Questions tagged [layer2]

For questions about the OSI layer-2 (data-link layer).

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17
votes
6answers
54k views

What, exactly, is required to make Airplay work across VLANs? [closed]

It seems like AirPlay works out-of-the-box, vaguely speaking, only within a LAN. It's not clear to me exactly why, but it looks like at least discovery relies on broadcasts. Wikipedia states that ...
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?
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
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
1answer
2k views

Can a router send ARP requests to hosts?

I play with libpcap in C recently to create an arp sniffer program that captures arp requests on the air. I came up with a strange effect. I captured arp requests from my router. I don't know if this ...
11
votes
2answers
8k views

VXLAN vs VLAN over layer 3

I read that one of the benefits of VXLAN over VLAN is that it can spawn across WAN and multiple layer 3 networks by creating overlay layer 2 networks. However AFAIN VLAN can also do the same using ...
10
votes
3answers
18k views

What is the relationship between the bandwith on a wire and the frequency?

I am trying to learn networking (currently Link - Physical Layer); this is self-study. I am very confused about one particular thing: Suppose I want to send a data on the wire something like this: ...
10
votes
1answer
10k views

RIB vs FIB differences?

Can someone explain the differences between a RIB and FIB a little further? I didn't see any similar questions on the site so I thought this would be a good addition. The RIB routes and the FIB ...
8
votes
4answers
8k views

How do I set up a VLAN between an ESXi VM and Physical Hardware?

First of all, I am a total noob. The extent of my VLAN experience is setting up a guest network on a wireless router. I have recently gotten a request to set up a public network requiring filtering ...
7
votes
7answers
3k views

How LAN works in video games if only one station is permitted to transmit?

I've read the in a LAN the data is transmitted in packets so as to allow only one station to transmit at a given time, so how could this be applied in video games LAN parties if only one device is ...
7
votes
4answers
4k views

Why preamble is not considered a part of the Ethernet Header?

If I search where does my data start in an Ethernet frame, I get a common answer which says TCP header( 20 bytes) + IP header ( 20 bytes) + Ethernet header( SA + DA + Type) i.e 14 bytes. So, in short ...
7
votes
2answers
419 views

How is the Groupwise Transient Key used in WiFi networks?

In Wifi WPA, I understand that during association, a 802.11 client and AP negotiate a Pairwise Transient Key (PTK), using which the Groupwise Transient Key(GTK) is provided to the station. I ...
7
votes
3answers
4k views

Does a switch understand packets? If yes, what is the frame terminology for?

I was a little confused between packet and frame. The confusion was based on, if a switch is a layer 2 device, how does it reads a packet from a layer 3 device, and where is the 'frame' thing which is ...
7
votes
1answer
872 views

Cisco: Is it possible to see an EFP path within an EVC?

The topology diagram below shows multiple CPE devices, each has multiple WAN sub-interface that are dot1q tagged as VLAN 10 and 20 in this example. The PE ME3600 devices has an SVI with an IP address ...
7
votes
4answers
205 views

Is ethernet a term describing just the protocol or the interfaces also?

Is ethernet describing the protocol and therefore cat5 cables became labelled ethernet cables because they are used for this protocol?
6
votes
2answers
6k views

Should I disable STP on my access ports?

I'm going to replace some old "central" access switches to some Cisco Catalyst 2960X, and I'm not sure what to do on their STP configurations. For this network, we have some "central" access switches,...
6
votes
4answers
32k views

What OSI layer do Access Points operate on?

I am studying for CCNA. In my book I read that wireless access points belong to OSI layer 2. I did some research and I found that is correct. This raised lots of questions in my head so I hope I don't ...
6
votes
1answer
4k views

Cisco: storm-control unicast

Within a typical service provider environment I can see how broadcast and multicast storm control can be useful. I am confused regarding unicast storm control though. Reading the Cisco docs online ...
6
votes
2answers
403 views

How to connect a special device in a network

I have a question which is puzzling me. I have a couple of custom electronic devices (are identical, one is a master and the other is a slave) which implement only up to level 2 of the ISO/OSI layer, ...
6
votes
1answer
1k views

How does priority work in token ring?

Token rings have an optional priority feature, for which the 8 access control bits in the token ring non-token frame are used. Suppose we are working with the delayed token reinsertion scheme. How ...
6
votes
3answers
1k views

Media Access Control for WAN protocols (equivalent to IEEE 802.3 Ethernet sublayer)

Ethernet can be divided into 2 sublayers; MAC and LLC. For WAN protocols like PPP, HDLC and frame relay, it is not discussed in regards to protocol sublayer for media access control. On serial ...
5
votes
2answers
9k views

Why each and every single port on Layer 2 switches need to have it's own MAC address?

A switch doesn't alter the frame, and source/destination mac addresses remain how sending host specified them,so let me say it again Layer 2 Switching never changes the frames means no changes in MAC ...
5
votes
3answers
2k views

Finding Source MAC Address of A Packet

Let's say a packet originates from an internal network (with a private IP). Even after this packet leaves the gateway router, the MAC address of the originating host is still somewhere in this packet (...
5
votes
5answers
282 views

What blocks a computer from another network in the same switch from answering an ARP request?

Let's suppose I have 3 computers A, B and C connected to the same switch with only the native VLAN active. A and B belong to the same network, let's say that their IP addresses are 192.168.1.1/24 for ...
5
votes
3answers
415 views

Why use 'switching' term for packet switching

On a packet switching network, a router is required to route packets to different networks. Why does packet switching uses the term 'switching' when a packet is a term for a layer 3 PDU and ...
5
votes
1answer
662 views

In Cyclic Redundancy Check, how does the receiver knows what the generator polynomial is?

Must the receiver know the generator polynomial in order to detect the errors in the received data? So how does it know what generator polynomial the sender has used?
5
votes
2answers
529 views

Network topology suggestions?

Looking for guidance on network topology changes. Currently our network has 2 different stacks of switches running in layer 2. These switches then connect to a stack of layer 3 switches to do internal ...
5
votes
1answer
8k views

How to trace a MAC address on a switched LAN?

How to trace a MAC address on a switched LAN? I want to be able to find out a switch port's MAC address. No device uses IP, therefore no address resolution and no SNMP. No device runs CDP, therefore ...
5
votes
2answers
817 views

What precisely is a link?

When reading about layer 2 the word "link" is frequently used but strangely I'm struggling to find a formal definition of it. What is the precise definition of a link? So in the image below, how many ...
5
votes
2answers
70 views

How the packet is routed between two host from same sub-net and associated with same AP?

Given host 'A' and 'B' have IP 10.10.0.5 and 10.10.0.8 respectively, the sub-net mask for both are 255.255.255.0, both 'A' and 'B' are associated and connected to the same AP, and the AP is connected ...
5
votes
2answers
211 views

Why is RTS/CTS optional for IEEE 802.11?

In wikipedia it is stated: Request to Send (RTS) frame: The RTS and CTS frames provide an optional collision reduction scheme for access points with hidden stations. A station sends a RTS frame as ...
5
votes
2answers
3k views

Ethernet Neighbor Discovery?

I am trying to familiarize myself at the moment with the Ethernet network protocol. However, when looking at the standard a Ethernet frame needs a destination MAC address, and the source address MAC ...
5
votes
1answer
334 views

How does STP convergence work?

Consider the IEEE 802.1D (first revision) Spanning Tree Protocol. I have read a lot of articles and forum entries about STP convergence. Often I do find contradictions about the convergence mechanics. ...
5
votes
1answer
1k views

Is Layer 3 web auth with Layer 2 PSK more secure?

I have configured a Cisco WLC to authenticate users using External Web Authentication at Layer 3. Are client wifi connections more resilient against eavesdroppers with a pre-shared key at Layer 2 ...
5
votes
1answer
159 views

ETH II vs 802.2 LLC/SNAP

I have a project where there is a custom implemented layer 2 protocol. An engineer before me decided to use what I'm used to as the Ethertype field as a size field. For awhile this looked right ...
5
votes
2answers
3k 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?
5
votes
1answer
3k views

Cisco 6509 as PPPoE BRAS

I have read that SIP-400 and SPA are compatible with 6509 series switch. These SIP and SPA are used to terminate PPP sessions on 7600 routers. If I installed this SIP and SPA (SPA-5X1GE-V2 for example)...
4
votes
4answers
1k views

Do layer 3 protocols use layer 2 protocols?

I saw the STP on layer 2. The guy that made the tutorial says at the end : Even though it seems that Layer 2 access layer solutions are slowly being replaced by blazing fast Layer 3 solutions. ...
4
votes
3answers
515 views

'interface vlanX' or 'interface loopbackX'

VLANs are a layer 2 concept. We can all agree on that but why, then, do we use the 'interface vlanX' to allow routing on a switch. If we want to setup a default gateway for VLAN X on a Layer 3 switch ...
4
votes
2answers
1k views

Connecting Layer 2 switchport to layer 3 port

I have a config that I am trying to bring up, and in my mind it should work, but perhaps I am missing something. The idea is that one end of the link is a layer 3 interface on a router. The other end ...
4
votes
2answers
413 views

Nexus 3172T MTU

I am attempting to set the L3 mtu on a Nexus port. The mtu option is only available if I set "no switchport" which then prevents me configuring VLAN related settings. Am I missing something obvious? ...
4
votes
3answers
1k views

Error Detection across OSI layers confusion

Confusion: How does the transmitter know that his information( bits in physical layer, frames in data link layer, packet in network layer ) was transmitted correctly to the receiver? I know that in ...
4
votes
1answer
3k views

What's use of Port Isolation vs traditional VLANs?

I've seen several switch products advertising port isolation feature as "way to limit ports communication without configuring VLANs" and it sounds quite... sketchy? If i understand it correctly port ...
4
votes
2answers
24k views

Enabling flow control on my ethernet NIC

When running "ethtool eth0", I get a line in the output that states -> Supported pause frame use: No Does this mean my Ethernet card does not support flow control at all? I try to enable the flow ...
4
votes
1answer
12k views

What does this “no switchport” command do?

I came across this command at a Layer 2 Switch, and I don't know what it does. no switchport Ip Addr 10.2.3.4 255.255.0.0
4
votes
2answers
470 views

Spanning Tree on the interface that resides at end users

I do not understand why we use portfast and in fact STP on the interface of a switch that is directly connected to end users. Indeed, a user can interfere with the STP election and perform DDoS (that ...
4
votes
1answer
972 views

Do we send ack frames in ethernet? if not then how does a router know if its neighbors are down or not?

So first off, do ethernet sends ack frames or not? if it doesn't : then how does a router know whether its neighbors are down or not? if it does : then is it cumulative ack with an ack timer or one ...
4
votes
2answers
365 views

How is the length of a PPP frame known?

How does the receiver determine the length of a PPP frame (not PPPoE or PPPoA)? Structure of a PPP frame shows no length field. Sure, at the end of the frame there is a 2-byte checksum followed by a ...
4
votes
1answer
5k views

Why are there checksums for different OSI layers?

Can anyone clarify the question "why we need checksum in both layer-2 and layer-3 of OSI layer", Layer-2 would encapsulate the rest of OSI layers into a single unit. If Layer-2 has a checksum, ...
4
votes
3answers
545 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 ...