80
votes
Accepted
Why does DHCP use UDP and not TCP?
DHCP cannot use TCP as the transport protocol because TCP requires both end-points to have unique IP addresses. At the time a host is required to use DHCP, it does not have an IP address it can source ...
- 1,700
34
votes
Accepted
Why is broadcast used at the DHCPREQUEST step?
https://www.rfc-editor.org/rfc/rfc2131#page-16
The servers receive the DHCPREQUEST broadcast from the client.
Those servers not selected by the DHCPREQUEST message use the
message as notification ...
- 993
26
votes
Why does DHCP use UDP and not TCP?
Since the source has no IP address (0.0.0.0) and the destination is everyone (255.255.255.255), it's hard to see how you would identify a particular session. But even if you could, what would be the ...
- 67.1k
21
votes
Why does DHCP use UDP and not TCP?
There are multiple reasons why TCP wouldn't work for DHCP(v4.)
First of all, TCP is connection-oriented. A TCP connection is defined between two particular hosts. However, when a DHCP client first ...
- 469
11
votes
Why is broadcast used at the DHCPREQUEST step?
You can potentially have multiple DHCP servers - the request is sent as broadcast to notify the other DHCP servers that have potentially sent offers that their offer isn't taken.
- 82.6k
10
votes
Accepted
Should a router use SLAAC for IPv6 address assignment?
In RFC 7084, it also states:
W-1: When the router is attached to the WAN interface link, it MUST
act as an IPv6 host for the purposes of stateless [RFC4862] or
stateful [RFC3315] interface address ...
- 9,421
10
votes
Accepted
dhcpv6 - stateful VS stateless, what is difference between it?
A stateful address assignment involves someone keeping track of the State. Which is to say, some system exists that provides a log that certain IP addresses were assigned to certain MAC addresses. ...
- 14.9k
7
votes
Should a router use SLAAC for IPv6 address assignment?
RFC 4862 really describes routers within a single administrative control. RFC 7084 clarifies how customer routers should behave. This gives an ISP some freedom in how it supports IPv6 to its customers....
- 98.7k
7
votes
Accepted
How IP helper address works?
Using the ip helper-address command sets up a DHCP relay. DHCP only works on the LAN to which it is connected because it uses broadcast, which cannot cross a router. To remedy that situation, DHCP ...
- 98.7k
7
votes
Accepted
Importance of DHCP option 12, client host name
You wouldn't set this option on the whole scope. Rather, you'd set it for a reserved address, so that a specific host gets a defined IP address and a defined host name.
- 82.6k
7
votes
Why does DHCP use UDP and not TCP?
When the host starts up it has no knowledge of the network addresses or masks that it should use. The only way for it to communicate is via an IP broadcast (255.255.255.255) to the local network. The ...
- 91
6
votes
Accepted
Can I use PPPoE and DHCP simultaneously?
According to Cisco, you cannot use DHCP with PPPoE, :
PPPoE is not supported in conjunction with DHCP because with PPPoE the
IP address is assigned by PPP.
Juniper has a whitepaper about ...
- 98.7k
6
votes
Accepted
How to setup a lease to 1 hour
In a nutshell, yes.
For the future reference of others, a snippet from this document:
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html#wp1001095
Configuring ...
- 776
6
votes
Should a router use SLAAC for IPv6 address assignment?
Why does RFC 4862 say "Since host autoconfiguration uses information advertised by routers, routers will need to be configured by some other means."
The people who wrote that document (and it's ...
- 13.1k
6
votes
How IP helper address works?
To add to Ron's great answer:
The key piece of this is that the Router sending the DHCP request on behalf of the client will unicast the request with a source address from the address space the ...
- 636
6
votes
Why is broadcast used at the DHCPREQUEST step?
Because until the server sends the DHCPACK, the client still doesn’t have an IP address. It is possible for a DHCP server to respond to a request with a DHCPNACK.
- 27.2k
6
votes
Accepted
What is the difference between DHCP and NAT?
Missing from the current answers is
DHCP is a protocol for configuring hosts, and runs periodically to keep their addresses and other configuration up to date. The traffic is between the host and a ...
- 16.2k
6
votes
Accepted
Why are DHCP ACK and REQ messages broadcast in Link Layer?
From RFC 2131 "Dynamic Host Configuration Protocol"
Remembering that there can be multiple DHCP servers, the DHCPREQUEST is broadcast because:
Those servers not selected by the DHCPREQUEST ...
- 16.2k
6
votes
Does a router always have two IP addresses?
Welcome to Network Engineering! Routers route between networks, so they need interfaces in the networks they route to. Otherwise, they're like a "bridge to nowhere."
- 67.1k
6
votes
Accepted
Does a router always have two IP addresses?
As others have said -- ignoring misconfigurations, bugs, errors states and so on -- a router needs to be on multiple networks and so most naturally has multiple IP addresses.
But there are a couple ...
- 16.2k
6
votes
Routing DHCP into different LANS
You need on your router a feature called DHCP relay
You configure it with the IP address of your DHCP server.
When the router receive a DHCP request it will send (as unicast) a request to the DHCP ...
- 19.5k
6
votes
Accepted
Client timeout for Cisco IOS Static DHCP Pool
It looks like your configuration for the DHCP reservation is incorrect. It seems you only have the hardware address (used for BOOTP) missing the network type, but not the client identifier (used for ...
- 98.7k
6
votes
Accepted
pfsense DHCP lease remember time settings?
You need to change the DHCP lease duration.
It is a standard option in all DHCP servers.
In pfsense you'll find related settings under the Services / DHCP server page, in the "other options" part:
...
- 19.5k
6
votes
Accepted
Do Directly Connected Routers Need Routing Protocols For DHCP Relay To Work?
Routers need to know how to reach networks in order to send packets to those networks. DHCP relay is sending packets to other networks. Routers learn routes in three ways:
Directly connected networks
...
- 98.7k
5
votes
How we can assume which network protocol is working in which OSI Layer?
In addition to what others have said regarding the services each layer provides, different layers typically also differ in terms of how far they are encapsulated within packets (and in what order they ...
- 315
5
votes
Accepted
Assign IP address using Router and Switch (DHCP)
Switches don't get involved in layer-3 (e.g. IP). A switch is a transparent layer-2 device, so hosts connected to a switch will have no problem getting to a DHCP server on the same LAN.
- 98.7k
5
votes
L3 leaf-spine architecture and DHCP ip assignment
My guess would be to use a DHCP relay agent on your core switch/router. Configure this under each vlan interface and point it to your DHCP server. The DHCP discoveries will only stay on the LAN ...
- 2,016
5
votes
How does a DHCP server differentiate between 2 requesting devices?
The DHCP server knows the layer-2 (MAC) address of the requesting host because of the CHADDR (Client hardware address) field in the DHCPDISCOVER message. The DHCPOFFER message also contains a CHADDR ...
- 98.7k
5
votes
Accepted
Difference Between Number of CPEs and Number of hosts
CPE stand for Customer Premise Equipment
It is mainly used in the context of a connection provided by an ISP to a customer site. The CPE is the router put on the customer site on which the line is ...
- 19.5k
5
votes
dhcpv6 - stateful VS stateless, what is difference between it?
Stateful configuration
Stateless configuration (also known as SLAAC-StateLess AutoConfiguration)
The stateful version of DHCPv6 is pretty much the same as for IPv4. Our DHCPv6 server will assign IPv6 ...
- 51
Only top scored, non community-wiki answers of a minimum length are eligible