We currently operate two LNSs (the boxes that handle all of our broadband traffic). We have one live system and one as a hot standby backup. They are designe to handle a gigabit of internet traffic to customers, and as we saw when IOS5 was released they can even exceed that a little.
When we want to upgrade software, after testing on our test LNSs we load the new code in to the backup LNS. We then do an LNS switch - causing all lines to drop, one at a time, and reconnect a few seconds later to the new LNS.
During this month we are upgrading our core network and we now have four LNSs. The new ones are being tested now and once they are fully deployed we will operate with three live LNSs. These will share the load, but we are taking care to ensure a customer with line bonding is connected to just one LNS so that the bonding stays working. The 4th LNS is for upgrades.
What this means is that when we want to upgrade, we will upgrade the 4th LNS, and re-designate one of the other three as the new spare LNS. We will clear sessions from that LNS which will reconnect to the new LNS. This means an LNS switch will only affect a third of the customers. However, to upgrade the live LNSs to new code we have to do all three. It means an LNS switch will be in three distinct stages and may not even be done on the same day if this is just a routine upgrade.
However, during January (and possibly in to Februrary if BT drag their heels) we will need to do the existing LNS switches to move traffic to the right boxes while we actually deploy the new equipment and move BT links. This should be no more disruptive than a normal LNS switch, and we will try and schedule these for Saturday nights as usual.
Thank you all for your patience during this essential upgrade work.