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77 votes

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 ...
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32 votes
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Why is broadcast used at the DHCPREQUEST step?

https://www.rfc-editor.org/rfc/rfc2131#page-13 The servers receive the DHCPREQUEST broadcast from the client. Those servers not selected by the DHCPREQUEST message use the message as notification ...
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25 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 ...
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20 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 ...
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15 votes
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How broadcasting works on different networks?

How Routers Handle Limited and Directed Broadcasts The first thing to understand to answer your questions is that limited broadcast frames are not routed. By default when a router receives a frame ...
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15 votes
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How does DHCP assign an IP address?

The basic process is quite simple. I'll only cover that and omit scenarios where several DHCP servers exist, error conditions crop up or discovery has to cross network boundaries. A new client on a ...
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15 votes
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SVI as a DHCP client

You can configure an SVI as a dhcp client by using the following command within the SVI. ip address dhcp The same command is used on physical interfaces as well.
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15 votes
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Stateful vs Stateless IPv6?

Stateful autoconfiguration of IPv6 is the equivalent to the use of DHCP in IPv4. It requires a DHCPv6 service to provide the IPv6 address to the client device and that both client device and server ...
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13 votes
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Is a DHCP offer packet a broadcast or unicast?

Be aware of the broadcast flag, see RFC2131 (page 24): A client that cannot receive unicast IP datagrams until its protocol software has been configured with an IP address SHOULD set the ...
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13 votes

Is a DHCP offer packet a broadcast or unicast?

The answer is that it can be either broadcast or unicast - and in some cases both unicast and broadcast before it reaches the client when an ip helper-address is used. A client doesn't actually have ...
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12 votes
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USE of ICMP echo request in DHCP

An ICMP Echo request from the DHCP server to the IP address its about to allocate is used to determine if the IP address about to be assigned is already in use on the network. If a response is ...
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12 votes
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Assign an IP address based on switch port

is it possible to assign a preconfigured IP address based on a port a device is connected to? What devices do I need for this? You can use a Cisco switch and an IOS that supports DHCP Server Port-...
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11 votes
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IPv6 DHCP Server vs Router

IPv6 has more options for configuring addresses than IPv4. The process works as follows: A new client joins the network and sends a Router Solicitation (RS) Each router (can be multiple) sends a ...
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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.
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10 votes
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Which DHCP server will work if there are two of them in a network?

DHCP (Dynamic Host Configuration Protocol) utilizes four key steps as illustrated below: DHCP Discovery (DHCPDISCOVER) - Sent by the Client seeking a DHCP Server. DHCP Offer (DHCPOFFER) - Sent by ...
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10 votes
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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 ...
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9 votes
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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 ? You can't "impose" an IP configuration mode to an host from the switch (or any other ...
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9 votes
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How DHCP OFFER unicast works?

This is because the DHCP server must reside or have a relay/proxy on the same L2 network as the client. The DHCP OFFER is sent to the L2 address of the client (i.e. it's MAC address). If the request ...
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9 votes
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Can Cisco Catalyst switch get IP from DHCP server?

You can specify an interface to receive an address from DHCP like this. configure terminal interface (int-name-num) ip address dhcp This guide gives way more detail into DHCP features - Cisco DHCP ...
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7 votes
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DHCP - Address management during a disconnection

If the client disconnect correctly through a DHCP Release (Graceful shutdown), then the lease ends upon receipt at the DHCP server, else the lease will expire upon the end of its actual "lease time"....
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  • 364
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....
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7 votes
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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 ...
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7 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.
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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 ...
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  • 91
6 votes
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ignore/block upstream DHCP, instead use switch DHCP?

Your scenario is somewhat confuse. If I uderstand well you want to filtered DHCP request on L2. You can achieve it by using the function DHCP Snooping features available on your platform. You ...
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6 votes
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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 ...
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6 votes
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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 ...
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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 ...
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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 ...
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  • 636
6 votes
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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. ...
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