DHCP

DHCP

ip dhcp excluded-address 10.1.50.1 10.1.50.50
ip dhcp pool Guests
	network 10.1.50.0 /24
	default-router 10.1.50.1
	dns-server 10.1.50.1
	domain-name example.com
	lease 0 12

DHCP Relay

interface fa0/0
	ip helper-address 10.1.50.1

Manual Address Binding

ip dhcp pool PC2Pool
	host 10.1.50.200 255.255.255.0
	client-identifer 0100.50b6.5bc0

DHCP Options

43
WLC address for lightweight AP
69
SMTP server
70
POP3 Server
150
TFTP Server (IP Phones)

IPv6 Stateless Address Auto-configuration (SLAAC)

Plug and play for network devices
Link prefix is obtained via router advertisement
Extended Unique Identifer address is based on MAC address
7th bit from left gets flipped (UL bit)
Subnet Prefix (64 Bits)
ㅤ
EUI-64 Interface ID (64 bits)
ㅤ
Subnet Prefix (64 Bits)
MAC (high 24 bits OUI)
FFFE
MAC (low 24 bits)
  1. Client generates a link-local address
  1. Client joins all-host multicast group
  1. Client sends Router solicitation (from LLA)
  1. Router sends advertisement
  1. Client generates IPv6 for every on-link prefix using MAC Address
  1. Duplicate address detection
  1. Client sets default router

DHCPv6

Allows more control than SLAAC
Operates the same as IPv4, with these exceptions,
  • Client first detects presence of routers on the link
  • Client examines router advertisements to determine if DHCP can be used
  • If no router found or DHCP can be used, client sends a DHCP SOLICT message
      • DHCPv6 four-way negotiation
DHCPv6 also has two-way version. Only SOLICIT and REPLY are used. Both client and server need to be configured for rapid-commit.

DHCPv6 Relay

interface fa0/0
	ipv6 dhcp relay destination 2001:DB8:11::1
DHCPv6 Relay Agent

Troubleshoot DHCP

  • Server misconfiguration (pool, gateway, DNS, excluded addresses)
  • Duplicate IPv4 Addresses (IPv6 used DAD)
  • DHCP pool runs out of addresses
  • Router not forwarding addresses
  • Client is not requesting and IP address
show ip dhcp binding
show ip dhcp conflict

debug ip dhcp server events

show ipv6 interface
Reference
Message
Description
0x01
DHCPDISCOVER
The client looks for available DHCP servers.
0x02
DHCPOFFER
The server response to the client DHCPDISCOVER.
0x03
DHCPREQUEST
The client broadcasts to the server, requests offered parameters from one server specifically, as defined in the packet.
0x04
DHCPDECLINE
The client-to-server communication, indicates that the network address is already in use.
0x05
DHCPACK
The server-to-client communication with configuration parameters, along with committed network address.
0x06
DHCPNAK
The server-to-client communication, refuses the request for configuration parameter.
0x07
DHCPRELEASE
The client-to-server communication, surrenders network address and cancels the remainder lease.
0x08
DHCPINFORM
The client-to-server communication, asks for only local configuration parameters that the client already has externally configured as an address.

RA Message Flags

  • A flag - This is the Address Autoconfiguration flag. Use Stateless Address Autoconfiguration (SLAAC) to create an IPv6 GUA.
  • O flag - This is the Other Configuration flag. Other information is available from a stateless DHCPv6 server.
  • M flag - This is the Managed Address Configuration flag. Use a stateful DHCPv6 server to obtain an IPv6 GUA.
To enable the sending of RA messages, a router must join the IPv6 all-routers group using the ipv6 unicast-routing global config