70
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 the packets from, nor does it have the IP address of the DHCP server. So it uses 0.0.0.0 as the source IP address and 255.255.255.255 (broadcast) as the ...
30
https://tools.ietf.org/html/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 that the client has declined that server's
offer.
The protocol assumes there may be multiple DHCP servers. By broadcasting the request message,...
24
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 benefit? The data in a DHCP message is quite small (~300 bytes), so it can easily fit into a single segment. It's not worth the overhead to establish a TCP ...
18
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 starts up, it doesn't know which host(s) it wants to talk to. Its only option is to broadcast a DHCP DISCOVER message to all hosts on the local network. ...
17
IPv4:
It will "flap". On the sending host (another host or the gateway/router) there will be an ARP entry for the IP pointing to the MAC address of one of the hosts. Packets will go to one of the hosts, wherever the ARP entry currently points. This will effectively disrupt connectivity for both hosts.
IPv6:
The neighbor discovery protocol will do a ...
16
Is it possible to allot IP address based on MAC address on CISCO CE 500
No you need a real Cisco IOS switch for DHCP server functionality with Maunal Bindings; the Catalyst Express series switches do not run IOS
15
Some work has been done to determine subtle differences in DHCP packets from different OSes, resulting in DHCP fingerprints. Examples include the options present in DHCP request and their order, and the content of certain options like option 55 (parameter list).
Have a look at the papers and signatures at fingerbank.org. This suggests (have not tested it ...
15
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.
15
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 with a destination address that is broadcast at either layer 2 or layer 3, the router simply drops the frame. That's why routers are said to be the boundary of ...
15
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 network sends a DHCPDISCOVER via udp from
address 0.0.0.0 to 255.255.255.255:67 (broadcast, port 67).
If there is at least one DHCP servers listining in the ...
14
Well, one way to do this would be to exclude the entire pool from assignment. This would leave the configuration of the pool in there and you would just remove the exclusion if you wanted it to assign addresses again.
ip dhcp excluded-address <first address> <last address>
However, I would just archive a copy of the config and remove the pool.
13
Cisco routers will use the primary subnet configured on the interface for identifying forwarded DHCP requests.
The only other option is the DHCP smart relay feature, which allows a router to use the secondary subnet, but only after multiple DHCP offers for the primary subnet are not seen.
12
The ip helper-address command designates an address which broadcasts are forwarded to; it will send the original packet to each of those listed addresses.
If both reply, both replies are forwarded back to LAN.
12
Cisco LWAPs will do this process in order to try and find a controller:
CAPWAP broadcast on the local subnet
Check NVRAM for previous controllers/mobility groups and try those.
OTAP (remove now though)
DHCP Option 43/60
DNS Lookup for "CISCO-CAPWAP-CONTROLLER.localdomain"
If the controllers are on the local subnet, then it will find those via broadcast, if ...
12
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-Based Address Allocation on your switch; you also must issue DHCP from your switch. Assume that you have got Fa0/1 and Fa0/2 in Vlan120.
ip dhcp use subscriber-...
12
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 an IP address until the DISCOVER-OFFER-REQUEST-ACK exchange is completed. It is possible (although very unlikely) to have a situation arise where the server ...
11
This only deals with how the DHCP server responds to a client DHCP REQUEST for a certain IP address that is not provided by the DHCP server configuration.
If it isn't part of the DHCP scope on the Mikrotik and the setting is yes, then it responds with a DHCP NACK message, which should prompt the client to do a DHCP DISCOVER.
If set to no, then it will just ...
11
We handle them on the same server. If the server is not in the same VLAN/Network as the clients, you can use dhcp relay on the layer3 device (normally the gateway) of said network.
For example, if you have wireless going to vlan 10, wired vlan 15, and your server is on vlan 20; you'll configure the dhcp relay agent on vlans 10 and 15 interfaces to point to ...
11
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 Router Advertisement (RA)
This happens both on request (when receiving an RS) as well as periodically
The RA contains a lot of information on how the network is ...
11
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 received from that IP the DHCP server will assign a different address.
The ARP request on the newly received address from the client would do the same, check if ...
11
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.
10
Assuming the replacement router uses the same internal IP and assigns from the same IP range, this won't be much of a problem. Clients will keep the IP assigned via DHCP and will request a renewal of the DHCP lease after some time. The new router will either honor the renewal or if the IP is assigned to another device refuse it, after which the client will ...
10
The router will forward all DHCP requests to all servers configured with ip helper. The first server to respond with a usable address wins. I'm unaware of a way to round-robin from the router.
10
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 maintain the "state" of that address (i.e. lease time, etc).
Stateless autoconfiguration of IPv6 allows the client device to self-configure its IPv6 address and ...
9
I opened a case with HP concerning this issue. After escalating past the useless Level 1 tech, the Level 2 tech very alertly spotted something that I had not.
The SRX is sending its DHCPDISCOVER packet with a TTL of 1. The Procurve's apparently will decrement the TTL and use the resulting TTL in the relay'ed packet to the DHCP server. In this case, the ...
9
You have two options.
Option One: Configure DHCP smart relay (see more on this in the answers from Dave and Mierdin): ip dhcp smart-relay
Option Two: Set up a shared-network statement in your dhcpcd.conf (assuming you're running a Linux-based DHCP server - see Peter's answer below if you aren't)
shared-network "testing" {
subnet 192.168.2.0 netmask 255....
9
If you configure an IP address on an interface the router will use the corresponding DHCP pool to answer DHCP requests. In your case you would do something like this:
interface Fa0/0
ip address 192.168.0.1 255.255.255.248
!
interface Fa0/1
ip address 172.16.0.1 255.255.255.248
!
ip dhcp pool 0
network 192.168.0.0 255.255.255.248
...
!
ip dhcp pool 1
...
9
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 device). You can prevent the communication between the host if he doesn't have an IP from the DHCP server.
In other words, using the Cisco access layer switch, I ...
9
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
BROADCAST bit in the 'flags' field to 1 in any DHCPDISCOVER or
DHCPREQUEST messages that client sends. The BROADCAST bit will
provide a hint to ...
9
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 the DHCP Server to offer an address (and other DHCP options) to the Client.
DHCP Request (DHCPREQUEST) - Sent by the Client to accept the lease for the offered ...
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