SEP Network

In Figure 17-2, LSW1 through LSW5 constitute a ring and are dual-homed to an upper-layer or a Layer 2 network. Two edge devices (LSW1 and LSW5) are indirectly connected. This networking is called open ring networking. This mode will cause a loop on the entire network. To eliminate redundant links and ensure link connectivity, a mechanism used to prevent loops is required.

Figure 17-2 shows the typical networking of an open ring running SEP. The following describes the basic concepts of SEP.

Figure 17-2  Open ring running SEP

The following includes basic SEP concepts:

• SEP segment

  A SEP segment is the basic unit of SEP. A SEP segment consists of interconnected Layer 2 switching devices configured with the same SEP segment ID and control VLAN ID.

  A SEP segment is a ring or linear Ethernet topology. Each SEP segment has a control VLAN, edge interfaces, and common interfaces.

• Control VLAN

  In a SEP segment, the control VLAN is used to transmit only SEP packets.

  Each SEP segment must have a control VLAN. After an interface is added to a SEP segment that has a control VLAN, the interface is automatically added to the control VLAN.

  Different SEP segments can use the same control VLAN.

  Different from a control VLAN, a data VLAN is used to transmit data packets.

• Node

  Each Layer 2 switching device in a SEP segment is a node. Each node can have a maximum of two interfaces added to the same SEP segment.

• Interface role

  As defined by SEP, there are two interface roles: common and edge interfaces.

  In Table 17-1, edge interfaces are further classified into primary edge interfaces, secondary edge interfaces, no-neighbor primary edge interfaces, and no-neighbor secondary edge interfaces.

  It is not recommended to configure primary edge interfaces and no-neighbor edge interfaces in the same SEP segment.

  It is not recommended to configure secondary edge interfaces and no-neighbor edge interfaces in the same SEP segment.

  Table 17-1  Interface roles

  Interface Role	Sub-role	Description
  Edge interface	Primary edge interface	A SEP segment has only one primary edge interface, which is determined by the configuration and election. The primary edge interface initiates blocked interface preemption, terminates packets, and sends topology change notification messages to other networks.
  	Secondary edge interface	A SEP segment has only one secondary edge interface, which is determined by the configuration and election. The secondary edge interface terminates packets and sends topology change notification messages to other networks.
  	No-neighbor primary edge interface	An interface at the edge of a SEP segment is a no-neighbor edge interface, which is determined by the configuration and election. The no-neighbor primary edge interface terminates packets and sends topology change notification messages to other networks. No-neighbor primary edge interfaces are used to interconnect Huawei devices and non-Huawei devices or interconnect Huawei devices and devices that do not support SEP.
  	No-neighbor secondary edge interface	A SEP segment has only one no-neighbor secondary edge interface, which is determined by the configuration and election. The no-neighbor secondary edge interface terminates packets and sends topology change notification messages to other networks. No-neighbor secondary edge interfaces are used to interconnect Huawei devices and non-Huawei devices or interconnect Huawei devices and devices that do not support SEP.
  Common interface	-	In a SEP segment, all interfaces except edge interfaces are common interfaces. A common interface monitors the status of a directly-connected SEP link. When the status of a link changes, the interface sends a topology change notification message to notify its neighbors. Then the topology change notification message is flooded on the link until it finally reaches the primary edge interface. The primary edge interface determines how to process the link change.

• Blocked interface

  In a SEP segment, some interfaces are blocked to prevent loops.

  Any interface in a SEP segment may be blocked if no interface is specified for blocking. A complete SEP segment has only one blocked interface.

• SEP interface status

  In a SEP segment, a SEP interface has two working states:

  • Forwarding: The interface can forward user traffic, receive and send SEP packets.
  • Discarding: The interface can receive and send SEP packets but cannot forward user traffic.

  An interface may be in Forwarding or Discarding state regardless of its role.

SEP Packet

Table 17-2 shows the types of SEP packets.