Questions tagged [ethernet]

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

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8
votes
4answers
22k 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 ...
29
votes
3answers
66k 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 ...
84
votes
4answers
421k 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 ...
19
votes
6answers
10k 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 ...
13
votes
3answers
25k 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
122k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
32
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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
79k 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 ...
40
votes
5answers
37k views

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

Why was ethernet MTU calculated as 1500 bytes? What specific calculation was done to arrive at 1500 byte ethernet MTUs, and what factors were considered for that calculation?
19
votes
4answers
14k 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
3answers
7k 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
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/...
11
votes
1answer
11k views

How can a TCP window size be allowed to be larger than the maximum size of an ethernet packet?

I know that TCP window sizes can be scaled to over 64KB, but looking at an ethernet packet datagram, such as this one: it looks like a layer 2 packet is limited in size to be much smaller than that. ...
12
votes
1answer
26k 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 ...
10
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3answers
11k 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?
17
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6answers
31k 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 ...
12
votes
2answers
38k 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 ...
9
votes
3answers
12k views

Network Byte Order

According to big endian byte ordering or network byte order the bits are transmitted in this order: bits 0-7 first, then bits 8-15, then 16-23 and bits 24-31 last. Does this means that bits from ...
5
votes
1answer
11k 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 ...
5
votes
2answers
5k 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?
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 ...
9
votes
3answers
25k 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 ...
7
votes
2answers
2k views

ARP Questions (Packet Types, Ethernet, Cache, Gratuitous)

Lately I have been doing some research on the Address Resolution Protocol, and even though I know the idea behind it I still have some questions. Q1: Are there more types of ARP messages that I am ...
6
votes
3answers
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 ...
4
votes
2answers
690 views

How does an Ethernet hub work internally?

I have a Cisco WS-C412 Ethernet hub. Am I correct that internally all the Rx and Tx ports connect to a single wire/bus? Like here, where three NIC's are connected to first, fifth and ninth hub port:
4
votes
2answers
3k 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 ...
3
votes
1answer
574 views

What does MTU depend on?

I'm trying to understand what factors determine the MTU. For instance, why 802.11 has MTU of 2304 while Ethernet MTU of 1500? Are there restrictions of the physical medium or arbitrary design ...
3
votes
1answer
357 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 ...
0
votes
2answers
3k views

Confusions regarding byte and bit ordering of Ethernet Frame

I understand the concept of Endianess regarding numbers because there the bit significance is well defined. But when applied to things like Ethernet frame or Mac address, things are so clear to me, ...
42
votes
15answers
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.
36
votes
2answers
96k 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 to ...
25
votes
5answers
64k 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 ...
21
votes
7answers
32k views

Introductory level explanation of VLANs

What's the basic use case(s) for VLANs? What are the basic design principles? I'm looking for something like a two paragraph executive summary style answer so I can determine if I need to learn ...
12
votes
2answers
14k 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 ...
20
votes
7answers
64k views

Minimum Ethernet Cable Length?

I have heard from a few people that cables less than 1 meter are prone to transmission problems. Is there any minimum copper cabling length? Please explain if this is true or not and share any ...
19
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4answers
5k views

Jumbo Ethernet Frame Deployment

What are some general guidelines for the successful deployment of jumbo frames in a routed environment?
16
votes
5answers
100k 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 ...
16
votes
3answers
66k 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?
14
votes
2answers
2k views

What does the naming convention for Ethernet standards mean: 1000BASE-T, BASE-TX, BASE-SX, etc.? What is the meaning of the components of the name?

I understand the number in the naming convention. The number represents how many Megabits per second can the standard support. However, I do not understand the rest of the naming convention. What do "...
12
votes
2answers
19k views

Shielded Twisted Pair (STP) termination and grounding

I have knowledge terminating CAT5 UTP. But, for an installation in a remote office, the cable will be routed near a power source which we already know causes a substantial amount of 'noise' on ...
16
votes
6answers
16k 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 ...
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?
13
votes
4answers
18k 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
3answers
2k views

Why does PPP need an underlying protocol?

The Point-to-Point Protocol is (at least according to Wikipedia) a Datalink Layer Protocol. I know that different use cases exist for it but on examining the image below from Wikipedia one can see ...
10
votes
4answers
174k 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 ...
7
votes
3answers
6k views

Why can a packet sniffer in an Ethernet LAN obtain all packets sent over the LAN?

Why can a packet sniffer in an Ethernet LAN obtain all packets sent over the LAN? In a limited broadcast environment, such as in many Ethernet LANs, a packet sniffer can obtain all packets sent ...