Multiple Master SMs
Scenario That Could Yield Multiple Master SMs
A Master SM may discover another Master SM during a sweep of the subnet. This can happen when a device is hot-plugged into a subnet, connecting two subnets together (or even when just connecting a link cable between two switches or a switch and a CA or router). The two subnets are then combined into a single subnet, and there may then be two Master SMs. When a device detects that a link that was down goes to the up state, its SMA sends a SubnTrap(Notice) to its Master SM reporting the event.
Assume that two fully operational subnets are hot-connected by a link cable connecting together ports of two switches at the fringe of each of the two subnets.
After the Physical Layers of the two ports have completed training, the Link Layer of each port transitions to the fully active LinkActive state.
Upon detecting that the link has transitioned from down to up, both switch SMAs sends a SubnTrap(Notice) to their respective Master SM to report the event. The Notice reports Subnet Management trap number 128 (see Table 29-7 on page 845) and the switch's management port LID address.
Each of them determines that a port that was down is now up.
Each of them issues a directed-route SubnGet(NodeInfo) SMP (because the LID address of the port on the other end of the link is not known; directed-route SMPs are covered in “Discovery” on page 871) to discover the type of device on the other end of the link.
The port at the other end of the link may or may not have been assigned an PortInfo.M_Key by its Master SM.
- If no key was assigned, then the attribute access is permitted.
- If a key was assigned, a read access will be permitted if the port's PortInfo.M_KeyProtectBits are set to 00b, or 01b (as long as the access isn't to read the port's M_Key).
- If a key was assigned, a read access will not be permitted if the port's PortInfo.M_KeyProtectBits are set to 10b or 11b. In this case, the Master SM cannot discover the devices that reside in the other subnet, and it would have to report this as a problem to upper layer software through an interface outside the scope of the specification.
- For more information on M_Key checking, refer to “Management Key (M_Key)” on page 324.
Assuming that the Master SM can read attributes from the devices in the other subnet, it can read the PortInfo.MasterSMLID to discover the location of the other subnet's Master SM (and, likewise, the other Master SM can find this subnet's Master SM).
The actions taken by the two Master SMs are covered later in this chapter.
SM Priority
As stated earlier, only one SM is permitted to manage a subnet. If a Master SM detects another Master SM during its periodic sweep of the subnet, they must have some way of resolving which of them will be the master of the new subnet created by the merge while the other transitions to the Standby state. This is accomplished by a comparison of the 4-bit priority value assigned to each SM.