VCS Zones between Control/Expressway

[Patrick Sparkman]

How does the VCS Control and Expressway behave when you have either a Neighbor or Traversal zone search rules pointing at each type of server and a search is made.  If a search starts at the Control, and gets sent to the Expressway, could it ever for whatever reason end up back at the Control due to either the Neighbor or Traversal zone search rules setup between the two?

Basically, does this type of search ever happen, and if so how to prevent it?

VCS-C > Neighbor Zone > VCS-E > Neighbor Zone > VCS-C

Thanks, Patrick

[Tomonori Taniguchi · Accepted solution]

Yes, possible to experience what you have described on both neighbor and traversal link depend on search rules (and/or transform configuration) that potentially signaling loops between VCS-C and VCS-E.
An example of this is a structured dial plan, where all systems are neighbored together in a mesh. In such a configuration, if the hop counts are set too high, a single search request may be sent repeatedly around the network until the hop count reaches 0, consuming resources unnecessarily.

The VCS have feature to detect search loops (VCS Configuration > Call) and terminate such searches through the Call loop detection mode setting, thus saving network resources. The options for this setting are:

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On: the VCS will fail any branch of a search that contains a loop, recording it as a level 2 "loop detected" event. Two searches are considered to be a loop if they meet all of the following criteria:

• have same call tag
• are for the same destination alias
• use the same protocol
• originate from the same zone

Off: the VCS will not detect and fail search loops. You are recommended to use this setting only in advanced deployments (or have very small value of hop count configuration).

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[Patrick Sparkman]

Another method of possibly preventing search loops when directing to a zone between devices would also be to designate the appropirate source that is supposed to use that zone, correct?  So for the VCS-C you might only allow the LocalZone to do searches through either the Neighbor or Traversal zones, to prevent any searces that come in from the VCS-E (which as their zones configured to allow Any) to go back out to the VCS-E, hense creating the loop.  In essence, whichever server is the most inward facing, in this case the VCS-C would have some of it's search rules have a defined source, to allow/disallow it's use.

[Martin Koch]

Short comment, you should only use a neighbor zone in between the VCS-C and -E if you exactly know why you are using that. If not it should be a traversal zone. But a search loop should happen on both in specific scanarios.

You now best what has to be found where and how calls are set up. With this knowledge you can design

your search rules and you can prevent looped calls.

Anyhow I would keep the loop detection on unless you know exaxtly what you want to prevent by truning it off!

Some third party sip proxies / h323gk or transforms / search rules might break the loop detection.

[Patrick Sparkman]

This is really for two VCS Controls that are internal to our network, one that is production and the other that is development.  It's so I can make calls to either system for testing.

The Control/Expressway was just an example as knowing how in relation to traversal zones behavior in terms of search loops is a benefit to knowing later on.

[Patrick Sparkman]

Something I have come accross just now doing some testing, our production VCS-C has a neighbor zone pointed to the develpoment VCS-C, and vise versa for searching each other's registered endpoints for testing. 

How could it be possible to allow incomng searchss to development from external sources via production neighbor zone with source "Any".

• VCS-C prod > (neighbor zone) > VCS-C dev

However, at the same time prevent a search loop when calling from development to production and it then get sent back to dev via the prod > dev neighbor zone.

• VCS-C dev > (neighbor zone) > VCS-C prod > (neighbor zone - loop) > VCS-C dev

Right now I have the production neighbor zone setup with source "LocalZone".  Suppose I could let the VCS detect the loops when set to source "Any", and allow the VCS to prevent them.  Wasn't sure if there would be a way to help detere this so that a loop isn't made since I know one would be in that configuration.

[Patrick Sparkman]

Figured out my problem, why my VCS-C developer searches and why other normal production searches kept getting routed to the dev neighbor zone!  I had the neighbor zones set to use Infrastructure device for their Zone profile, as well as one of the VCS's was set to use Optimal for it's Call routed mode.  So neither VCS was communicating to each other for searches, they were just sending everything back and forth between each other.