SANET infrastructure consists of multiple interconnected fibre rings, which create a highly resilient network. In case of fibre cut, traffic is immediately rerouted over the remaining links to ensure high reliability.
At IP layer, SANET backbone forms a single ethernet segment, where TRILL protocol performs shortest-path routing for all L2 packets. Thus for IP, the distance between all POPs is exactly 1 hop, which means there’s no need to run any IGP protocol at L3. Exchange of routing information is done solely by BGP protocol.