Questions tagged [ethernet]

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

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79
votes
20answers
69k 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 ...
78
votes
4answers
374k 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 ...
46
votes
13answers
105k views

Does having a longer Ethernet cable slow your connection?

As the title suggests, Does a longer Ethernet cable slow your connection down?
43
votes
5answers
13k 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 ...
42
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.
38
votes
5answers
34k 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.
35
votes
3answers
104k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
34
votes
5answers
8k 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 ...
33
votes
1answer
63k 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 ...
32
votes
2answers
74k 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 ...
29
votes
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 ...
24
votes
5answers
50k 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 ...
23
votes
2answers
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 ...
22
votes
3answers
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, ...
21
votes
7answers
29k 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 ...
20
votes
3answers
53k 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 ...
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?
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/...
18
votes
6answers
46k 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 ...
17
votes
4answers
7k 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 ...
17
votes
4answers
7k 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 ...
17
votes
4answers
6k 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 ...
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
3answers
60k 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?
16
votes
3answers
3k 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 ...
15
votes
4answers
5k 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
6answers
8k 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 ...
15
votes
5answers
89k 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.
15
votes
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 ...
14
votes
6answers
22k 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 ...
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
2answers
6k views

Troubleshooting low Metro Ethernet TCP throughput

The setup We have rented a few leased lines that present themselves as a layer 2 network, i.e. you have one big pipe at the datacenter and the remote sites have smaller pipes. Inside the layer 2 ...
13
votes
5answers
3k 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 ...
13
votes
4answers
16k 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 ...
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
11k views

10GBase-T vs. SFP+ Direct Attach

What are the advantages and disadvantages of using 10GBase-T vs. SFP+ Direct Attach to interconnect devices where distance is not a determining factor?
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
4k 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?...
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
3answers
33k 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 ...
12
votes
2answers
2k 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 ...
12
votes
1answer
23k 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 ...
12
votes
1answer
921 views

Why can't we find Ethernet checksum in Wireshark?

I am having a small question regarding Ethernet Frame Header Length. As per my knowledge Ethernet Header size is 18 bytes without VLAN tag and 22 bytes if there is a VLAN tag. But in Wireshark ...
11
votes
4answers
7k views

What's the point of auto-sensing 10/100 Mbit/s Ethernet?

According to the notes I'm reading: 10/100 Mbit/s Ethernet refers to the standard that can autosense which speed it needs to run between speeds of 10 Mbit/s or 100 Mbit/s. Why would autosensing be ...
11
votes
2answers
946 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 "...
11
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 ...
11
votes
2answers
16k 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 ...
11
votes
2answers
6k views

Tracing TCN Messages in STP

We have about 20 vlans on a L2 network running Rapid PVST+ where the root bridge is a stack of Cisco's 3750 switches. I am a bit puzzled by the number of TCN notifications I receive on the switch.. ...
11
votes
2answers
3k views

Extend MACSec encryption over provider bridge

I already asked this question on SF, but figured it might be a better fit here. Is it at all possible to extend MACSec encryption over a provider bridge? Will the typical 802.1ad implementation be ...
10
votes
4answers
15k 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?