Virtual Private LAN Service176FD 100/320Gbps NT and FX NT IHub Services Guide3HH-11985-AAAA-TQZZA Issue: 13Unknown destinations, broadcasts, and multicasts are flooded to all other SAPs inthe service. If SAPs are connected together, either through misconfiguration or forredundancy purposes, loops can form and flooded packets can keep flowing throughthe network. Nokia’s implementation of the Spanning Tree Protocol (xSTP) isdesigned to remove these loops from the VPLS topology. This is done by putting oneor several SAPs in the discarding state.5.4.7.1 Spanning Tree Operating ModesA preferred STP variant can be configured for the m-VPLS. The following STPvariants supported on the ISAM.• rstpRapid Spanning Tree Protocol (RSTP) compliant with IEEE 802.1D-2004 - defaultmode.• dot1wCompliant with IEEE 802.1w.• comp-dot1wOperation as in RSTP but backwards compatible with IEEE 802.1w (this modewas introduced for interoperability with some MTU types).• mstpCompliant with the Multiple Spanning Tree Protocol specified in IEEE802.1Q-REV/D5.0-09/200.• pmstpProvider MSTP mode is implemented according to 802.1ad (provider bridges)The provider functionality is implemented and the provider edge functionality isnot.While the ISAM initially uses the mode configured for the m-VPLS, it will dynamicallyfall back (on a per-SAP basis) to STP (IEEE 802.1D-1998) based on the detection ofa BPDU of a different format. A trap or log entry is generated for every change inspanning tree variant.• Some older 802.1W compliant RSTP implementations may have problems withsome of the features added in the 802.1D-2004 standard. Interworking with theseolder systems is improved with the comp-dot1w mode. The differences betweenthe RSTP mode and the comp-dot1w mode are: the RSTP mode implements theimproved convergence over shared media feature, for example, RSTP willtransition from discarding to forwarding in 4 seconds when operating over sharedmedia. The comp-dot1w mode does not implement this 802.1D-2004improvement and transitions conform to 802.1w in 30 seconds (both modesimplement fast convergence over point-to-point links).• In the RSTP mode, the transmitted BPDUs contain the port's designated priorityvector (DPV) (conforms to 802.1D-2004). Older implementations may beconfused by the DPV in a BPDU and may fail to recognize an agreement BPDU
