Questions[routing](server)

This is a Canonical Question about Redundant DHCP Servers.

Is it possible to have more than one DHCP server on the same LAN? What are the implications of doing this?

1. What happens if there is more than one DHCP server available? How do my clients know which one to use?
2. How can I have DHCP servers supplying addresses to more than one subnet\network segment?
3. How can I configure multiple DHCP servers to supply addresses for the same subnet.

I have never had the privilege of working in an environment that required complicated routing or if it did require it, it was handled upstream of me. I've always used very simple static routing configurations and never needed to do any multipath routing -- hence my general confusion regarding this subject. I would like to understand multicasting and anycasting better.

• What is the difference between unicast, anycast, broadcast and multicast traffic?
• What situations are they generally used in and why (e.g., what applications use which method)?
• How do you calculate how much broadcast traffic is too much for a given network segment or broadcast domain?
• What are the security implications of allowing broadcast and multicast traffic?

This turns out to be harder than I thought. The routes I want to delete are the "!" rejected routes, but I can't seem to formulate the right "route del" command to pull it off.

Here is the routing table...

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
67.40.227.206   *               255.255.255.255 UH    0      0        0 ppp0
192.168.46.79   *               255.255.255.255 UH    0      0        0 ipsec0
192.168.46.79   -               255.255.255.255 !H    2      -        0 -
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
10.1.0.0        *               255.255.0.0     U     0      0        0 ipsec0
10.1.0.0        -               255.255.0.0     !     2      -        0 -
default         *               0.0.0.0         U     3      0        0 ppp0
default         *               0.0.0.0         U     4      0        0 ppp0

I have two entries for 192.168.46.79 and 10.1.0.0. These are auto-generated by the little Linux based router I'm using. I can ping the IPSEC tunnels from the shell itself, but traffic from the LAN takes the second route (the rejected "!" or "!H" route) for reasons I simply don't understand.

Running CentOS 5.4

Why do I have route to 169.254.0.0 although it does not appear in Network > Ethernet Device > Route configuration dialog?

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth2
169.254.0.0     *               255.255.0.0     U     0      0        0 eth2
default         192.168.1.1     0.0.0.0         UG    0      0        0 eth2

I'd never heard of anycast until a few seconds ago when I read "What are some cool or useful server/networking tricks?".

The wikipedia "Anycast" article on it is quite formal and doesn't really evoke a mental picture of how it would be used.

Can someone explain in a few informal sentences what "anycast" is, how you configure it (just in a general sense), and what its benefits are (what does it make easier)?