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BSCI – Cisco IOS DHCP server

One of the topics Cisco removed from the ROUTE exam is DHCP;  it’s about time they fix this weird decision. Why it is on the BSCI to begin with is a mystery but it does not change the fact that I have to study this topic.
Here are few notes about using Cisco IOS as DHCP server:

There are three DHCP assignment methods:

There are two binding methods:
Manual binding – MAC address to IP address
Automatic binding – not kept on the DHCP server (router), usually kept on TFTP server
DHCP database agent (TFTP) is configured using ip dhcp database command

When you enable the DHCP service using the service dhcp command, DHCP service uses the following packet types:
DHCPDiscover ->broadcast from the client (always the same type)
DHCPOffer ->DHCP server send back IP address and parameters (coming from IOS)
DHCPRequest ->client say which IP address it accepted (choose from all the responses)
DHCPAck ->DHCP server approve the IP assignment and provide detailed parameters

DHCP discovery requests are broadcast and we know that routers block it. To bypass this router limitation the command ip helper-address is used.
ip helper-address
translate broadcast to multicast to allow traffic via router, it should be configured on the interface that receive the broadcast.
This is a useful command out of the DHCP world so keep it in mind!

This is a basic example of DHCP configuration (include basic DHCP values):
ip dhcp pool IOS_DHCP_SERVICE
domain-name mydomain.com
ip dhcp excluded-address

If we want an interface (DSL or Cable) to receive a DHCP address:
ip address dhcp

Other than the extra overhead on the router, the main downside for using a router as DHCP server is the day-to-day management. I do not think many people would argue that using Microsoft’s DHCP server is much easier. Microsoft servers present an easy to use interface that provides information on all the clients and Cisco does not. The following command allow you to export the binding information from the Cisco router to a TFTP server as a text file:
ip dhcp database tftp://

I hope my exam will have basics only on DHCP and as long as they keep in the above scope, I think I’ll do just fine.

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