Network Architecture of SEP
As shown in Figure 1, LSW1, LSW2, LSW3, LSW4, and LSW5 are connected to access a Layer 2 network. On the Layer 2 network, two edge devices LSW1 and LSW5 are indirectly connected. This networking is called open-ring networking. Such networking mode results in a new loop on the entire network. To eliminate redundant loops on the network and ensure the connectivity of links, a protection mechanism is required. Figure 1 shows the typical networking of an open ring running SEP. The following describes the basic concepts of SEP.
Figure 1 Networking diagram of an open ring running SEP
SEP segment
SEP takes an SEP segment as a basic unit. An SEP segment is composed of multiple interconnected Layer 2 switching devices that are configured with the same SEP segment ID and control VLAN ID.
An SEP segment physically corresponds to a ring-shaped or line-shaped Ethernet topology. Each SEP segment has a control VLAN, edge interfaces, and common interfaces.
Control VLAN
In an SEP segment, the control VLAN is used to transmit only SEP packets.
Each SEP segment must be configured with a control VLAN. After an interface is added to an SEP segment that is configured with a control VLAN, the interface is added to the control VLAN automatically.
Different SEP segments can use the same control VLAN ID.
Different from a control VLAN, a data VLAN is used to transmit data packets.
Node
A node refers to a Layer 2 switching device that is added to an SEP segment. A maximum of two interfaces on a node can be added to the same SEP segment.
Interface role
As defined by SEP, there are two interface roles: common interfaces and edge interfaces.
As shown in Table 1, edge interfaces are further classified into primary edge interfaces, secondary edge interfaces, no-neighbor primary edge interfaces, and no-neighbor secondary edge interfaces.
Normally, edge interfaces and no-neighbor edge interfaces do not reside in the same SEP segment.
The interfaces connected to a primary edge interface and a secondary edge interface must be SEP interfaces. The interfaces connected to a no-neighbor primary edge interface and a no-neighbor secondary edge interface can not be SEP interfaces.
Interface Role | Sub-role | Description |
Common interface | - | In an SEP segment, all interfaces except edge interfaces are common interfaces. A common interface monitors the status of its directly connected SEP link and sends a message about link status changes to a neighboring interface in time. The neighboring interface constantly floods the message to other interfaces in the SEP segment until the message reaches the primary edge interface. The primary edge interface then processes the message. |
Edge interface | Primary edge interface | There is only one primary edge interface in an SEP segment. The primary edge interface is elected after being configured. The primary edge interface initiates blocked-interface preemption, terminates packets, and sends messages about topology changes to other networks. |
Secondary edge interface | There is only one secondary edge interface in an SEP segment. The secondary edge interface is elected after being configured. The secondary edge interface terminates packets and sends messages about topology changes to other networks. | |
No-neighbor primary edge interface | The interface at the edge of the SEP segment is a no-neighbor edge interface. The no-neighbor edge interface is configured by a user. The no-neighbor primary edge interface terminates packets and sends messages about topology changes 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. NOTE: Whether the device where a no-neighbor edge interface resides sends preemption packets is determined by whether the brother interface of the no-neighbor edge interface is blocked. o If the brother interface of the no-neighbor edge interface is blocked, the device does not need to send preemption packets. o If the brother interface of the no-neighbor edge interface is unblocked, the brother interface sends preemption packets. | |
No-neighbor secondary edge interface | There is only one no-neighbor secondary edge interface in an SEP segment. The no-neighbor secondary edge interface is elected after being configured. The no-neighbor secondary edge interface terminates packets and sends messages about topology changes 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. | |
Blocked interface
A blocked interface refers to the interface that is blocked to prevent loops in an SEP segment.
In an SEP segment, no interface is specified as a blocked interface. Each interface in the SEP segment may become as a blocked interface. When an SEP segment works normally, there is only one blocked interface in the SEP segment.
Status of interfaces enabled with SEP
In an SEP segment, the status of interfaces enabled with SEP is classified into two types, as shown in Table 2.
Interface Status | Description |
Forwarding | In the Forwarding state, an interface can forward user traffic, and receive and send SEP packets. |
Discarding | In the Discarding state, an interface just receives and sends SEP packets. |
Interface status and interface roles are not necessarily related. Interfaces playing different roles support Forwarding and Discarding states.
SEP Packet
Table 3 shows the types of SEP packets.
Packet Type | Packet Subtype | Description |
Hello packet | - | After an interface is added to an SEP segment, the neighbor negotiation mechanism is started on the interface. By exchanging Hello packets, the interface and its neighboring interface establish the neighbor relationship. After neighbor negotiation succeeds, the interfaces continue to exchange Hello packets to detect the neighbor status. |
LSA | LSA Request packet | After an interface is enabled with SEP, the interface periodically sends Link Status Advertisements (LSA) to its neighboring interface. After the state machine of the neighboring interface is Up, the two interfaces update their link status databases, that is, all topology information. |
LSA ACK packet | ||
TC packet | - | When the topology of an SEP segment changes, a Topology Change (TC) packet is sent to notify the upper-layer network. Then, all nodes on the upper-layer network need to update their MAC address forwarding tables and ARP tables. The TC packet is sent by the device where the SEP segment and the upper-layer network are intersected. |
GR packet | - | If a device sends an SEP Graceful Restart (GR) packet, it indicates that active/standby switchover occurs on the device. A GR packet is sent by a device to instruct other nodes to prolong the aging time of the LSA received from the device. After active/standby switchover is complete, the device needs to send another GR packet to instruct other nodes to restore the aging time of the LSA received from the device to the previous value. |
Primary edge interface-election packet | - | After an interface is enabled with SEP, the interface sets its interface role as the primary edge interface if it has the right to participate in the election of the primary edge interface. Then, the interface periodically sends primary edge interface-election packets without waiting for the success of neighbor negotiation. A primary edge interface-election packet contains the interface role (primary edge interface, secondary edge interface, or common interface), bridge MAC address of the interface, interface ID, and status of the topology database. |
Preemption packet | Preemption Request packet | A preemption packet is used to block a specified interface. Preemption packets are sent by the selected primary edge interface or the brother interface of a no-neighbor primary edge interface. |
