The SEP protocol is a dedicated link layer protocol for use on Ethernet ring networks. A SEP segment is the basic unit of the protocol. Only two interfaces on a Layer 2 switching device can be added to the same SEP segment.
In a SEP segment, loops can be prevented by starting a protection mechanism to selectively block certain interfaces and eliminate Ethernet redundant links. When a fault occurs on a ring network, a device running SEP can quickly unblock the blocked interface to perform link switching. This maintains normal communication between nodes on the ring network.
Figure 1 shows a typical SEP application. CE1 is connected to NPEs through a semi-ring formed by switches. A VRRP backup group is deployed on the NPEs. Initially, the status of NPE1 is master and the status of NPE2 is backup. When the link between NPE1 and LSW5 or a node on the link becomes faulty (it is assumed that the link between LSW1 and LSW5 becomes faulty), the status of NPE1 changes from master to backup and the status of NPE2 changes from backup to master, and the following situations occur:
If SEP is not deployed on the semi-ring, CE1 still forwards traffic along the original path. NPE1 that becomes backup does not forward traffic, causing traffic interruption.
If SEP is deployed on the semi-ring, the blocked interface on LSW5 becomes unblocked and enters the forwarding state. In addition, it sends Link Status Advertisements (LSAs) to instruct other nodes on the SEP segment to refresh their LSA databases. CE1 sends traffic along the backup link LSW5->LSW2->LSW4->NPE2. This ensures proper traffic transmission.
Figure 1 Schematic diagram for SEP
In ordinary SEP networking, a physical ring can be configured with only one SEP segment in which only one interface can be blocked. If an interface in the SEP segment in the complete state is blocked, all user package is transmitted only along the path where the primary edge interface resides. The path where the secondary edge interface resides is idle, wasting bandwidths.
SEP multi-instance developed by Huawei is a solution to limit bandwidth waste and to implement traffic load balancing. SEP multi-instance allows two SEP segments to be configured on a physical ring network. All devices, interface roles, and control VLANs in each SEP segment must be configured by conforming to basic SEP configuration principles. Each of the two SEP segments has a blocked interface. The blocked interfaces detect whether the physical ring network is complete. The blocked interfaces in the two SEP segments are independent of each other.
