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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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43
votes
15answers
77k 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.
40
votes
4answers
35k 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 ...
33
votes
1answer
65k 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 ...
28
votes
8answers
9k 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?
24
votes
4answers
30k 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, correct? Is this a cisco only ...
22
votes
8answers
7k 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 ...
22
votes
3answers
55k 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 ...
21
votes
7answers
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 ...
19
votes
5answers
17k 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 ...
19
votes
2answers
23k 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/...
19
votes
1answer
3k 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 ...
18
votes
3answers
27k 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 ...
17
votes
7answers
52k 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 ...
16
votes
3answers
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 ...
16
votes
8answers
17k 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 ...
15
votes
4answers
6k 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 ...
15
votes
5answers
71k 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 ...
15
votes
4answers
1k 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 ...
15
votes
2answers
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,...
14
votes
5answers
680 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.
14
votes
7answers
8k 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 ...
14
votes
3answers
41k 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 ...
14
votes
4answers
4k 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 ...
14
votes
3answers
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 ...
14
votes
2answers
11k 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....
13
votes
2answers
3k 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 ...
13
votes
2answers
15k 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: ...
13
votes
3answers
15k 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 ...
12
votes
3answers
2k 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 ...
12
votes
5answers
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 ...
12
votes
2answers
17k 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 -...
12
votes
1answer
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 ...
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 ...
11
votes
4answers
35k 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 ...
11
votes
6answers
226k 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?
11
votes
1answer
25k 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 ...
11
votes
2answers
230 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 (...
11
votes
1answer
5k 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 ...
11
votes
2answers
1k 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 ...
10
votes
6answers
5k 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 ?
10
votes
3answers
35k 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 ...
10
votes
3answers
2k views

DHCP assign IP address based on MAC address with Cisco CE 500 platform?

I have very little knowledge about CISCO and its switches, but now I have to configure it myself. Is it possible to allot IP address based on MAC address on CISCO CE 500 (Cisco Catalyst Express 500-...
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 ...
10
votes
2answers
1k views

How many stack cables would be needed if we want to stack two 3850 switches

How many stack cables would be needed if we want to stack two 3850 switches : We have one 3850 switch and planning to add one more. So my guess is only one Power & One data stack cable would be ...
10
votes
3answers
2k views

Trying to get 10 gigabit connection over a switch with 100 megabit standard gateway

I have limited knowledge of this and I'm still learning, so I hope someone can give me advice on this. I have a 10GbE NAS, a 10GbE NIC in my PC and a switch with two 10GbE ports, which connect all ...
10
votes
4answers
17k 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 ...
10
votes
4answers
5k views

Redundancy between router and servers

In the past, I was using a NetGear GS724T between my router and my servers. Unfortunatly the switch suddenly died. Now I've changed it for a GS748T and also bought one more and I am wondering how I ...
10
votes
2answers
2k views

Why can't I ping from one computer to another using L3 cisco 3550 switch?

I have configured my 3550 L3 switch to redirect HTTP traffic to a significant Machine: This is my scenario ( PCs instead of Routers): This is the switch configuration: CAT3550# show running-config ...
10
votes
2answers
4k views

Port mirroring just the Rx (or Tx) direction of a port?

Something on my Ethernet LAN is occasionally sending out spurious duplicate packets and I need to track it down. The culprit is likely a switch or other bridge (such as a Wi-Fi AP) and not an endpoint,...
10
votes
1answer
6k views

If two NICs are bonded and connected to different switches how does a router decide which switch to forward the packets to?

I am trying to understand how the architecture works in case of NIC Bonding. The scenario is: There are 2 NICs on the same system. Both are bonded to support failover and load balancing. Hence the IP ...