Infrastructure Services · Topic 4 of 6
Troubleshooting IPv4 and IPv6 DHCP
Troubleshoot DHCP on IOS XE: client, server and relay roles, DHCP options, the DORA exchange, and IPv6 SLAAC, stateless and stateful DHCPv6 with relays.
- 5 min
- DHCP uses UDP 67 for the server and UDP 68 for the client.
- The four messages are Discover, Offer, Request and Acknowledgement.
ip helper-addresson the client interface relays the broadcast to a server.
What it is
DHCP gives hosts an address and other network settings automatically, and a Cisco router can act as the client, the server or the relay.
A client asks for settings. A server answers from a pool. A relay forwards requests from a subnet that has no server to a server on another subnet.
IPv6 can hand out settings in three ways, and the router advertisement flags tell the host which one to use.
How it works
IPv4 DHCP uses a four step exchange, and the relay is the usual point of failure.
The IPv4 exchange
The client and server use UDP port 68 and port 67. The four messages are Discover, Offer, Request and Acknowledgement, often called DORA.
The client broadcasts Discover. The server replies with an Offer. The client sends a Request for that offer, and the server confirms it with an Acknowledgement.
IOS DHCP server
R1(config)# ip dhcp excluded-address 10.1.1.1 10.1.1.10
R1(config)# ip dhcp pool LAN
R1(dhcp-config)# network 10.1.1.0 255.255.255.0
R1(dhcp-config)# default-router 10.1.1.1
R1(dhcp-config)# dns-server 10.1.1.2
R1(dhcp-config)# domain-name example.com
R1(dhcp-config)# lease 7The excluded range stops the server handing out addresses that are used by routers and servers. Lease 7 means seven days. A router interface can also be a client with ip address dhcp.
DHCP relay
A broadcast does not cross a router. The relay is configured on the interface that faces the clients.
R2(config)# interface GigabitEthernet0/1
R2(config-if)# ip helper-address 10.2.2.2The router receives the broadcast, and sends it as a unicast to the server, adding its own interface address as the gateway address. The server uses that address to choose the right pool. A missing or wrong ip helper-address is a very common fault.
DHCP options
Options carry extra information.
| Option | Use |
|---|---|
| 3 | Default router |
| 6 | DNS servers |
| 15 | Domain name |
| 43 | Vendor information, such as a controller address for access points |
| 150 | TFTP server address, used by Cisco IP phones |
R1(dhcp-config)# option 150 ip 10.1.1.5Troubleshooting IPv4
Check each step of the path.
- Is the client getting a Discover through to the server?
- Does the relay on the client interface point to the correct server?
- Does the pool match the subnet, and are addresses left?
- Is a conflict stopping an address from being used?
- Does a firewall or ACL block UDP 67 and 68?
R1# show ip dhcp binding
R1# show ip dhcp pool
R1# show ip dhcp conflict
R1# show ip dhcp server statistics
R1# debug ip dhcp server packetshow ip dhcp binding lists leases. show ip dhcp conflict lists addresses the server found already in use. show ip dhcp pool shows how many addresses are leased.
IPv6 address assignment
IPv6 hosts learn from router advertisements (RA). Two flags in the RA choose the method.
| Method | RA flags | Address from | Other settings from |
|---|---|---|---|
| SLAAC | Neither flag | Host builds it from the prefix | RA, for example the prefix |
| Stateless DHCPv6 | Other config flag | SLAAC | DHCPv6 server |
| Stateful DHCPv6 | Managed config flag | DHCPv6 server | DHCPv6 server |
R1(config)# ipv6 dhcp pool V6POOL
R1(config-dhcpv6)# address prefix 2001:DB8:1::/64
R1(config-dhcpv6)# dns-server 2001:DB8:2::53
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ipv6 address 2001:DB8:1::1/64
R1(config-if)# ipv6 nd managed-config-flag
R1(config-if)# ipv6 dhcp server V6POOLFor stateless DHCPv6, use ipv6 nd other-config-flag instead of the managed flag. The pool then only gives the other settings, such as DNS.
DHCPv6 clients use UDP port 546, and servers and relays use UDP 547. Clients find a server using the multicast address FF02::1:2.
A DHCPv6 relay is configured on the client interface.
R2(config-if)# ipv6 dhcp relay destination 2001:DB8:2::2
R2# show ipv6 dhcp binding
R2# show ipv6 dhcp interfaceThe IPv6 checks follow the same pattern.
- Does the RA carry the right flag?
- Does the relay point to the server?
- Is the pool right for the prefix?
Start with the client side. If show ip dhcp binding shows nothing, the request probably never reached the server.
What to remember for the exam
Eight facts cover most questions on this topic.
- DHCP uses UDP 67 for the server and UDP 68 for the client.
- The four messages are Discover, Offer, Request and Acknowledgement.
ip helper-addresson the client interface relays the broadcast to a server.ip dhcp excluded-addresskeeps addresses out of the pool.- Option 150 gives a TFTP server address to phones.
- The managed config flag means stateful DHCPv6. The other config flag means stateless.
- DHCPv6 uses UDP 546 for clients and 547 for servers.
show ip dhcp bindingandshow ip dhcp conflicthelp find faults.
Place ip helper-address on the interface that faces the clients, not on the one that faces the server.
Common mistakes
These five mistakes appear often.
No helper address. The client broadcast never reaches the server.
Helper on the wrong interface. The router does not see the client broadcast on that interface.
Pool does not match the subnet. The server finds no pool for the relayed request.
Missing exclusions. The server hands out the address of the default gateway.
Wrong RA flag. The host uses SLAAC and never asks DHCPv6.
Check yourself
Work through these three questions.
What are the four messages in the DHCP exchange?
Discover, Offer, Request and Acknowledgement.
Where is
ip helper-addressconfigured?On the router interface that faces the clients, pointing to the DHCP server.
Which RA flag makes a host use stateful DHCPv6?
The managed config flag, set with
ipv6 nd managed-config-flag.
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