21.1.3 What to do when the prefix information is changed

In a system that uses RAs to distribute terminals with prefixes, if a prefix is changed, addresses are suddenly changed and communication might stop. To avoid this problem, previous prefixes remain for 604800 seconds (7 days) by default. To delete an old prefix, routers need to advertise both the old and new prefixes and gradually shorten the life time of the old prefix. The following figure shows how RAs are used to do this.

Figure 21-3: RA Usage example

1. Configure Ethernet interface Ia to send an RA to the network.

   • Prefix of Ia = 3ffe:501:811:ff01::/64

2. Change the prefix of Ia from 3ffe:501:811:ff01::/64 to 3ffe:501:811:ff22::/64 as follows:

   • Set a short advertisement interval for the new prefix 3ffe:501:811:ff22::/64 to be advertised on Ia, and start advertising.

   • Set a short preferred lifetime and valid lifetime for the old prefix 3ffe:501:811:ff01::/64 of Ia, which is going to become unavailable, and advertise it.

   • Restore the advertisement interval of the new prefix 3ffe:501:811:ff22::/64 of Ia to the default value.

   • Stop advertising the old prefix 3ffe:501:811:ff01::/64.