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Questions tagged [layer3]

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

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33
votes
3answers
97k views

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 ...
18
votes
3answers
7k 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 ...
17
votes
6answers
55k 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 ...
13
votes
6answers
10k 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 ...
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
9k 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
1answer
11k 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 ...
9
votes
3answers
1k 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 "...
8
votes
3answers
23k 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 ...
8
votes
2answers
9k 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 ...
8
votes
3answers
29k 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 ...
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
4answers
499 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 ...
6
votes
4answers
2k 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 ...
6
votes
4answers
3k views

Advantage of layer 3 switch over Router

What is the advantage of Layer 3 switch over the Router ??
6
votes
4answers
3k 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....
6
votes
3answers
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 ...
6
votes
1answer
992 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: ...
6
votes
5answers
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 ...
6
votes
1answer
3k 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 ...
5
votes
5answers
2k 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 ...
5
votes
3answers
423 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
2answers
537 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
3answers
21k 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 ...
5
votes
3answers
6k 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 ...
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
2answers
4k 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
578 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 ...
5
votes
1answer
290 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 ...
5
votes
1answer
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....
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
547 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
4answers
44k views

Intervlan routing using two Layer 3 Switches in Packet Tracer

I am having trouble when I am using two Layer 3 switches and two PCs to create a simple network for Inter Vlan routing, First of all have a look at this network, The configuration for PC1 is, IP: ...
4
votes
3answers
2k 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 ...
4
votes
3answers
301 views

Leaf-Spine network without routing

If I have a leaf-spine network with all the servers on the same subnet, say 10.10.0.0/16, do I need a router (or l3) since everything is on the same network and the network is flat?
4
votes
2answers
2k 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
3answers
958 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 ...
4
votes
1answer
6k 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
2answers
1k views

Difference between router leg and static routes

I am looking a L2/L3 switch spec. which says it supports 256 Router Legs and maximum of 32 Static routes. Can somebody explain with an example, what is the difference between router leg and static ...
4
votes
3answers
569 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 ...
4
votes
3answers
775 views

Need for network layer

I was reading about the 'IP' in TCP/IP. Most of its functions seem to be for connecting different (types of) networks. Does this mean that network layer is not required for communication between ...
4
votes
2answers
9k 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)?
4
votes
3answers
998 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 ...
4
votes
3answers
4k views

Layer 3 Switch Routing vs Router on a Stick

CORE: Dell N4032 (6.3.0.18) Access: Dell N2048 (6.3.1.13) We are in the process of setting up a serviced office that will serve multiple businesses. Each "customer" will naturally be on their own ...
4
votes
3answers
5k views

Layer 2 vs Layer 3 core - with HP IRF

The SMB organization I work for is migrating to HP/3COM Comware L3 switches and we will be implementing the standard Core>Distribution>Access design. Please refer to this diagram for clarification: ...
4
votes
1answer
708 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 ...
4
votes
1answer
844 views

Inter-VLAN Routing with VTP Server and Clients in Packet Tracer

I am new to the networking world and am attempting my first few projects in Packet Tracer. The first few were easy but I've gotten to a road block. My goal is to have three separate VLANs with at ...
4
votes
2answers
578 views

Can I Use BGP as an IGP - Multisite w Redundant Layer 3 MPLS WANs

The requirements are Hundreds of sites. Each has a router and firewall and a switch stack. Layer 3 MPLS WAN with BGP to provider VRF. BGP as an IGP? iBGP Would you distribute a public ASN across ...
4
votes
1answer
2k views

Help with VLANs, ACLs, DHCP on SG-300 Layer 3 switch

I have to do the following configuration on an cisco Sg-300 small business L3 switch as in the figure. Since i don’t have access to this switche for practice before doing this live in production i ...
4
votes
1answer
82 views

How does Junos distinguish that route is learned in Layer 3 or Layer 2 VPN environment?

As I understand from Understanding Junos OS Routing Tables document, then bgp.l3vpn.0/bgp.l3vpn-inet6.0 or bgp.l2vpn.0 tables contain routes learned from other PE routers in Layer 3 or Layer 2 VPN ...