Netgear XCM8806 Chassis User Manual - Page 552

NETGEAR 8800 User Manual Another situation may arise if you have more than one bridge and you lower the port cost for the alternate port, which makes it the new root port. The previous root port is now an alternate port. Depending on your STP implementation, STP may set the new root port to the forwarding state before setting the alternate port to the blocking state. This may cause a loop. To prevent this type of loop from occurring, the recent root timer starts when the port leaves the root port role. The timer stops if the port enters the blocking state. RSTP requires that the recent root timer stop on the previous root port before the new root port can enter the forwarding state. Designated Port Rapid Behavior When a port becomes a new designated port, or the STP priority changes on an existing designated port, the port becomes an unsynced designated port. For an unsynced designated port to rapidly move into the forwarding state, the port must propose a confirmation of its role on the attached LAN segment (unless the port is an edge port). Upon receiving an "agree" message, the port immediately enters the forwarding state. If the receiving bridge does not agree and it has a superior STP priority, the receiving bridge replies with its own BPDU. Otherwise, the receiving bridge keeps silent, and the proposing port enters the forwarding state and starts the forward delay timer. The link between the new designated port and the LAN segment must be a point-to-point link. If there is a multi-access link, the "propose" message is sent to multiple recipients. If only one of the recipients agrees with the proposal, the port can erroneously enter the forwarding state after receiving a single "agree" message. Receiving Bridge Behavior The receiving bridge must decide whether or not to accept a proposal from a port. Upon receiving a proposal for a root port, the receiving bridge: • Processes the BPDU and computes the new STP topology. • Synchronizes all of the designated ports if the receiving port is the root port of the new topology. • Puts all unsynced, designated ports into the blocking state. • Sends down further "propose" messages. • Sends back an "agree" message through the root port. If the receiving bridge receives a proposal for a designated port, the bridge replies with its own BPDU. If the proposal is for an alternate or backup port, the bridge keeps silent. Propagating Topology Change Information When a change occurs in the topology of the network, such events are communicated through the network. In an RSTP environment, only non-edge ports entering the forwarding state cause a topology change. A loss of network connectivity is not considered a topology change; however, a gain in network connectivity must be communicated. When an RSTP bridge detects a topology 552 | Chapter 18. STP