Questions tagged [layer3]

For questions about the OSI layer-3 (network layer).

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What is the difference between MSS and MTU?

I'm confused as to the difference between maximum segment size and a maximum transmission unit. Can someone please explain in relation to layers 2 and 3? If I had a packet of 800 bytes in the ...
NetworkNinja's user avatar
22 votes
3 answers
10k views

What do TCP/UDP add to "raw ip"?

I know that both TCP and UDP are built on top of IP, and I know the differences between TCP & UDP, but I'm confused about what exactly "raw ip" is. Would it be fair to say that TCP & UDP both ...
John Dorian's user avatar
22 votes
6 answers
72k 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 ...
alx9r's user avatar
  • 904
13 votes
6 answers
14k views

How does BGP work over TCP?

As far as I know we are using routing protocols to achieve Layer 3 connectivity, I am confused with the fact that BGP works over TCP which is L4. How can L4/TCP be established without prior L3 ...
blended's user avatar
  • 375
13 votes
4 answers
37k views

What layer of the OSI model does NAT work?

Simple question and the answer is layer 3 of course, but can we say that PAT is working on layer 4 also because it deals with port numbers which is a layer 4 function? Or is it only a layer 3 ...
aia87's user avatar
  • 133
13 votes
1 answer
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 ...
user avatar
13 votes
1 answer
34k 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 ...
stets's user avatar
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12 votes
2 answers
11k 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 ...
mart's user avatar
  • 123
12 votes
4 answers
143k views

What are the reasons for seeing an incomplete ARP?

As was mentioned in this post: The reason for seeing an incomplete ARP is that "An ARP request was sent for that address, but the host with that address is not up and running on the LAN, so there is ...
kamokoba's user avatar
  • 483
11 votes
3 answers
2k views

What is a "Layer 3 Switch"?

What is a "Layer 3 Switch"? I essentially understand the OSI model, well maybe I don't and maybe that is why I don't understand. Its just that I don't see how/when/if a "managed switch" becomes a "...
Mark Christopher's user avatar
10 votes
3 answers
22k views

What is the difference between layer-2 and layer-3 broadcasts? Please explain with an example

What is the difference between broadcast addresses ff.ff.ff.ff.ff.ff (Layer 2) and 255.255.255.255 (Layer 3)?
Rakesh Nittur's user avatar
9 votes
5 answers
3k views

Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?

Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page (same layer as IP) when the ICMP protocol data is a payload of the IP packet? This is the article on Wikipedia that states ...
yoyo_fun's user avatar
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9 votes
4 answers
3k views

Advantage of layer 3 switch over Router

What is the advantage of Layer 3 switch over the Router ??
Abiral's user avatar
  • 489
9 votes
2 answers
11k views

Cisco ASA Routing Issue

I am working on a lab right now and am having some issues with ASA routing. Here is the current network topology: Currently if I login to the console on the ASA or Router I can get outside to the ...
thefiddler's user avatar
8 votes
6 answers
5k views

How Does A Layer 2 Switch Differentiate Between Different Networks?

I know that different IP-based networks can be connected to the same layer-2 switch. and I know each network's nodes can communicate just fine, however, no normal traffic can occur between two nodes ...
Shadi's user avatar
  • 285
8 votes
3 answers
7k 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 ...
Taha's user avatar
  • 83
8 votes
5 answers
754 views

Why is the IP layer aware of higher layers in the network stack?

In the IP packet header there is an 8-bit Protocol field (at offset 9) that holds the "next level protocol", with assigned numbers given in RFC 790. For example, TCP is 6. My question is, why is the ...
I. J. Kennedy's user avatar
8 votes
4 answers
47k views

Access Multiple VLANs from One Computer

I inherited a bit of mess and am preparing to redo our IP addressing scheme and also split our network into multiple VLANs. Possibly something like: 10.1.10.0/24 VLAN 10 VOIP 10.1.20.0/24 VLAN 20 ...
netnewb's user avatar
  • 81
7 votes
4 answers
14k views

Is it possible to perform a Layer 2 (MAC address) traceroute?

I understand the difference between layer2 & layer3 and MAC & IP. I understand address resolution protocol, etc. But is there any way to do a traceroute that will show Layer 2 information? For ...
voices's user avatar
  • 313
6 votes
6 answers
34k views

Difference between routing, forwarding, and switching

I feel like the three terms are often used as synonyms but actually all mean something different. I did look at Wikipedia sites but I'm still unsure about the definitions of the three. From my ...
stefanbschneider's user avatar
6 votes
2 answers
4k views

How do I know the switch is layer 2 or layer 3?

Is it possible to find the switch is Layer3 or Layer2 in (Switch)CLI? Is there any command to check whether the switch Layer(Layer 3 or 2) in CLI?
sjarasan's user avatar
  • 125
6 votes
3 answers
12k views

When is 'Timestamp' and 'Timestamp Reply' are used in ICMP protocol?

I started the packet capture in my laptop through Wireshark application. Now through cmd, I pinged some server (say, www.google.com). And i stopped packet capture and analysed the packets being ...
Deepak's user avatar
  • 697
6 votes
3 answers
1k views

When is a layer 3 switch more appropriate than a router in a branch office?

It seems that as time passes, vendors have added almost all the features of a router to a layer 3 switch. For branch office deployments, how do other people who design networks choose between ...
Sam A's user avatar
  • 126
6 votes
4 answers
5k views

In the TCP/IP model, how does the application layer inform the Internet layer of the destination IP address?

New to networking. How does the application layer inform the Internet layer of the destination IP address? Let's say I make a Http request to www.google.com, which the application layer resolves to 8....
Kim Akool's user avatar
6 votes
3 answers
3k views

Quagga OSPF DR/BDR Mismatch: Losing Routes

I am experiencing a problem with lost routes in an OSPF over DMVPN environment. I am running ubuntu server 14.04 with Quagga 0.99.22.4, and OpenNHRP 0.14.1. While technically the DMVPN network is an ...
Travis Hegner's user avatar
6 votes
1 answer
1k views

Connect Multiple VLANs which reside on same L3 Switch

Introduction The steps described in this manual have been applied in order to enable the connection between multiple VLANs which reside on the same switch. Two VLANs have been configured. VLAN 1: ...
030's user avatar
  • 553
6 votes
5 answers
1k views

Inter-VLAN Routing

I am trying to configure inter-VLAN routing, but I can't route using MLS. I attached the image. Please check and give the answers. In switch0 I created VLANs 2 and 4; in switch1 only VLAN 3; in ...
Robichayanzz's user avatar
6 votes
1 answer
6k views

Assistance with routing between layer 3 switches with ospf

I have two Cisco 3560's in a branch office. One acts as an access switch configured with two vlans: interface Vlan10 ip address 192.168.0.254 255.255.255.0 interface Vlan20 ip address 172.16.0.254 ...
SwitchArt's user avatar
5 votes
4 answers
2k 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. ...
Rom's user avatar
  • 163
5 votes
3 answers
5k views

How does Deep Packet Inspection work with encrypted packets?

I've been reading up on Deep Packet Inspection for Software Defined Networks. However, I know that some applications use encryption to evade Deep Packet Inspection. Is there any alternative (such as ...
boomselector's user avatar
5 votes
3 answers
2k 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 ...
Michael May's user avatar
  • 1,005
5 votes
3 answers
5k views

Does a router send frames or packets?

Perhaps it's a novice question, but does a router send/receive frames? When I read about this subject, it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in ...
Smbbo's user avatar
  • 51
5 votes
5 answers
5k views

Why can't two hosts (in two different logical networks) connected to each other through cross-over cable communicate?

Let us say, i have two computers (hosts) A and B. Where A has been assigned an IP address of 192.168.1.10/24 and host B has been assigned an IP address of 192.168.2.11/24. And both of them has been ...
Deepak's user avatar
  • 697
5 votes
2 answers
723 views

Is a datagram from an upper network layer converted 1:1 to one of the lower layer?

I know datagrams from upper layers are converted to those of lower layers by adding metadata, but I cannot seem to find this information anywhere. Which one of the below is correct? A single TCP ...
Livy's user avatar
  • 423
5 votes
5 answers
547 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 ...
caiokameda's user avatar
5 votes
3 answers
518 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 ...
Ron Vince's user avatar
  • 927
5 votes
2 answers
710 views

Can a jumbogram (giant data packet) be used at a large scale (in WAN for example)?

There are many scientific paper for Jumbo frame, even more, some real implementations and deployments. But it is not the case for Jumbogram !! Jumbo frame (geant frame) for Layer 2; Jumbogram (geant ...
Lee's user avatar
  • 63
5 votes
1 answer
9k 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, ...
user2720323's user avatar
  • 1,611
5 votes
2 answers
649 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 ...
DanielJay's user avatar
  • 453
5 votes
3 answers
2k views

When are MAC addresses used instead of IP addresses in the OSI model?

Okay, so I'm studying for the Network+ exam, and I've gone over the OSI model about a million times, and I understand most of it fairly well, and my studies are coming along fine. However, there is ...
golgothan3's user avatar
5 votes
2 answers
423 views

What does 'moving packets away from the source' in RPF means?

I recently started studying multicast and I am a bit confused by how reverse path forwarding is explained. Most of the sources describe RPF as 'moving packets away from the source' but I understand ...
Marcus's user avatar
  • 183
5 votes
3 answers
12k views

How to trunk VLANs over Layer 3 EtherChannel?

I am trying to setup a network that includes an EtherChannel link between two Cisco Layer 3 switches (Catalyst 3750). I am using a collapsed core hierarchical network. The picture above is my ...
Roy T.'s user avatar
  • 65
5 votes
2 answers
505 views

Does the IP header have a "Padding" field?

I found two images of the IP header, one image shows a field called "Padding", while the other image don't show the "Padding" field: Which one of these two images is correct?
user4582812's user avatar
5 votes
1 answer
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 ...
berto's user avatar
  • 153
5 votes
1 answer
748 views

Cisco Router Multiple Interfaces in the Same Broadcast Domain

I have a Cisco 3945E with two EtherSwitch modules (gi 1/0, gi 2/0) and a UCSE (ucse 4/0) server installed in it. The short of it is i'd like to trunk VLANs to the two EtherSwitch modules, and place ...
Justin Jahn's user avatar
5 votes
1 answer
3k views

Cisco 3172 asks for LAN_ENTERPRISE_SERVICES_PKG license to enable OSPF and PIM when they are LAN_BASE_SERVICES_PKG license features

I have several Nexus 3100 switches in which I use OSPF and PIM. Cisco says that these features are included in the LAN_BASE_SERVICES_PKG which is included by default in the switches. Switch1# sh ...
UaT's user avatar
  • 73
5 votes
1 answer
370 views

Optimizing Network Topology

Recently I wish to split the network by VLANs. Mainly for two reasons Exclude management interfaces from the commonly accessible lan Separate WiFi traffic (security and broadcasts) The current idea ...
Dex's user avatar
  • 205
5 votes
1 answer
1k views

Layer 3 network, multiple internet connections

I'm using a Procurve 5406zl (J8697A) layer 3 switch with multiple vlans: VLANs: v10 Main Network 10.1.4.1 v12 Wireless 10.1.8.1 v13 Restricted Wireless 10.0.0.1 v14 Tech 192.168.7.1 v50 Internet 192....
Jason Tucker's user avatar
4 votes
4 answers
1k views

TCP/IP and data cut

I would like to clarify my understand about TCP/IP model and packet cutting. I think it's done by layer 3 (Transport, eg. TCP) but I'm note sure about layer 2 (Network, eg. IP) or even layer 1 (...
PanCho's user avatar
  • 41
4 votes
3 answers
9k views

What is the difference between ff-ff-ff-ff-ff-ff (layer 2 broadcast) 255.255.255.255 (layer 3 limited broadcast)?

Please explain the flow of traffic in each case.
Vishnu Vivek's user avatar

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