Questions tagged [ethernet]

For questions relating to Ethernet networking, as defined by the IEEE 802.3 standards.

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6
votes
4answers
14k views

The maximum length of CAT6 for intranet network?

Recently I purchased CAT6 cable with a length of 150 meter. I bought another CAT6 with a length of 148 meter. I joined both the cables using RJ45 cable coupler. I tested it using LAN TESTER, and ...
81
votes
4answers
408k 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 ...
22
votes
3answers
60k 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 ...
18
votes
6answers
8k views

Why use IP if we already have a MAC address?

I am preparing for ICND1 exams and recently started to learn about different Cisco devices. I have just come to know how the packet is generated to be transmitted over a network, or outside the ...
12
votes
3answers
19k views

Why do I need a crossover cable to connect devices of the same type?

I know that a crossover cable should be used to directly connect devices of the same type and patch cable is used to connect devices of different types. But I never understood why is it so exactly? ...
39
votes
3answers
114k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
30
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5answers
5k views

Why do we still use Ethernet?

There's no doubt the vast majority of Ethernet frames are transporting IP packets. I know there are various other protocols that can be transported over Ethernet as well, but those can be transported ...
33
votes
1answer
70k 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 ...
38
votes
5answers
35k views

Why was the MTU size for ethernet frames calculated as 1500 bytes?

Is there any specific calculation that that was done to arrive at this number, and what were the factors that were taken into consideration for that calculation.
16
votes
4answers
8k 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 ...
3
votes
4answers
3k views

How does DHCP assign an IP address?

I understand that IP address is a unique address to identify a machine in a network. But for the first time when i Plug in say ethernet cable of a network to my PC's Ethernet Port, how the IP address ...
5
votes
1answer
8k views

Calculate the FCS number from a frame ethernet

I've the following frame ethernet: 08 00 20 0A 70 66 08 00 20 0A AC 96 08 00 45 00 00 28 A6 F5 00 00 1A 06 75 94 C0 5D 02 01 84 E3 3D 05 00 15 0F 87 9C CB 7E 01 27 E3 EA 01 50 12 10 00 DF 3D 00 00 20 ...
15
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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
2k views

What is the reason for the different order of the Source and Destination in a L2 header?

A TCP/UDP header (Layer 4) lists the Source Port, then the Destination Port: An IP header (Layer 3) lists the Source IP, then the Destination IP: An Ethernet header (L2) and a WiFi Header (L2) lists ...
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/...
33
votes
2answers
88k views

What is the actual size of an Ethernet MTU

I think I might be getting confused with terminology surrounding MTU. This definition from Wendell Odom's CCNA book on MTU: The IEEE 802.3 specification limits the data portion of the 802.3 frame ...
12
votes
1answer
24k views

What is the difference between Ethernet II and 802.3 Ethernet?

Can anyone explain to me what is the difference between Ethernet, Ethernet-II and 802.3 Ethernet. I referred to a lot of books, they are in high level language. Can anyone explain to me in a simple ...
15
votes
6answers
25k views

How are IP addresses mapped to MAC addresses?

I've been reading about the differences between MAC and IP addresses, and why we need both of them. To summarize, MAC addresses are physical unchangeable unique IDs for every single device while IP ...
5
votes
2answers
4k 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?
9
votes
3answers
19k 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 ...
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 ...
5
votes
5answers
196 views

How do two computers with different speeds communicate

This is one extremely basic and probably equally dumb question that I just couldn't ever find a definite answer to. Suppose we have 2 computers one that can transmit data at say 10 bits per second (A)...
4
votes
2answers
1k views

Ethernet's frame format, length or EtherType?

Ethernet defines a 2 byte header for the payload's length in its frame format. This header has an ambiguous meaning: if smaller than or equal to 1500 it represents the payload length. if ...
4
votes
3answers
2k 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 ...
3
votes
1answer
352 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 ...
43
votes
15answers
79k 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.
25
votes
5answers
57k 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 ...
10
votes
3answers
10k views

Why don't switches rewrite mac-addresses?

Is there any particular reason why Ethernet switches don't change the MAC address of a packet? Is it for end host identification using the MAC address, or anything else?
19
votes
4answers
5k views

Jumbo Ethernet Frame Deployment

What are some general guidelines for the successful deployment of jumbo frames in a routed environment?
15
votes
5answers
95k views

What's the difference between an Ethernet cable and a UTP cable?

Is there such a thing as an Ethernet cable or technically no? Are they actually called UTP cables? From wikipedia UTP cables are found in many Ethernet networks and telephone systems.
12
votes
7answers
3k views

Accurate strategy for measuring point-to-point link performance?

I have a requirement to provide an accurate means to demonstrate line performance over a flat layer2 1Gb point-to-point Ethernet circuit, approx 100 miles apart that seemingly goes through a number of ...
12
votes
2answers
13k views

Technical difference between a Collision and Broadcast Domain

Many people new to networking wonder what the technical differences between a broadcast domain and a collision domain are. Specifically: What criteria is used to know the boundaries of a collision ...
16
votes
3answers
65k views

How is the root bridge elected in STP?

How does STP/RSTP determine which bridge is the root bridge, and how are paths to the root bridge selected?
10
votes
4answers
16k views

speed/duplex mismatch

How do mismatches occur; what is the impact of a mismatch on network connectivity; is resolution of mismatches really worth the effort; what are some ways to detect mismatches on a large scale?
15
votes
6answers
11k views

Reason for half-duplex mode in Ethernet?

Half-duplex connections were popular at the time of 10Mbps and 100Mbps Ethernet, and, according to standards, it is allowed also in case of 1Gbps Ethernet. Am I correct that half-duplex mode support ...
13
votes
4answers
17k views

MTU and Fragmentation

Consider the following: NAS with 10G interface connected to 10G interface on switch. Client computers connected to switch via gigabit Ethernet. Will there be any gain in performance if MTU size is ...
10
votes
4answers
151k 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, ...
8
votes
1answer
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 ...
8
votes
4answers
78k views

NMap: Failed to open device eth1

EDIT: I Have found that Nmap can work in unprivileged mode. A question related to that can be found here. I recently installed NMap 6.49BETA4, and the ZenMap GUI, on my Windows 7 Home Premium x64. ...
7
votes
2answers
7k views

ARP Replies can they be Broadcast

I have been informed by certain networking professionals that ARP replies can and sometimes actually are broadcast packets instead of unicast. If and when would you use a broadcast ARP reply?
3
votes
1answer
56k views

4 Wire (as compared to 8 wire) LAN connections and Bandwidth Loss when Splitting Ethernet? [closed]

This is my first time here, and if this isn't the right place for this question, my apologies. Please direct me to the right place. In order to get a wired LAN connection to a desktop computer and a ...
2
votes
3answers
340 views

How can 1000BaseT transmit on Rx & receive on Tx pins?

How can 1000-BaseT (Gigabit ethernet twisted copper pairs) technology transmit over the Rx pins & receive over the Tx pins? It's baseband technology (hence the name), right? Discrete digital ...
2
votes
1answer
2k 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 ...
-3
votes
4answers
11k views

How Layer-2 data will be fragmented?

Got a doubt while studying about LLC layer. From Network layer I came to know that IP fragmentation will be done, For L4 TCP will segment the data. Here, the question is Protocols starting from Layer-...
17
votes
4answers
8k views

Are there any reasons to not use BFD?

In looking to implement Bidirectional Forwarding Detection (BFD) it seems to be very flexible in terms of timer tuning, light weight regarding any overhead and it's flexibility in terms of overall ...
6
votes
2answers
5k views

Why maximum length of IP, TCP, UDP packet is not suit?

From many tutorials, I get follow knowledge(maybe I'm misunderstood) : maximum of an Ethernet packet is about 1500 bytes. maximum of an IP packet is about 65535 bytes. maximum of a UDP packet is ...
5
votes
2answers
1k views

Did the LSB-first method of transmitting the canonical MAC address in Standard Ethernet have any specific advantages?

When MAC addresses are sent out they are sent such that the Group Address Indicator is the first bit of the Ethernet frame but when stored in memory this forms the LSB of the leftmost byte of the MAC ...
4
votes
3answers
779 views

What practices/ mechanisms/ protocols can be used to secure communication on physically exposed Ethernet links?

Given that Ethernet cables are everywhere and use for all kinds of links and it is assumed they are physically guarded, hence typical installations do not leverage any "secure" mechanisms. Let's say ...
3
votes
1answer
883 views

NMap only works in --unprivilged mode?

This question originated from an earlier NMap question here. Anyways, my NMap will only work inside --unprivileged mode. How can I make NMap operate outside --unprivileged? The same thing persist ...
3
votes
1answer
503 views

What is meant by Jumbo frame ? what happen when a frame size is greater than the MTU size?

I know that the frames having size greater than the MTU size should be dropped . i.e, The Ethernet frame having size more than 1522 should be dropped . But my doubt here is what is Jumbo Frame ?