In Microsoft Lync Server you can configure bandwidth management such that media can use an alternate path, if adequate bandwidth is not available in primary path. It is done using the Set-CsNetworkRegion cmdlet and the boolean parameters AudioAlternatePath and VideoAlternatePath. The alternate path is via any deployed A/V Edges and the Internet.
In a topology as the above allowing AudioAlternatePath and VideoAlternatePath would enable Alice and Bob to communicate with media via the Internet in case bandwidth on the direct link between Odense and Copenhagen is unavailable.
Another way of looking at AudioAlternatePath and VideoAlternatePath is as parameters controlling, if media is allowed to flow via the Internet when bandwidth management is enabled. In a scenario as below with Alice being remote on the Internet she will not be able to establish a media session with Bob, if AudioAlternatePath and VideoAlternatePath are not True in the network region incorporating the A/V Edge used by Alice.
My colleague Keith pointed out that I’m not talking about PSTN re-routing in this post. The reason is that alternate path is only related to Internet. I covered PSTN re-routing in this post.