IPv6 General Prefix

This topic is to discuss the following lesson:

Hi Rene and staff,
just a typo in the first stanza: it is 16 bits instead 12 bits, is not it ?

But my question is about the way to change general-prefix
The lesson use the no form to suppress manually the old general-prefix
So i wonder

  1. can R1 configure a validtime for a general-prefix, then it could be suppressed automatically ?
  2. can the ISP router give a validtime for the general prefix that R1 could interpret ?
    In the router advertisement message format i dont find any field for transmitting this information neither in option. Could a DHCPv6 option do this ?

I wonder another question about preferred time and valid time for IPV6 address. Where could i configure different values from the default ? i google this a little but i do not find anything about a way to do this


1 Like

Hello Dominique

The specific general-prefix feature is one that functions statically. There are no parameters in which you can configure time sensitive validity of the prefix itself. However, if you do want to achieve this, there are a couple of ways that come to mind.

You can use IPv6 DHCPv6 Prefix Delegation which allows for a DHCPv6 server to provide the general prefix to the customer routers, which in turn provide the specific IPv6 addresses based on those general prefixes. These prefixes can be changed and advertised using DHCPv6 rather than using the static configuration suggested in this lesson.

Alternatively, you can cause a router to change its own configuration internally using EEM scripting. This is a feature that allows you to run scripts or commands when a certain event occurs, or a particular times.

A third possibility is to use device programmability, with tools such as NETCONF, RESTCONF, and gRPC to enable network automation with which you can achieve what you need.

I’m not quite sure if this answers your question, but if not, feel free to clarify!

I hope this has been helpful!


thanks a lot LAZ,
i will do a lab to test the EEM solution (and if i succeed i will do a post)
I accept also the solution with device programmability although i need to dive deeper in network programmability to config this type of solution

But for DHCPv6 and prefix delegation, it is not quite clear; suppose the ISP has to change the binding of just one customer

It has to change C1’s binding with a given value for some reason; for example 1100 has to be replaced with 1109. No change for C2.

How can the ISP do this at a specific time ? assuming the customer has nothing to do to get the new prefix at this specific time.
This will be beautiful feature, but not quite sure it is possible ?

Hello Dominique

The idea behind my mentioning Prefix Delegation was the fact that using DHCPv6, the ISP could indeed change the prefix that is provided to each individual customer whenever they would like. They can do it manually, but they can also set a valid-lifetime and a preferred-lifetime. The command would be issued like so:

prefix-delegation pool poolname [lifetime valid-lifetime preferred-lifetime]

Notice this context sensitive help:

ISP(config-dhcpv6)#prefix-delegation pool GLOBAL_POOL lifetime ?
  <60-4294967295>  Valid lifetime (seconds)
  at               Expire prefix at a specific time/date
  infinite         Infinite valid lifetime

ISP(config-dhcpv6)#prefix-delegation pool GLOBAL_POOL lifetime 600 ?
  <60-4294967295>  Preferred lifetime (seconds)
  infinite         Infinite preferred lifetime

You can specify the number of seconds, or you can specify a fixed duration of time for hosts to remember router advertisements using the at keyword, where you can actually input a specific date.

You can find out more about this command and all of its features at the following Command Line Reference link:

To find out the difference between preferred and valid status of prefixes, take a look at the following RFC4862 link. Although it is speaking about IPv6 addresses, this is valid for general prefixes as well.

I hope this has been helpful!