Auto-configuration
One of the new features IPv6 brings to the table is auto-configuration.
Specifically, address auto-configuration. This means that a host can deduce it's
own IPv6 address without resorting to DHCP. Now whilst DHCP is a perfectly valid
solution to the address distribution problem, it requires some degree of
configuration and somewhere to store the state of which host has which
address.
Auto-configuration is more light-weight allowing the host to simply concatenate it's MAC address with an advertisement from the local router. The MAC address is extended out to fill the 64bit host portion of an IPv6 address and puts the routers advertised prefix in the first 64 bits.
This is obviously a bare bones solution but it works with a few caveats. Firstly, the only thing that gets configured this way is the address but we are used to DHCP providing much more information to a host. Currently there is proposals in the works to fix this so that the router advertisement contains things like DNS settings but for now there is a compromise. The router, whilst advertising it's prefix for a network will set a flag telling the host to get other information via DHCP.
Sure, this is a kludge of sorts but it does work and it has the benefit that the information given out via DHCP is static so no information needs to be stored. It's trivial to configure a DHCP server on a Cisco router to serve up IPv6 DNS information. Here is an example:
On the routers interface we have two statements:
ipv6 nd other-config-flag
ipv6 dhcp server V6DNS
The first sets the flag mentioned earlier; ND is the neighbour discovery protocol which issues the router advertisements.
The second binds the interface to a specific IPv6 DHCP server which is a little different to how it works for IPv4.
Then we have the DHCP pool definition:
ipv6 dhcp pool V6DNS
dns-server 2400:F900:0:CEA::6
dns-server 2400:F900:0:CF3::9
domain-name lc.alphawest.com.au
These two bits of configuration enable an IPv6 only LAN to be configured, without using DHCP. Of course there may be many reasons to stick with DHCP for IPv6 addresses but this is just an example of how IPv6 enables other solutions.
Auto-configuration is more light-weight allowing the host to simply concatenate it's MAC address with an advertisement from the local router. The MAC address is extended out to fill the 64bit host portion of an IPv6 address and puts the routers advertised prefix in the first 64 bits.
This is obviously a bare bones solution but it works with a few caveats. Firstly, the only thing that gets configured this way is the address but we are used to DHCP providing much more information to a host. Currently there is proposals in the works to fix this so that the router advertisement contains things like DNS settings but for now there is a compromise. The router, whilst advertising it's prefix for a network will set a flag telling the host to get other information via DHCP.
Sure, this is a kludge of sorts but it does work and it has the benefit that the information given out via DHCP is static so no information needs to be stored. It's trivial to configure a DHCP server on a Cisco router to serve up IPv6 DNS information. Here is an example:
On the routers interface we have two statements:
ipv6 nd other-config-flag
ipv6 dhcp server V6DNS
The first sets the flag mentioned earlier; ND is the neighbour discovery protocol which issues the router advertisements.
The second binds the interface to a specific IPv6 DHCP server which is a little different to how it works for IPv4.
Then we have the DHCP pool definition:
ipv6 dhcp pool V6DNS
dns-server 2400:F900:0:CEA::6
dns-server 2400:F900:0:CF3::9
domain-name lc.alphawest.com.au
These two bits of configuration enable an IPv6 only LAN to be configured, without using DHCP. Of course there may be many reasons to stick with DHCP for IPv6 addresses but this is just an example of how IPv6 enables other solutions.
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