Troubleshooting DHCP and NAT Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Understanding DHCP

1. DHCP is used to dynamically assign hosts with IP addressing information
2. DHCP can provide IP address, mask, default gateway, and DNS servers
3. DHCP uses UDP port 68
4. Cisco IOS routers and some switches can be configured as DHCP clients and servers
5. DHCP is a client / server protocol
6. DHCP clients transition through a series of states upon initialization
7. DHCP clients transition through the following states:

1. Initializing
2. Selecting
3. Requesting
4. Bound
5. Renewing
6. Rebinding

1. DHCP clients send out a DHCPDISCOVER message during the initializing phase
2. DHCP servers respond with a DHCPOFFER message
3. DHCP clients transition to the selecting state after the DHCPOFFER is received
4. DHCP clients then send a DHCPREQUEST and transition to the requesting phase
5. DHCP servers respond with a DHCPACK message
6. After the DHCPACK is received, the client transitions to the bound state
7. Additional DHCP messages that can be sent by the client or server include the following:

1. DHCPNAK or DHCPNACK
2. DHCPDECLINE
3. DHCPINFORM
4. DHCPRELEASE

1. Cisco IOS devices can be configured as DHCP / BOOTP Relay Agents
2. DHCP / BOOTP Relay Agents forward DHCP messages to servers
3. Messages between Relay Agents and servers are Unicast between the two
4. By default, the ip helper-address will forward the following UDP Broadcasts:

1. Trivial File Transfer Protocol (TFTP) (port 69)
2. DNS (port 53), time service (port 37)
3. NetBIOS name server (port 137)
4. NetBIOS datagram server (port 138)
5. Boot Protocol (DHCP/BOOTP) client and server datagrams (ports 67 and 68)
6. Terminal Access Control Access Control System (TACACS) service (port 49)
7. IEN-116 name service (port 42)

Troubleshooting DHCP

1. The most common cause of problems with DHCP is device misconfigurations
2. Additional issues that can cause DHCP problems include the following:

1. NIC compatibility issue or DHCP feature issue
2. Faulty NIC or improper NIC driver installation
3. Operating System behavior or defect
4. Spanning Tree issues
5. CDP Is Disabled On Ports Connected to IP Phones
6. DHCP Server DHCP Relay Information Option Incompatibility
7. Cisco IOS DHCP/BOOTP Software Bugs

Understanding NAT

1. NAT enables hosts on private networks to access resources on the Internet / public networks
2. NAT is an IETF standard
3. NAT converts packet headers for incoming /outgoing traffic and keeps track of each session
4. The inside interface is the border interface of the organization administrative domain
5. The inside local address is the IP address of a host residing on the inside network
6. The inside global address is the IP address of an internal host as it appears to the outside
7. The outside interface is the boundary for the administrative domain
8. The outside local address is the address of an outside host as it appears to inside hosts
9. The outside global address is an address that is legal and can be used on the Internet

Troubleshooting NAT

1. NAT has several limitations. These limitations include the following:

1. Breaking the end-to-end IP model
2. The need to maintain connection state issues
3. The inhabitation of end-to-end security
4. Applications that are not NAT-friendly
5. Address space collision
6. Ratio of internal and reachable IP addresses

1. Most NAT problems can be attributed to general device misconfigurations
2. Common misconfigurations include the following:

1. Misconfigured NAT Access Control Lists
2. Asymmetric Routing
3. Misconfigured NAT Address Pool
4. Router Interfaces Missing NAT Commands

1. In addition to misconfigurations, the following problems can also result in NAT issues:

1. Layer 1 and Layer 2 Issues
2. Layer 3 Issues
3. Device Resource Depletion
4. Incompatible Applications