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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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51 votes
6 answers
112k 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 ...
stets's user avatar
  • 1,030
44 votes
15 answers
84k 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.
nos's user avatar
  • 783
41 votes
4 answers
42k 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 ...
Daniel Yuste Aroca's user avatar
34 votes
1 answer
91k 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 ...
sosytee's user avatar
  • 695
30 votes
4 answers
75k views

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 ...
Celeritas's user avatar
  • 835
29 votes
8 answers
17k views

What does the 1000M on a network switch mean?

I thought the 1000M ment that it was capable of Gigabit speed but it isn't, I noticed then that it stated 10/100Mbps. What does this 1000M mean?
Jeroentetje3's user avatar
27 votes
9 answers
31k 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 ...
Jim Pap's user avatar
  • 419
22 votes
4 answers
19k 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 ...
yoyo_fun's user avatar
  • 1,541
21 votes
7 answers
7k views

Building new network and need 200+ wireless devices

I'm planning the network for my company's new office space. I'm not an expert so I'd like to present my plan and ask y'all nice folk to tell me if and where I'm making mistakes. Assumptions: Office ...
SAR622's user avatar
  • 693
19 votes
5 answers
19k 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 ...
Tim Brigham's user avatar
19 votes
2 answers
27k 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/...
BrunoMCBraga's user avatar
19 votes
1 answer
4k views

Finding transparent firewall packet loss

We use the Cisco ASA 5585 in a layer2 transparent mode. The configuration is just two 10GE links between our business partner dmz and our inside network. A simple map looks like this. 10.4.2.9/30 ...
user2096's user avatar
  • 285
18 votes
3 answers
28k views

How do you know if a Cisco switch is handling traffic well?

I have just read Jeremy's question about 2960-S and iSCSI traffic handling, which drove me to ask this question: We all have lots of switches deployed in distinct infrastructures, how can you know ...
laf's user avatar
  • 795
18 votes
7 answers
67k views

Cisco command to show which interfaces an ACL is applied to

For Cisco routers and switches, is there a show command, or something similar, that will display what physical and logical interfaces an ACL is implemented on and what direction it is applied in? I ...
Adam Loveless's user avatar
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 ...
generalnetworkerror's user avatar
17 votes
5 answers
87k views

What causes total output drops on a cisco switch interface?

I have an HP c7000 blade chassis which contains Cisco 3120X and Cisco 3120G switches running ios 12.2(58)SE1. The blades themselves are very lightly loaded yet many interfaces on different blade ...
User123456's user avatar
16 votes
3 answers
5k views

Why would a switch be connected to a router using three ethernet cables?

In our rented office, we have a router (BT smart hub) connected to a large switch (model unknown). The two are connected using three ethernet cables, thus: ----------------------- | 1 2 3 4 ...
EvilDr's user avatar
  • 273
16 votes
4 answers
6k views

What happens when 2 computers transmits at the same time to a 3rd one in a full duplex switch?

Consider the following scenario: I have 2 PC's (PC1 and PC2) that wants to transmit at the same time to PC3 in a full duplex ethernet switch. Let's consider that all ports are in the same VLAN, so ...
Juan Jose Polanco Arias's user avatar
15 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 ...
Marcos Gonzalez's user avatar
15 votes
8 answers
11k views

How to connect 5 different networks together?

I have a network consisting of three servers in a data center, all of which are connected to each other via a switch. In addition to this I need this network to access 4 additional networks, most of ...
Kranar's user avatar
  • 153
15 votes
3 answers
29k views

How does a switch know where to route packet, when there is another switch in front of the destination?

I am just wondering how a switch does know how to route packets to destinations "hidden" behind another switch. In my understanding a switch has a table that maps each of its hardware ports to exactly ...
Jonas Eschmann's user avatar
14 votes
3 answers
51k 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 ...
user avatar
14 votes
5 answers
729 views

In a multi-tenant environment what should be done to make your switchports silent on Cisco and Juniper switches?

For example preventing it from sending arp, stp, etc and to reveal as little as possible about the rest of the network. Example use case would be connecting to a peering exchange.
SimonJGreen's user avatar
  • 1,695
14 votes
3 answers
50k 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 ...
cgasp's user avatar
  • 2,008
14 votes
2 answers
25k views

Is daisy chaining switches increasing latency?

This is a general question not related to specific hardware. If we have the following configuration of daisy chained switches: internet ---- core network ----- switch 1 ----- switch 2 ----- switch 3 -...
vladiz's user avatar
  • 546
14 votes
3 answers
3k views

Why we get high fluctuation on bandwidth measurement Cacti graph?

We were on redundancy test of Etherchannel and Routing on our network. During this intervention we made some measurement. Our monitoring tool is Cacti for graph. The equipment monitored is a 4500-X ...
cgasp's user avatar
  • 2,008
14 votes
4 answers
2k views

Is Nexus 7000 series line card "OIR" impact-free?

I have read many data sheets on the Nexus 7000 series and it's cards purporting to be capable of OIR (Online Insertion and Removal). However I have never needed to replace/reseat/add any Nexus line ...
Brett Lykins's user avatar
  • 8,278
14 votes
2 answers
25k views

Does the source MAC address of a frame change when it passes through several switches? [duplicate]

If PC1 sends an Ethernet frame to Switch1 which forwards the frame to Switch2 which in turn forwards the frame to PC2, does any decapsulation happen that changes the source MAC address?
Rana Mallah's user avatar
14 votes
2 answers
21k views

STP blocking port selection

I want to make sure I'm understanding how this works. Specifically the term "Sending Port ID" has me very confused. Consider this diagram, I want to make sure I have root port selection figured out: ...
red888's user avatar
  • 459
14 votes
2 answers
2k views

In what mode does the 2960-S IOS prompt ends with a colon ":"

QUESTION For a Cisco 2960-S switch, when connecting on the console port, normally the prompt is: SwitchName> then you issue an enable command and end-up with a prompt like: SwitchName# However,...
Philibert Perusse's user avatar
14 votes
2 answers
17k 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....
Mike Marotta's user avatar
  • 2,057
13 votes
4 answers
59k views

Why do switches need ARP tables when the translation are done on the machines side?

As it is clear from the title, why do switches need ARP tables when the translation are done on the machines side? Roughly saying, why there are two ARP tables on machines and on switches? Is not the ...
محمد جعفر نعمة's user avatar
13 votes
2 answers
47k 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 ...
Vistance's user avatar
  • 233
13 votes
6 answers
348k 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?
John's user avatar
  • 812
13 votes
2 answers
4k views

Switchport in half duplex - Download speed suffering but upload was fine

A user had issues with download speed from the Internet. The connection to the Internet is 100 Mbit/s. The user got around 7 Mbit/s downstream and around 80 Mbit/s upstream. I tested from my computer ...
Daniel Dib's user avatar
  • 7,498
13 votes
4 answers
151k views

What are the reasons for seeing an incomplete ARP?

As was mentioned in this post: The reason for seeing an incomplete ARP is that "An ARP request was sent for that address, but the host with that address is not up and running on the LAN, so there is ...
kamokoba's user avatar
  • 503
13 votes
3 answers
18k views

Tracking down an invalid source mac address

I have inherited support of a remote site which contains a Cisco 4500 and is connected to ~2 dozen cisco access switches - primarily 2960s with a couple of 3750s and 3560s. Not all access switches ...
User123456's user avatar
12 votes
4 answers
2k views

Which specs should I look for (or avoid) when searching for a iSCSI 10Gbit ethernet switch, independent of brand?

I've been tasked with selecting two new switches intended to be our new iSCSI SAN backbone. Currently, we are using two Dell N4064 improvised for the task since that's what we had when the need ...
ÜM Mineiro's user avatar
12 votes
5 answers
2k views

Do faster segments of a network mean faster flow rate or lower latency?

This is another one of those hypothetical questions. I've been trying to figure out whether or not having a faster 'segment' of a network in between host A and host B will translate into a faster flow ...
Libbux's user avatar
  • 515
12 votes
4 answers
24k 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 ...
ranger's user avatar
  • 123
12 votes
3 answers
20k views

Does Switch need its own MAC address?

I saw a post say Switch do not need its MAC address, unless it's an administrable Switch for managing. but I found the spanning-tree protocol need a MAC address of Switch to form STP Root Bridge ID. ...
three-blocks's user avatar
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 ...
Abhishek Balaji R's user avatar
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 ...
hyussuf's user avatar
  • 467
11 votes
6 answers
7k views

When does switch talk to router

I have read a lot on how switches forward within LAN, but little about how switch talks to router. When does a switch send packet to router ? Is it when it does not find recipient on the same LAN ?
JavaDeveloper's user avatar
11 votes
3 answers
2k 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 "...
Mark Christopher's user avatar
11 votes
1 answer
30k 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 ...
HAL's user avatar
  • 1,679
11 votes
1 answer
6k views

Cisco IOS built-in SSH client default version

You issue the command ssh 192.168.97.97 from a IOS device and the destination IOS router/switch SSH server is accepting both SSH versions. Being that you did not specify a version to use on the SSH ...
Adam Loveless's user avatar
11 votes
2 answers
302 views

IPv6 Subnetting - Block RA on Switches?

I'm currently planning a v6 only small datacenter network. Imagine for now one deployment consisting of 6 racks with 12 servers per rack. Each server has 2 NICs for two physically separate networks (...
paraa's user avatar
  • 113
11 votes
2 answers
966 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....
modest's user avatar
  • 419
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 ...
mfinni's user avatar
  • 243

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