Questions tagged [dhcp]

For questions about DHCP in in relation to network devices. For instance, you are setting up DHCP on a layer 3 switch.

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27
votes
8answers
18k views

Why does DHCP use UDP and not TCP?

DHCP uses UDP as its transport protocol. DHCP messages that a client sends to a server are sent to well-known port 67 (UDP—Bootstrap Protocol and DHCP). DHCP Messages  that a server sends to a client ...
20
votes
3answers
7k views

Why is broadcast used at the DHCPREQUEST step?

This is the process of DHCP operation, My question is at the 3rd step why does the Client send a Broadcast and not a Unicast as after the previous two operations the address of the DHCP server / ...
18
votes
4answers
8k views

Can DHCP Server determine client's OS? [closed]

Is it possible for the DHCP Server to determine the client operating system? I'm working on a monitoring tool for gateways on local networks that is web-based and would like to be able to somehow ...
18
votes
4answers
24k views

Cisco APs ignore DHCP Option 43 and use CAPWAP UDP broadcasts

Background I am trying to redeploy Cisco 1242 LWAPs on new Cat4507R+E switches, which means the IP address of the WLC is changing for that LWAP. For some reason, I have to configure ip helper-...
16
votes
4answers
68k views

Best practice using Cisco's “ip-helper” for DHCP?

Our topology is such that we have two 4510's in our IDF closets. Each switch has a data VLAN and a voice VLAN. The switches are layer 2 trunked to the core, where the VLAN interfaces are, routing ...
15
votes
4answers
39k views

How are two “ip helper-address” statements handled?

I've discovered a subnet with two "ip helper-address" statements. This is not our normal configuration. interface Vlan300 ip address 10.1.3.2 255.255.255.0 ip helper-address 10.2.1.10 ip helper-...
14
votes
5answers
11k views

Duplicate IP Address - which one wins?

If there's a duplicate IP address, which one "wins"? First, last, flaps, neither?
14
votes
3answers
32k views

How broadcasting works on different networks?

I was reading DHCP ( RFC 2131 ), I have basic knowledge about Relay Agent and Broadcasting. But for Understanding DHCP in depth, I felt like I have to know Broadcasting and Relay Agent ( RFC 1542 ) in ...
13
votes
3answers
4k views

Should a router use SLAAC for IPv6 address assignment?

I am a little confused by the following 2 RFCs relating to IPv6: RFC 4862 (IPv6 Stateless Address Autoconfiguration) RFC 7084 (IPv6 CE Router Requirements) RFC 4862 states: The autoconfiguration ...
12
votes
6answers
37k views

Is a DHCP offer packet a broadcast or unicast?

DHCP OFFER is a layer3 broadcast because the server don't know client Ip, but server knows the client mac address. So how it would be the broadcast? If anyone ask me about this? how should I answer? ...
12
votes
7answers
15k views

DHCP relay problem on interfaces secondary network

A secondary network was configured on an interface of a Cisco 2621 router and the ip helper-address commands put in place but no clients of that secondary network could pull DHCP addresses. We swapped ...
11
votes
2answers
3k views

IPv6 DHCP Server vs Router

I am setting up a testing environment for dualstack. I currently have a server (Windows 2008 R2), 2 switches and a router installed and configured. The server is the Primary Domain Controller and it ...
10
votes
3answers
2k views

DHCP assign IP address based on MAC address with Cisco CE 500 platform?

I have very little knowledge about CISCO and its switches, but now I have to configure it myself. Is it possible to allot IP address based on MAC address on CISCO CE 500 (Cisco Catalyst Express 500-...
10
votes
4answers
6k views

Is it possible to disable a single DHCP pool in IOS?

I am working on a Cisco L3 switch where most of SVIs are having ip helper-address configured for DHCP relay but also a couple of DHCP pools have been configured in the switch itself to serve dynamic ...
10
votes
4answers
18k views

Setting up DHCP server on the router or on the switch?

We're using the Cisco 1921/K9 with a SG300 L3 switch. Does one set up DHCP server on the router or on the SG300? If both is capable, what is the reason to choose one over the other? Note: I have 4 ...
10
votes
3answers
3k views

SRX DHCP client compatibility with HP Procurve DHCP Relay

I am trying to bootstrap the config on some Juniper SRX100s and am having some DHCP issues. Specifically, I am connecting the 0/0 port (fe-0/0/0 in the software) to my existing network, where DHCP ...
10
votes
2answers
5k views

Does Cisco *ip dhcp snooping limit rate* apply if DHCP snooping is not configured for the access VLAN?

Came across a situation where DHCP snooping was enabled on a Cisco switch, but only for certain VLANs. However, all access ports had a ip dhcp snooping limit rate 15 applied whether or not DHCP ...
9
votes
4answers
7k views

Changing the DHCP server

We have a LAN with 20 machines on it. They are connected to a switch which also connects to our primary Westell DSL modem for internet access. The modem does the NATing and is the DHCP server, (so ...
9
votes
2answers
58k views

Stateful vs Stateless IPv6?

What's the difference here? Are stateful IPv6 entries retained in the DHCPv6 server?
9
votes
3answers
23k views

Assign an IP address based on switch port

Short: I want DHCP server to assign a preconfigured IP address based on a port (on a switch, ideally) a device is connected to. Long: My situation: I am building an automated indoor farm (for ...
9
votes
1answer
12k views

How DHCP OFFER unicast works?

In the DHCP discover packet, the source IP address field is 0.0.0.0 which means the client does not have an IP address yet. But, in the DHCP offer packet, an unicast address (which will be allocated ...
8
votes
3answers
1k views

DHCP Relay by MAC Address of CPE

We are testing a new Provisioning Server in DOCSIS/CMTS environment and we need to setup a lab with 5 models of CPE. I'm wondering if we can configure on Cisco Routers the dhcp relay (ip helper) ...
8
votes
1answer
1k views

DHCP Snooping Confusion

I'm trying to work out how to enable dhcp snooping and dynamic arp inspection on our guest network and I'm a little confused.... Here's the situation: We have a hub and spoke topology, with a ...
8
votes
3answers
3k views

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

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 ...
8
votes
1answer
2k views

DHCP-Snooping on 3com S4210

I'm setting up dhcp snooping with IP source guard and DAI on some 3com S4210 switches. I found problem with recording bindings in snooping database. In some cases I know that there is PC connected to ...
8
votes
3answers
592 views

Map the underlying structure of a VLAN

Is there any way to detect the underlying layout physical layout of a VLAN. For instance, to know whether two devices in the same VLAN are connected via the same wifi AP. To clarify, this would be ...
7
votes
2answers
3k views

SVI as a DHCP client

Can a Cisco switch be configured to have a management interface (or SVI, or VLAN interface) become a DHCP client, therefore obtaining its own IP address on the respective network? This is out of ...
7
votes
3answers
20k views

Does authoritative DHCP server mean no static IP setting? [closed]

I've recently switched my Mikrotik Router's DHCP server to authoritative mode. It fixed several problems with some clients remembering bad IP addresses. But what would happen if I connected a device ...
7
votes
2answers
41k views

dhcpv6 - stateful VS stateless, what is difference between it?

Can someone explain me, what is difference between DHCPv6 Stateful and Stateless? I was looking on the net and, surprisingly, I found only the remains of information but some general comparison or ...
7
votes
1answer
789 views

Is possible to configure a Cisco 881 by DHCP?

I'm wondering if is possible to configure a Cisco Router 881 from a file on a tftp instruct on a DHCP option. I know is possible to configure the wan port through DHCP by configuring "ip address dhcp" ...
7
votes
1answer
570 views

What behavior results from both ip dhcp pool and ip helper-address?

If a device has a dhcp pool defined: ip dhcp pool DATA network 192.0.2.0 255.255.255.0 default-router 192.0.2.1 domain-name test.net But it also has a DHCP helper setup on the interface: ...
7
votes
2answers
20k views

Devices behind Catalyst 2960 not receiving DHCP addresses

I am having an issue with DHCP not traversing to devices behind a switch. I have many other switches deployed with nearly the same configuration that are able to receive a DHCP address. On this switch ...
7
votes
3answers
56k views

dhcp failed. apipa is being used

I'm testing a network and I use the vlan among 20 vlan but in this packet tracer I just use 10 Vlan on my network, In this case, I use the DHCP server on my multilayer switch, not on my router. so ...
6
votes
2answers
7k 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
1answer
9k views

Specific DHCP pool on specific interface - IOS

Is it possible for a specific interface to only allow access to specific dhcp pools? This is assuming a cisco router(IOS) with dhcp server capabilities. E.g. For some router, hosts on fa0/0 can ...
6
votes
1answer
9k views

Mobile devices behind wireless bridge not getting IP address

I'm trying to get a wireless link set up to a building about 1/2 mile away from the rest of my network. I'm using a pair of Engenius EOC2611P radios, and as far as I can tell everything is set up ...
6
votes
1answer
9k views

Why do DHCP servers lease IP address based on Option-61 (Client-ID)?

Why do DHCP servers (specifically Cisco) use "Client-ID Options", not the CHADDR (client MAC) in the DHCP request, for static binding? From: Configuring DHCP All DHCP clients send a client ...
6
votes
1answer
5k views

Wireless (Ruckus) and Dynamic VLAN Assignment via Microsoft NPS

Our current 802.11 setup has a large number of SSIDs to segregate traffic by subnet. This isn't ideal, and I've been attempting to consolidate to a single SSID but use dynamic VLANs instead. This is ...
5
votes
3answers
21k views

Can Cisco Catalyst switch get IP from DHCP server?

This may be stupid question as I don't have much knowledge in networking. Anyways, I have Cisco Catalyst 2960 switch and I'm curios if I can configure it to get IP configuration from DHCP server, ...
5
votes
3answers
4k views

How to force the use of a DHCP-issued IP on Cisco switch

What would be the correct way to force a client on an access port to use a dynamically assigned IP address ? In other words, using the Cisco access layer switch, I would like to prevent the client's ...
5
votes
5answers
989 views

How we can assume which network protocol is working in which OSI Layer?

For a long time, I have this kind of silly question: How we can say some network protocols are working in X layer of the OSI model? For example, HTTP working in L7 of OSI, OSPF is L3, ICMP is L4, TCP ...
5
votes
2answers
6k views

Obtain Static IP set in DHCP on Cisco ASA

I am working with an ASA-5505. I am trying to set aside a set of 10 IP addresses. Currently I have internally 10.1.10.100 - 254 set for my current vlan -- vlan 1, and all equipment attached is ...
5
votes
1answer
6k views

DHCP with Brocade

I have the following config on a brocade icx with the premium license for L3 features: stack unit 1 module 1 icx6450-24p-poe-port-management-module module 2 icx6450-sfp-plus-4port-40g-module ! ...
5
votes
1answer
810 views

HP 4210G DHCP relay

I have an HP 4210G, which is a rebranded 3com 4200G. I have set up DHCP relay on the switch, but I'm only able to select one server group aka one IP at a time. Is it possible to set up multiple DHCP ...
4
votes
3answers
2k views

Where to properly put the DHCP server? [closed]

In a network that's being designed to provide wireless and wired access in a library, where there will be more or less 50 users at the same time... Where would be better to handle the DHCP requests? ...
4
votes
4answers
21k views

DHCP server for multiple VLANs

Good afternoon, I'm learning how to build a network, and I'm trying to add a DHCP server for all VLANs, but I don´t understand how to make it work for all hosts regardless of whether they are on the ...
4
votes
2answers
1k views

DHCP - Address management during a disconnection

When a host is disconnected, the assigned IP remains unavailable until the end of "lease time" or immediately becomes available to be reassigned? How DCHP manages this?
4
votes
3answers
151 views

Interface VLAN IP address not on exclude list?

Does an IP address need to be included in a ip dhcp excluded-address x.x.x.x if its already included in an interface vlan? Examples: ip dhcp excluded-address 10.3.8.1 10.3.8.20 interface Vlan30 ...
4
votes
3answers
943 views

can a cisco ASA be configured as a DHCP server for a VLAN on a subinterface? and route packets between VLANs?

We are in the process of planning to phase out a Sonicwall tz205 in our office. i am hoping to replace it with a CISCO ASA 5516 specifically. We have three networks in house the main production ...
4
votes
2answers
2k views

Some question about the relationship between VLAN , DHCP and subnet

I am new here and very interested in computer networks. From a book, I have some questions, for which I need help. question1: Can there be only one working DHCP server in one VLAN? question2: Can ...

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