Chapter 1 - IPV4/IPv6 Addressing and Routing Review

Official Cert Guide Chapter 1

Describe the DHCP DORA process.

1. Discover - DHCP client boots, notices it lacks an address. Sends discover to 255.255.255.255 w/ destination MAC of FFFF.FFFF.FFFF and a source IP of 0.0.0.0 and its own MAC address.

2. Offer - Server receives the discover (either via relay or broadcast) and transmits an offer. More than one server may reply, so the client usually responds to the first one.

3. Request - Client receives offer and replies with a broadcast saying it’ll use the offered IP.

4. Acknowledge - Server replies with acknowledgement stating the address is leased to the client and provides DHCP options, gateway, lease duration, DNS, and so on.

What command enables DHCP service on a router? Is it on by default?

service dhcp

It is enabled by default but is also required for relays to work.

What is the command to enable a DHCP relay on an interface?

ip helper-address ___ (ip address)

What additional protocols can be relayed to a client using a helper?

TFPT, DNS, ITS (internet time service), NetBIOS name server, NetBIOS datagram server, BootP, and TACACS.

Which ports are used for DHCP Discover and Offer?

Discover uses UDP 67

Offer uses UDP 68

What is a DHCPDECLINE?

Sent from the client to server stating the issued IP is already in use.W

What is a DHCPNAK?

Sent from the server to client declining to provide the client with its requested IP configuration.

What is a DHCPRELEASE?

Sent from the client to the server indicating it is giving up its lease and the address can be reassigned.

What is a DHCPINFORM?

Sent by the client to the server requesting IP configuration parameters. For example, an access server asking for IP information of a remote client.

How do you configure DHCP on an interface?

ip address dhcp

How does a Cisco device acting as a DHCP server determine if an address is in use?

It sends a ping and a gratuitous ARP. If there’s a conflict, the address is removed from the pool until it is addressed by an administrator.

What command shows if the Cisco device is listening on port 67?

show socket detail or show ip socket detail

What is a DHCP database agent?

Any host (can be an FTP, TFTP, RCP server) or local media (USB or flash) that stores the DHCP bindings database. You can configure multiple and adjust their update interval.

Where are manual DHCP bindings stored?

In NVRAM on the DHCP server.

Where are automatic DHCP bindings stored?

in RAM on the DHCP server.

What option do all clients sent in their DHCP packets?

Clients send option 61, client ID.

What is the process for creating a DHCP pool?

1. ip dhcp pool ___(name)

2. network ___ ___ (network and mask)

3. default-router ___

4. dns-server ___

5. Configure options

What command creates an excluded address in DHCP?

ip dhcp excluded address ___ (lower bound) ___ (upper bound)

What should be examined when troubleshooting DHCP issues?

1. Is the router forwarding broadcasts/is there a relay?

2. Is the DHCP pool exhausted?

3. Is the pool misconfigured? Wrong IPs, lease time too long, incorrect exclusion, DNS servers missing or incorrect?

4. Are there duplicate IPs?

5. Are redundant DHCP servers not communicating and sending duplicate DORA?

6. Is the client just… stupid?

What command displays IP conflicts, time detected, and method detected?

show ip dhcp conflict

What command clears a DHCP conflict after it’s been resolved?

clear ip dhcp conflict *

What debug will show updates to the DHCP database?

show ip dhcp server events

What debug shows the DHCP message exchange?

debug ip dhcp server packet