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Questions tagged [ethernet]

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

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86 votes
21 answers
79k views

When to choose fiber instead of twisted pair (copper)

These days both fibre and twisted pair are supporting 1000BASE-T and 10GBASE-T, but when do you choose one over the other? The obvious one is maximum cable length, but what are other factors which ...
Lucas Kauffman's user avatar
86 votes
4 answers
426k 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 ...
Vishwanath gowda k's user avatar
52 votes
14 answers
251k views

Does having a longer Ethernet cable slow your connection?

As the title suggests, Does a longer Ethernet cable slow your connection down?
SidS's user avatar
  • 623
47 votes
5 answers
21k views

Is Ethernet port blinking really useful?

This question may not look as an important one and in fact it is just out of curiosity. But to the point: Is Ethernet port blinking really useful? Ethernet ports usually have two light indicators ...
Stanowczo's user avatar
  • 581
43 votes
15 answers
83k 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
  • 763
41 votes
3 answers
126k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
Trojan's user avatar
  • 1,408
41 votes
5 answers
39k 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?
Padmaraj's user avatar
  • 511
37 votes
2 answers
102k 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 ...
Josh's user avatar
  • 473
36 votes
5 answers
9k views

Why are Ethernet Standards written in the form of 10/100/1000? Why not just 1000?

I always have seen Ethernet Standards written in the form "low value/middle value/max value", and always wondered why. Won't a router just reproduce a lower speeds anything below its maximum if it's ...
user3121651's user avatar
33 votes
1 answer
90k 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
  • 685
32 votes
5 answers
6k 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 ...
rfb's user avatar
  • 329
30 votes
4 answers
74k 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
27 votes
5 answers
72k 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 ...
maxxxou's user avatar
  • 373
24 votes
2 answers
2k views

Emulate Ethernet frame loss for testing purposes [closed]

Maybe I am missing something rather basic, but I don't know so I figured I would ask. In terms of testing, how sensitive various TDMoE are to BER and packet delay, I am looking into how to simulate ...
Chris Travers's user avatar
23 votes
3 answers
9k views

What is "industrial ethernet"?

When a teacher introduced it to us he gave the impression that it specifically covers some Siemens technology; that it's even a Siemens trademark. I believe the term covers something more general, ...
Mads Skjern's user avatar
23 votes
7 answers
75k 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 ...
Jim's user avatar
  • 313
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
  • 1,531
22 votes
7 answers
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 ...
Craig Constantine's user avatar
19 votes
3 answers
8k views

Why does an Ethernet cable have four pairs?

I'm curious about why 10Base-T and then 100Base-T Ethernet networks used cables which had four pairs if they only needed two? Were there some cables that only had two pairs? If we wanted to increase ...
UnderAnOpenSky's user avatar
19 votes
4 answers
5k views

Jumbo Ethernet Frame Deployment

What are some general guidelines for the successful deployment of jumbo frames in a routed environment?
Dennis Olvany's user avatar
19 votes
6 answers
11k 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 ...
Sufiyan Ghori'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
18 votes
6 answers
35k 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 ...
CodyBugstein's user avatar
18 votes
6 answers
21k 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 ...
Martin's user avatar
  • 1,033
18 votes
4 answers
9k 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 ...
MattE's user avatar
  • 2,087
18 votes
4 answers
11k views

MAC address recycling?

Something I've always wondered about is MAC address exhaustion. Do hardware vendors keep track of MAC address allocation to their platforms, and then subsequently "reclaim" MAC addresses when a ...
John Jensen's user avatar
  • 9,037
17 votes
6 answers
103k 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.
Celeritas's user avatar
  • 835
17 votes
3 answers
61k views

On which layer of the OSI Model does the arp protocol belong? [closed]

Can anyone explain which layer of the OSI model ARP belongs to and which socket it uses? I referred to lots of books before asking this question. In some books, Authors are saying that it belongs to ...
user2720323's user avatar
  • 1,611
17 votes
2 answers
29k 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 ...
user2720323's user avatar
  • 1,611
16 votes
5 answers
11k views

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

I am trying to understand how a collision occurs in Ethernet, especially when a duplex mismatch exists or when on a legacy Ethernet network two nodes transmit simultaneously. Everyone explains the ...
Christos Dalamagkas'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
3 answers
67k 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?
nos's user avatar
  • 763
16 votes
4 answers
5k 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
16 votes
3 answers
4k views

class-default matches control traffic?

I'm seeing an issue with BFD on a link that's being policed egress where it appears during times where the policer is maxed BFD packets are not getting to the other side. I'm wondering if BFD hellos ...
Mud's user avatar
  • 335
15 votes
3 answers
28k 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
15 votes
3 answers
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 ...
Mike Pennington's user avatar
15 votes
3 answers
40k views

What is the purpose of an Ethernet magnetic transformer, and how are they used?

I have been told that Ethernet magnetic transformers are used for base-t Ethernet when transmission is sent over a lengthy cable. What is the purpose of the transformer? (is it for signal filtering or ...
Benjamin's user avatar
  • 151
14 votes
1 answer
14k 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. ...
Zach Smith's user avatar
14 votes
4 answers
30k 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? ...
syntagma's user avatar
  • 349
14 votes
2 answers
4k 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 "...
yoyo_fun's user avatar
  • 1,531
14 votes
2 answers
24k 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
22k 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 ...
HaydnWVN's user avatar
  • 245
14 votes
1 answer
7k views

Troubleshooting low Metro Ethernet TCP throughput

Background We rented a few layer 2 leased lines, i.e. you have one big pipe at the datacenter and the remote sites have smaller pipes. Inside the layer 2 network you can do whatever you like. Probably ...
Marki's user avatar
  • 1,211
13 votes
2 answers
4k views

Why does Ethernet use EtherType field to determine what type of packet is in a frame instead of just looking at the packet header?

I'm studying for some networking concepts and have learned that a receiving host looks at an Ethernet frame's header to determine what protocol being used by the packet. It makes me wonder though, ...
ComputerWhacker's user avatar
13 votes
4 answers
42k views

Minimum ethernet frame is 64 bytes, Why the payload must be padded to at least 46 bytes

What is reason to pad payload to mininum 46bytes, to form a ethernet frame of 64 bytes. Following says min would be 41 bytes. https://stackoverflow.com/questions/14526139/what-is-the-minimum-packet-...
Vasu's user avatar
  • 155
13 votes
4 answers
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 ...
sardean's user avatar
  • 562
13 votes
2 answers
7k views

Is the MTU of Ethernet still 1500 Bytes?

So basically do the modern routers in the internet still use 1500 as the MTU of Ethernet? or the 1500 Byte was for the old days? What I'm asking here is what is the MTU of Ethernet in modern Networks?...
John P's user avatar
  • 531
13 votes
2 answers
45k 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
2 answers
3k views

Detecting different Ethernet frames

How can someone distinguish between different packets in the Ethernet protocol? It has no "length" field/area as higher-level protocols use to do so. Because this protocol has handling in both the ...
Reflection's user avatar
13 votes
4 answers
4k views

Raw-Ethernet Frames

Recently I was puzzled by the following articles about Raw-Ethernet frames: RAW ethernet vs. UDP Message Passing Using Raw Ethernet Frames Briefly, both of them discusses that it is possible to ...
Mark Messa's user avatar

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