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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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ERPS Multi-ring Principles

ERPS Multi-ring Principles

Ethernet Ring Protection Switching version 1 (ERPSv1) supports only single ring topology, whereas ERPSv2 supports single and multi-ring topologies.

A multi-ring network consists of one or more major rings and sub-rings. A sub-ring can have a virtual channel (VC) or non-virtual channel (NVC), depending on whether R-APS PDUs on the sub-ring will be transmitted to a major ring.

This section describes how ERPS is implemented on a multi-ring network with sub-rings that have NVCs when links are normal, when a link fails, and when the link recovers.

Links Are Normal

On the multi-ring network shown in Figure 12-11, Device A through Device E constitute a major ring; Device B, Device C, and Device F constitute sub-ring 1, and Device C, Device D, and Device G constitute sub-ring 2. The devices on each ring can communicate with each other.

  1. To prevent loops, each ring blocks its RPL owner port. All other ports can transmit service traffic.
  2. The RPL owner on each ring sends R-APS (NR) messages to all other nodes on the same ring at an interval of 5s. The R-APS (NR) messages on the major ring are transmitted only on this ring. The R-APS (NR) messages on each sub-ring are terminated on the interconnection nodes and therefore are not transmitted to the major ring.

Traffic between PC1 and the upper-layer network travels along the path PC1 <-> Device F <-> Device B <-> Device A <-> PE1; traffic between PC2 and the upper-layer network travels along the path PC2 <-> Device G <-> Device D <-> Device E <-> PE2.

Figure 12-11 ERPS multi-ring networking (links are normal)

A Link Fails

As shown in Figure 12-12, if the link between Device D and Device G fails, the ERPS protection switching mechanism is triggered. The ports on both ends of the faulty link are blocked, and the RPL owner port on sub-ring 2 is unblocked to send and receive traffic. In this situation, traffic from PC1 still travels along the original path. Device C and Device D inform the other nodes on the major ring of the topology change so that traffic from PC2 is also not interrupted. Traffic between PC2 and the upper-layer network travels along the path PC2 <-> Device G <-> Device C <-> Device B <-> Device A <-> Device E <-> PE2. The process is as follows:

  1. After Device D and Device G detect the link fault, they block their ports on the faulty link and perform a Filtering Database (FDB) flush.
  2. Device G sends three consecutive R-APS (SF) messages to the other LSWs and then sends one R-APS (SF) message at an interval of 5s afterwards.
  3. Device G then unblocks the RPL owner port and performs an FDB flush.
  4. After the interconnection node Device C receives an R-APS (SF) message, it performs an FDB flush. Device C and Device D then send R-APS Event messages within the major ring to notify the topology change in sub-ring 2.
  5. After receiving an R-APS Event message, the other LSWs on the major ring perform an FDB flush.

Then traffic from PC2 is switched to a normal link.

Figure 12-12 ERPS multi-ring networking (unblocking the RPL owner port if a link fails)

The Link Recovers

After the link fault is rectified, either of two situations may occur:

  • If the ERPS ring uses revertive switching, the RPL owner port is blocked again, and the link that has recovered is used to forward traffic.
  • If the ERPS ring uses non-revertive switching, the RPL remains unblocked, and the link that has recovered remains blocked.

The following example uses revertive switching to describe the process after the link recovers.

  1. After the link between Device D and Device G recovers, Device D and Device G start a guard timer to avoid receiving out-of-date R-APS PDUs. The two routers do not receive any R-APS PDUs before the timer expires. Then Device D and Device G send R-APS (NR) messages within sub-ring 2.
  2. Device G on which the RPL owner port resides starts the WTR timer. After the WTR timer expires, Device G blocks the RPL owner port and unblocks its port on the link that has recovered and then sends R-APS (NR, RB) messages within sub-ring 2.
  3. After receiving an R-APS (NR, RB) message from Device G, Device D unblocks its port on the recovered link, stops sending R-APS (NR) messages, and performs an FDB flush. Device C also performs an FDB flush.
  4. Device C and Device D, the interconnection nodes, then send R-APS Event messages within the major ring to notify the link recovery of sub-ring 2.
  5. After receiving an R-APS Event message, the other LSWs on the major ring perform an FDB flush.

Then traffic changes to the normal state, as shown in Figure 12-11.

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Updated: 2019-01-02

Document ID: EDOC1100058405

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