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  • 401
14 votes
2 answers
14k views

Why does "show interface" show "Last input never" when packets input is incrementing?

Why is it that when I perform a show interface command I see that Last input always shows never? FastEthernet1/31 is up, line protocol is up (connected) Hardware is C6k 100Mb 802.3, address is 000d....
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  • 2,057
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
5 votes
2 answers
6k views

How does a layer-2 switch work with UDP?

If UDP and/or TCP send packets via IP to an Internet Protocol Address, how can a layer-2 switch forward these? Is this possible or do you need a layer-3 switch?
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  • 614
5 votes
2 answers
5k 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 ...
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  • 205
1 vote
1 answer
5k views

HSRP issue both device active stats

Here is the Diagram. Ip address: ISP: [VIP]- 10.107.25.5 [R1] - 10.107.25.6 [R2] - 10.107.25.7 Me: [VIP] - 10.107.25.10 [SW1] - 10.107.25.11 [SW2] - 10.107.25.12 Configuration on port: SW1 ...
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  • 4,247
9 votes
3 answers
26k views

Assign an IP address based on switch port

Short: I want DHCP server to assign a preconfigured IP address based on a port (on a switch, ideally) a device is connected to. Long: My situation: I am building an automated indoor farm (for ...
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  • 193
6 votes
3 answers
2k views

Circuit and Packet Switching

I'm struggling to understand 1 or 2 things about packet and circuit switching. In circuit switching, resources are not shared and a it's guaranteed full capacity to the call right? They are "reserved"...
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  • 63
6 votes
3 answers
4k views

Is there any reason to use a hub over a switch?

Given that hubs are essentially the same as switches, as far as I know, except they share bandwidth between all connected devices, while switches do not, resulting in faster connections. I am simply ...
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5 votes
2 answers
330 views

Why FastEthernet STP Link Selected over EtherChannel?

What are the possible reasons why STP would choose to use a FastEthernet link over an EtherChannel? Below is a network diagram where this is happening.
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5 votes
5 answers
451 views

What blocks a computer from another network in the same switch from answering an ARP request?

Let's suppose I have 3 computers A, B and C connected to the same switch with only the native VLAN active. A and B belong to the same network, let's say that their IP addresses are 192.168.1.1/24 for ...
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4 votes
3 answers
1k views

Collision domain question

I'm studying for my CCENT exam, and not quite sure why the book (official Cisco) says that there are 5 collision domains in this example. Isn't it supposed to be 4? The hub on the left, the hub in the ...
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3 votes
2 answers
9k 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 ...
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3 votes
1 answer
364 views

question about switches and frame and forwarding table

hey guys was wondering if someone can potentially explain this to me but lets say switch-A just learned a Ethernet address on port 4 (00:0f:4b:02:63:ae) based on the forwarding table now a station ...
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3 votes
1 answer
2k views

What is the point of a subnet mask on a local network?

I have a question regarding subnet masks and my understanding (or misunderstanding of them). So when a subnet mask is used, a bitwise AND is performed using the subnet mask and the IP address(es). ...
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2 votes
6 answers
2k views

Is a "LAN" a layer 2 or a layer 3 concept, in TCP/IP?

I think my question is purely theoretical and a matter of definitions; it derives from the fact that a L2-switch with VLAN capabilities is able to "create" different virtual LANs, and thus I am led to ...
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1 vote
2 answers
6k views

Forced full duplex and autonegotiation

Let's say I have two connected switches. I specifically configure (force) one of them to use full duplex and the other one I configure to use autonegotiation. Why does this config fail and the autoneg....
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  • 269
1 vote
1 answer
1k views

Layer 2 Switch Mac Address

I know a layer 2 switch floods the frame out asking who has a certain mac address. My question is how does it know what mac address to ask for in the first place? Scenario: A brand new switch is ...
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  • 11
42 votes
15 answers
81k views

How can I diagnose a bridging (ethernet) loop?

Given that spanning tree has failed (or you don't have any spanning tree) and get an ethernet loop, what's the best way to diagnose where the problem is? Which switch?, which cable? and so on.
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  • 753
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
9 votes
3 answers
7k views

Generate list of subnets on switch using SNMP?

Is anyone familiar with an approach to generate a list of subnets on a particular switch (let's pick a more popular one - Cisco) using SNMP? How about a list of VLANs? If so, can you point me to the ...
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19 votes
5 answers
18k views

Should portfast be used on a port potentially connecting to an unmanaged switch?

I understand the basics of how spanning tree works and why you would want to use portfast on user access ports. When dealing with a topology with a large number of dumb switches under desks and ...
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13 votes
6 answers
328k views

Finding Non-Cisco End Device IP Addresses Connected to Cisco Switch

I have Cisco catalyst switch, with many VLANs and address ranges. How do I find IP addresses connected to the switch that are not Cisco devices?
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  • 812
8 votes
6 answers
2k views

How Does A Layer 2 Switch Differentiate Between Different Networks?

I know that different IP-based networks can be connected to the same layer-2 switch. and I know each network's nodes can communicate just fine, however, no normal traffic can occur between two nodes ...
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  • 183
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
1 answer
28k views

Non-Cisco SFP's defaulting to an err-disable state in 2960s and 2960 PoE Switch

I recently came across an issue while setting up a stack to replace an older M switch setup. I stacked 3 switches - 2 Cisco 2960's and 1 C2960 PoE using SFP cables to their backplane. My plan was to ...
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  • 1,669
11 votes
2 answers
2k views

How did I break (half of) my network?

I'm looking for some post-event advice so this event doesn't happen again. We have a network core of two Cisco 4500x switches, configured for VSS redundancy. From those, we have iSCSI devices, our ...
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  • 243
11 votes
2 answers
938 views

Transaction based CLI on Ethernet Switches

I am familiar with CLI on managed Ethernet switches. However, recently I came across a term 'transaction based CLI' on switches. I am not exactly sure what is that and purpose of having it in switches....
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  • 419
10 votes
4 answers
180k views

What can cause "FastEthernet is up, line protocol is down" on a Cisco Switch?

I wonder what can cause the first line output of the command show interfaces to be: FastEthernet is up, line protocol is down. In other words, Is the cable that is connected to the local interface, ...
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  • 101
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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  • 1,441
9 votes
4 answers
3k views

Advantage of layer 3 switch over Router

What is the advantage of Layer 3 switch over the Router ??
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  • 489
8 votes
1 answer
2k views

Something is filling my force10 ARP table

I have 2 force10 s25 switches that are used for a dark fiber link between two school buildings. One of my switches is filling its ARP table to the brim and then failing. This can happen as often as ...
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  • 832
8 votes
5 answers
24k views

Copy running config in one line

I want to make a script to copy the running config of a switch 4506 in one line. So far, I see two option : copy run tftp: sw#copy run tftp://10.0.0.1/cfg.cfg Address or name of remote host [10....
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  • 2,018
8 votes
2 answers
12k views

Why each and every single port on Layer 2 switches need to have it's own MAC address?

A switch doesn't alter the frame, and source/destination mac addresses remain how sending host specified them,so let me say it again Layer 2 Switching never changes the frames means no changes in MAC ...
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  • 415
7 votes
4 answers
73k 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?
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  • 353