December 2006
Intermediate to advanced
1188 pages
72h 8m
English
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Limiting the Impact of IP Helper Addresses
Problem
After configuring your router to use IP helper addresses, you suffer from high link utilization or high CPU utilization on the DHCP server.
Solution
The ip helper-address command implicitly enables forwarding several different kinds of UDP broadcasts. You can prevent the router from forwarding the unwanted types of broadcasts with the no ip forward-protocol udp configuration command:
Router1#configure terminalEnter configuration commands, one per line. End with CNTL/Z. Router1(config)#no ip forward-protocol udp tftpRouter1(config)#no ip forward-protocol udp nameserverRouter1(config)#no ip forward-protocol udp domainRouter1(config)#no ip forward-protocol udp timeRouter1(config)#no ip forward-protocol udp netbios-nsRouter1(config)#no ip forward-protocol udp netbios-dgmRouter1(config)#no ip forward-protocol udp tacacsRouter1(config)#endRouter1#
Discussion
As mentioned in Recipe 20.1, if the DHCP client and server are on different network segments, the router on the client’s segment must be configured with a helper address for DHCP to work. However, the helper address configuration forwards a variety of different UDP broadcasts, not just DHCP packets. This can cause network loading problems, as well as CPU loading problems on the DHCP server.
By default, when you configure the ip helper-address command on an interface, the router will automatically forward UDP broadcasts for all of the protocols shown in Table 20-1.
Table 20-1. Default ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.