Cisco DHCPv6 Server Configuration

[lagapidis (Lazarus Agapidis)]

Hello CJ

@aiegacy

• Don’t global unicast prefixes need to come from an ISP to ensure they are unique? If so, why would there be a need to manually configure one anywhere? Shouldn’t a router already have this prefix from the ISP?

If we’re talking about a single IPv6 address that is assigned to the external interface of the edge router, then that will often be assigned directly by the ISP using their own DHCP server. Alternatively, you may assign it as a static address on the interface of the router. However, if you as an organization are provided with a block of global unicast IPv6 addresses by your ISP to use for your own Internet-facing services, (i.e. web server, email, VPN service etc…) then you must manage them yourself. So it is conceivable that you are given the 2001:1111:1111:1111::/64 prefix by the ISP, but you are using your own DHCP server to assign them to the hosts you choose.

@aiegacy

• What happens if you accidentally type in the wrong global unicast prefix and it doesn’t match what the ISP assigned you? Couldn’t this risk address collision?

ISPs will typically have safeguards against such errors. They will typically only accept packets from your network that have an acceptable source IPv6 address. So if you were to use a different address that already exists on the Internet, the ISP would be able to identify it and filter it out.

@aiegacy

• Assuming you are getting the prefix from the ISP, why are you manually assigning an IPv6 address to this interface? Can’t it just get one as a DHCPv6 client or use SLAAC from the ISP-provided prefix?

The DHCP server itself must have an IP address assigned to it, and this will typically be static. If the ISP has given us a block of global unicast addresses, then it is likely that the DHCP server itself will also be assigned one of those addresses. This however is not mandatory, as the DHCP server may be assigned an internal IPv6 address.

@aiegacy

• Why are you manually typing in the 2001:1111:1111:111::/64 prefix in the ipv6 nd prefix command? You already assigned an address to the interface. Couldn’t you just use the default parameter? Cisco NetAcad does this.

It is possible to issue the following command:

DHCPV6(config-if)#ipv6 nd prefix default

This will cause the prefix of the interface itself to be used for DHCP. However, you do not need to be confined only to this prefix. The DHCP server can assign DHCP addresses with prefixes other than that assigned to the specific server interface.

@aiegacy

1. What’s with the 4-hour lifespans on that prefix? What would happen when those 14,400 seconds expire?

The lifetime parameters of the ipv6 nd command are part of the fundamental operation of the ND protocol. The command as shown in this Cisco IOS command reference documentaiton indicates two parameters: the preferred lifetime and the valid lifetime. I have created a NetworkLessons note on the topic for more detail.

I hope this has been helpful!

Laz