>> Moreover, TTZ provides smooth transferring between a zone and its single >> pseudo node. That is that a zone can be smoothly transferred to a single >> pseudo node, and the pseudo node can be smoothly rolled back to the zone. > > This strikes me as the important difference from area proxy. It certainly > adds complexity to things, I wonder if it's worth it?
FWIW, we looked at this quite seriously a while ago. Turning abstraction on and off is simply not a daily occurrence. Doing it properly requires careful thought and planning. Where are the boundaries? What prefixes will be advertised and with what metrics? How will the affect traffic flow? The corner cases that happen when you are in this transition are large. We have to deal with the issue of the metrics that are abstracted away. We chose to go down the path of intra-area vs. inter-area metrics, exactly as OSPF does. But if you do this, then you have an issue: the metrics are different when abstraction is enabled or disabled and different nodes will compute different paths depending on which LSPs have propagated to them. This seemed like a wonderful opportunity for forwarding loops. This is especially problematic when abstraction is enabled, as there is now an entire area’s worth of LSPs that need to age out before you can be assured of consistency. Yes, you can try to purge them, but purges aren’t the most reliable thing in the world. Net net, we felt that the complexity exceeded the benefit. Yes, there is benefit there, but from the pragmatic viewpoint of making it work in production, the risks and costs seemed very high. We expect that operators would want to make the change in a maintenance window anyway, and as long as you’re having a maintenance window, you might as well do things the simpler way. Regards, Tony
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