RE: [Pce] Virtual Network Topology (VNT) changes and IP layer routingloops

["Don Fedyk" <dwfedyk@nortel.com>]

Hi  Greg

Sorry for the delay on this. 
> -----Original Message-----
> From: Greg Bernstein [mailto:gregb@grotto-networking.com] 
> 
> Hi Don, I'm using the term "Virtual Network Topology" related 
> to the data plane not the control plane.  For example, when I 
> interconnect IP routers via a WDM or SDH switching layer the 
> resulting IP layer topology is termed a "virtual network 
> topology" since the connectivity amongst the IP routers is 
> not necessarily the same as the fiber topology. It is these 
> changes to the IP layer topology that can cause the traffic 
> disrupting micro loops.  Note that other ways that micro loops occur
> are: (1) during IGP link weight adjustments and (2) 
> maintenance operations.
> 
> The connection to GMPLS/PCE is we are trying to make setting 
> up these virtual topologies quicker and potential for traffic 
> engineering purposes. There have been some good papers in the 
> literature on using GMPLS and advanced algorithms for the VNT 
> design problem.

OK In as much as these topologies represent nodes and links, some how
distributed, I would agree that: 

1) Proactive topology changes should be dealt with in a graceful manner.
One way is to have loop free alternates activated then make the topology
changes.  This may result in slowly stepwise shifting the topology when
turning on/off resources. 
More elaborate schemes can add coverage in this case. But you can just
as well design mechanisms for this particular case that explicit versus
algorithmic. 

2) When it comes to reactive topology changes that is where it gets a
bit trickier. You should have have loop free and perhaps a few more
advanced IPFRR techniques. But most of IPFRR can only protect single
failure protection, node or links. There are provisions for shared
resource correlation to maximize picking independent alternates still
biased by a single failure. 

While the simple IPFRR techniques are a no brainer I'm not convinced
that Virutal network topology is an agrument for more complex IPFRR. 

Regards,
Don 
<snip>