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Questions tagged [arp]

For questions about Address Resolution Protocol (ARP). ARP resolves OSI layer-3 addresses to OSI layer-2 addresses.

35
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3answers
104k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
20
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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 ...
18
votes
6answers
11k views

Best practice for the combination of HSRP and ECMP

The combination of ECMP (or other causes of asymmetric paths) and HSRP is broken by default in Cisco IOS; the default behaviour with this design floods unicast traffic excessively. What is the best-...
14
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6answers
21k 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
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5answers
9k views

Duplicate IP Address - which one wins?

If there's a duplicate IP address, which one "wins"? First, last, flaps, neither?
12
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3answers
32k 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
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 ...
12
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6answers
21k views

What happens when the ARP cache overflows?

In at least one implementation there is a hard limit on the capacity of the ARP table. What happens when the ARP cache is full and a packet is offered with a destination (or next-hop) that isn't ...
11
votes
1answer
4k views

Should ARP requests be VLAN tagged?

I'm wondering if the ARP requests should be tagged when being sent out of a VLAN tagged interface? It seems logical to me that the ARP request should be tagged, since it needs to travel all the ...
10
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4answers
32k views

Why do switches need ARP tables when the translation are done on the machines side?

As it is clear from the title, why do switches need ARP tables when the translation are done on the machines side? Roughly saying, why there are two ARP tables on machines and on switches? Is not the ...
10
votes
2answers
552 views

Why is adapter ARPing something it already knows?

I don't get why a machine on the network is asking the gateway for its MAC address when it already knows its MAC address: So here you can see the machine MAC ***80 (IP ..*.115) asks the gateway (...
10
votes
3answers
11k views

Find MAC address of a remote computer using IP

Is there a way to find out a computer's MAC address if we know its IP address, assuming this computer is outside our LAN? I know we can easily get the physical address of a computer in our LAN using ...
9
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2answers
2k views

Differences and functions of type codes 0800 and 0806

What are the primary differences and functions (or applications) of type codes 0800 and 0806? Through my own research, I found the 0800 was related to the 0800 to the IP/Layer 3 of the OSI model. ...
8
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1answer
2k views

Something is filling my force10 ARP table

I have 2 force10 s25 switches that are used for a dark fiber link between two school buildings. One of my switches is filling its ARP table to the brim and then failing. This can happen as often as ...
7
votes
4answers
1k views

Does every host on the LAN share the same ARP table, or do hosts keep them individually?

I'm just learning about ARP. I'm a little bit confused. My confusion is better explained in a scenario. Host A is communicating with Host B. Host A knows Host B's IP address, but not it's MAC address....
7
votes
3answers
23k views

ARP request outside of LAN; Target machine or router response?

If I sent an ARP request to the target machine, which is in over Internet and has 2 routers between, what will the target IP address on my request (0001) packet be? Will it be the Router IP address, ...
7
votes
2answers
1k views

Won't GLBP confuse a switch's mac address-table?

Say for example two routers (R1 and R2) are connected to the same switch (SW1) and both routers have Gateway Load Balancing Protocol (GLBP) correctly configured as the First-Hop Redundancy Protocol (...
7
votes
2answers
6k views

ARP Replies can they be Broadcast

I have been informed by certain networking professionals that ARP replies can and sometimes actually are broadcast packets instead of unicast. If and when would you use a broadcast ARP reply?
7
votes
3answers
1k views

Will Linux send the ARP gratuitous or ARP probe (defined in RFC 5227)

I did an experiment to check if Linux will send the gratuitous ARP or ARP probe before using the target IP. The experiment scenario is as follows: There are two Linux computers, both connected to ...
6
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2answers
5k views

USE of ICMP echo request in DHCP

In RFC2131 (page 12) it's been mentioned that The server should use whatever information is available in the configuration information repository to choose an address to reuse. For example, ...
6
votes
3answers
943 views

What is the characteristic of spoofed ARP packets?

Hi what is the characteristic of spoofed ARP packets? I want to know how to detect it but before that I have to know how can I call a packet a spoofed packet. I hope you can help me thank you.
6
votes
1answer
2k views

ARP Questions (Packet Types, Ethernet, Cache, Gratuitous)

Lately I have been doing some research on the Address Resolution Protocol, and even though I know the idea behind it I still have some questions. Q1: Are there more types of ARP messages that I am ...
5
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3answers
3k views

Why do we need MAC Address if we can uniquely identify each machine with an IP Address

I learnt that in ARP Protocol, to send a packet from one machine to another machine we need to know the MAC Address of the destination machine but since the destination has already been assigned a ...
5
votes
2answers
6k views

How ARP works in case of Virtual machine?

As far as my understading goes MAC address is used to recognize a computer node in a LAN where as internet communications happen over IPv4 address. When a request comes from the internet to the ...
5
votes
3answers
12k views

What to use as ARP request Target Hardware Address?

When sending ARP request, what must be filled in the target hardware address (THA)? This technet blog post says it is to be filled with zeros (0.0.0.0) -although doesn't look like a MAC address!, and ...
5
votes
3answers
1k views

IP Address vs MAC Address, how does that really work together illustration

My question is simple, i don't really know if I understand it so I'm asking. Why do we have both addresses, if we know the ip address of the target and source computer, why do we need some physical ...
5
votes
5answers
278 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
1answer
4k views

ARP request unicast MAC?

I would like to ask, with ARP request I got to believe that the destination MAC address was in all situations a broadcast address. Yet I got across a wireshark capture: I just cannot understand the ...
5
votes
3answers
411 views

How to prevent IP conflict when using VRRP

We are currently using VSRP (Layer2/3) where we have a single IP as the gateway per VLAN. We are considering moving to VRRP (Layer3) which requires 3 IPs, 1 virtual and 2 real ones. This means that ...
5
votes
1answer
235 views

malloc failures and ARP packed on a Cisco 2901

I have a 2901 between a network and the internet. It shipped with the routing defined as follows. ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/1 Using the interface like this instead of an IP struck ...
5
votes
1answer
838 views

Why are routers not answering ARP Broadcasts?

Host A wants to send something to Host B who is in a different network. Between them is Router 1. Host A sends an ARP broadcast but Router 1 is not answering it. Why is that so?
5
votes
3answers
12k 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
2answers
596 views

Learn MAC addresses via replies to other hosts requests

I have a question regarding ARP: If host A wants to send a message to host B and the ARP table of host A does not contain a MAC address for host B then he sends an ARP request and host B replies with ...
5
votes
3answers
2k views

Server link bond across 2 switches with LACP, inter-switch LAG dropping alternate pings?

I have a Debian Linux server with 4x 1Gb NICs, 2 onboard and 2 PCI. All NICs are configured into a single mode 4 LACP bonded interface: bond0 I have two switches: Cisco SG300-28 and Cisco SG300-10, to ...
5
votes
1answer
1k views

Cisco HSRP CAM vs ARP timeout

Today we received email from our ISP saying they are performing maintenance on their network so if you have HSRP connectivity with them in that case you need to clear ARP table or reduce ARP timeout ...
5
votes
3answers
9k views

When exactly is ARP protocol is used

I am a quite a beginner in network engineering so please be nice. I am not sure I understand how ARP protocol works in some situation. For example, if we have following problem: ARP cache is empty ...
5
votes
1answer
394 views

Pinging across different subnets? This shouldn't work but it does

I have two computers: PC1 and PC2. PC1 has an IP address of 10.0.0.1/30, and PC2 has an IP address of 10.0.0.6/30. They are connected directly to each other with a crossover cable. My understanding is ...
5
votes
1answer
164 views

What happens at the hardware level of a router when a packet is received and eventually triggers a process like ARP?

Please let me know where the gaps in my understanding are. There are many gaps, so please do not hesitate to fill them in or correct them. To my knowledge, what happens on the hardware level when ...
5
votes
2answers
9k views

Difference between proxy arp and default gateway

Firstly what I understand, Default Gateway--> When we set the default gateway in PC the network layer check the destination IP address and if it is not on same broadcast domain as the source IP ...
4
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3answers
3k views

How will be the ARP Request (Unicast/Broadcast) if Subnet Mask is Unknown

Please help me with one doubt regarding ARP requests. Based on my understanding the subnet of an IP is identified by it's Subnet Mask so consider this scenario:- Suppose Device1 with IP- 30.129.33....
4
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3answers
30k views

Understanding ARP and Routers

Let's say there are two PCs, PC A and PC B. PC A wants to communicate with PC B, such as sending a message. I need to describe the arp packets for this task. Known details: PC A -> SW 1 -> Router -> ...
4
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4answers
2k views

ARP table does not store devices responding to a broadcasted ping request

I have got a simple question, I am doing a broadcasted ping request on my local network, using a Mac with the latest Yosemite 10.10.4. So I ping 255.255.255.255, I get responses from a bunch of ...
4
votes
5answers
783 views

Do we really need ARP when only hubs are used to connect devices?

In the case only hubs are used to connect devices do we need ARP? Packets will be forwarded to every device. So, when a packet whose destination IP doest does not match with the device IP, it will be ...
4
votes
1answer
353 views

ARP table aging: What timer can I expect?

Is the hold time standardized? Is there a standard or reliable recommendation for a timeout? I am well aware that a SOHO non-managed switch requires a shorter timer than a data-center switch. That ...
4
votes
1answer
400 views

Why would a switch ever send an ARP request for a MAC address when it can just wait for the first packet to be received from a device?

I've been reading 'Windows Networking Fundamentals' and in that book it says that a switch can learn what MAC address is associated with a particular port when the connected device makes its first ...
4
votes
2answers
763 views

How does the ARP table work in case of the Gateway Router?

Suppose I am a host, I understand how ARP works in case I address an IP within my subnet. For each IP address there is a corresponding MAC address, and the owner responds to my broadcast ARP query. ...
4
votes
1answer
4k views

What happens if two hosts with the same MAC address are in the same broadcast domain?

I am aware that situation, where duplicate MAC addresses appear in the same broadcast domain, should never happen. However, if such situation happens, then how will the network work? I think one ...
4
votes
1answer
13k views

Sonicwall NSA 2600 routing issues with multiple LAN interfaces configured

We have a Sonicwall NSA 2400 that has been in service for some years now. We've never had an issue with any routing until we "upgraded" to a new NSA 2600 device. Below is a rough idea of the ...
4
votes
2answers
2k views

Wrong dynamic ARP entry - IP Address couldn't pinged

Situation: Our Stacked Switch (two WS-C2960X-48LPS-L - 15.2(2)E3 - 192.168.10.19 - VLAN 10) wont be reachable via ICMP or SNMP by the Cisco Prime Infrastructure (172.16.2.103 - VLAN1[nativ]) and back. ...
4
votes
1answer
76 views

ARP Packet format question

I am unable to understand the meaning behind the following extract from the ARP RFC 826: For the 10Mbit Ethernet, the value in the protocol field (ar$pro) is taken from the set ether_type$. This ...