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NE40E-M2 V800R010C10SPC500 Feature Description - LAN Access and MAN Access 01

This is NE40E-M2 V800R010C10SPC500 Feature Description - LAN Access and MAN Access
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MSTP Multi-process

MSTP Multi-process

Background

On the network shown in Figure 10-12:

  • UPEs are deployed at the aggregation layer, running MSTP.

  • UPE1 and UPE2 are connected by a Layer 2 link.

  • Multiple rings are connected to UPE1 and UPE2 through different ports.

  • The routers on the rings reside at the access layer, running STP or RSTP. In addition, UPE1 and UPE2 work for different carriers, and therefore they need to reside on different spanning trees whose topology changes do not affect each other.

Figure 10-12 Application with both MSTP and STP/RSTP

On the network shown in Figure 10-12, routers and UPEs construct multiple Layer 2 rings. STP must be enabled on these rings to prevent loops. UPE1 and UPE2 are connected to multiple access rings that are independent of each other. The spanning tree protocol cannot calculate a single spanning tree for all routers. Instead, the spanning tree protocol must be enabled on each ring to calculate a separate spanning tree.

MSTP supports MSTIs, but these MSTIs must belong to one MST region and routers in the region must have the same configurations. If the routers belong to different regions, MSTP calculates the spanning tree based on only one instance. Assume that routers on the network belong to different regions, and only one spanning tree is calculated in one instance. In this case, the status change of any router on the network affects the stability of the entire network. On the network shown in Figure 10-12, the routers connected to UPEs support only STP or RSTP but not MSTP. When MSTP-enabled UPEs receive RST BPDUs from the routers, the UPEs consider that they and routers belong to different regions. As a result, only one spanning tree is calculated for the rings composed of UPEs and routers, and the rings affect each other.

To prevent this problem, MSTP multi-process is introduced. MSTP multi-process is an enhancement to MSTP. The MSTP multi-process mechanism allows ports on routers to be bound to different processes. MSTP calculation is performed based on processes. In this manner, only ports that are bound to a process participate in the MSTP calculation for this process. With the MSTP multi-process mechanism, spanning trees of different processes are calculated independently and do not affect each other. The network shown in Figure 10-12 can be divided into multiple MSTP processes by using MSTP multi-process. Each process takes charge of a ring composed of routers. The MSTP processes have the same functions and support MSTIs. The MSTP calculation for one process does not affect the MSTP calculation for another process.

NOTE:

MSTP multi-process is applicable to MSTP as well as RSTP and STP.

Purpose

On the network shown in Figure 10-12, MSTP multi-process is configured to implement the following:
  • Greatly improves applicability of STP to different networking conditions.

    To help a network running different spanning tree protocols run properly, you can bind the routers running different spanning tree protocols to different processes. In this manner, every process calculates a separate spanning tree.

  • Improves the networking reliability. For a network composed of many Layer 2 access devices, using MSTP multi-process reduces the adverse effect of a single node failure on the entire network.

    The topology is calculated for each process. If a device fails, only the topology corresponding to the process to which the device belongs changes.

  • Reduces the network administrator workload during network expansion, facilitating operation and maintenance.

    To expand a network, you only need to configure new processes, connect the processes to the existing network, and keep the existing MSTP processes unchanged. If device expansion is performed in a process, only this process needs to be modified.

  • Implements separate Layer 2 port management

    An MSTP process manages parts of ports on a router. Layer 2 ports on a router are separately managed by multiple MSTP processes.

Principles

  • Public link status

    As shown in Figure 10-12, the public link between UPE1 and UPE2 is a Layer 2 link running MSTP. The public link between UPE1 and UPE2 is different from the links connecting routers to UPEs. The ports on the public link need to participate in the calculation for multiple access rings and MSTP processes. Therefore, the UPEs must identify the process from which MST BPDUs are sent.

    In addition, a port on the public link participates in the calculation for multiple MSTP processes, and obtains different status. As a result, the port cannot determine its status.

    To prevent this situation, it is defined that a port on a public link always adopts its status in MSTP process 0 when participating in the calculation for multiple MSTP processes.

    NOTE:

    After a routers normally starts, MSTP process 0 exists by default, and MSTP configurations in the system view and interface view belong to this process.

  • Reliability

    On the network shown in Figure 10-13, after the topology of a ring changes, the MSTP multi-process mechanism helps UPEs flood a TC packet to all routers on the ring and prevent the TC packet from being flooded to routers on the other ring. UPE1 and UPE2 update MAC and ARP entries on the ports corresponding to the changed spanning tree.

    Figure 10-13 MSTP multi-process topology change

    On the network shown in Figure 10-14, if the public link between UPE1 and UPE2 fails, multiple routers that are connected to the UPEs will unblock their blocked ports.

    Figure 10-14 Public link fault

    Assume that UPE1 is configured with the highest priority, UPE2 with the second highest priority, and routers with default or lower priorities. After the link between UPE1 and UPE2 fails, the blocked ports (replacing the root ports) on routers no longer receive packets with higher priorities and re-performs state machine calculation. If the calculation changes the blocked ports to designated ports, a permanent loop occurs, as shown in Figure 10-15.

    Figure 10-15 Loop between access rings

  • Solutions

    To prevent a loop between access rings, use either of the following solutions:

    • Configure root protection between UPE1 and UPE2.

      If all physical links between UPE1 and UPE2 fail, configuring an inter-board Eth-Trunk link cannot prevent the loop. Root protection can be configured to prevent the loop shown in Figure 10-15.

      Figure 10-16 MSTP multi-process with root protection

      Use the blue ring shown in Figure 10-16 as an example. UPE1 is configured with the highest priority, UPE2 with the second highest priority, and routers on the blue ring with default or lower priorities. In addition, root protection is enabled on UPE2.

      Assume that a port on S1 is blocked. When the public link between UPE1 and UPE2 fails, the blocked port on S1 begins to calculate the state machine because it no longer receives BPDUs of higher priorities. After the calculation, the blocked port becomes the designated port and performs P/A negotiation with the downstream router.

      After S1, which is directly connected to UPE2, sends BPDUs of higher priorities to the UPE2 port enabled with root protection, the port is blocked. From then on, the port remains blocked because it continues receiving BPDUs of higher priorities. In this manner, no loop will occur.

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Updated: 2019-01-02

Document ID: EDOC1100058405

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