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📢 Features of edge ports in RSTP

💬 Functions of the edge port

  • The edge port enters the forwarding state immediately after the port goes UP. You do not need to wait twice the forwarding delay to forward traffic
  • In the P/A mechanism, edge ports are not blocked and continue to forward traffic
  • When the edge port is UP, the TC mechanism is not enabled

💬 Problems encountered when using the edge port

Environment description

G0/0/1 and G0/0/2 of SW1 are configured with edge ports and connected to a non-nms switch

Description of loop formation

When the link is just connected, the port with the Edge port enters the forwarding state immediately. When the port does not receive a BPDU, the HUB’s E0/0/0 and E0/0/1, and SW1’s G0/0/1 and G0/0/2 are all in the forwarding state, resulting in a loop.

When an edge port receives a BPDU, it loses its edge port features and becomes a common STP port. In addition, it participates in STP calculation and the loop disappears. STP (determining the port role, port status, and forwarding delay) needs to be calculated, which causes network flapping.

Usage scenarios of edge ports

Connect to the DHCP client so that it can quickly obtain an IP address

If the Windows DHCP client does not obtain the IP address assigned by the DHCP server in a certain period of time, it automatically uses the 169.254.x. x /16 IP address to communicate with other devices. In this case, the DHCP client does not request an IP address. If the waiting time is shorter than 30 seconds, the DHCP client cannot obtain a valid IP address in a short time

Switches important server connection, as shown in figure, after the link between SW1 and SW2 recovery, will be P/A mechanism, the edge if the interface is not configured to answering the PC port, the port will block, did not enter the forwarding state after 30 s, communications between PC1 and PC2 will interrupt the 30 s, after edge port configuration, P/A mechanism of the port is not blocked, The communication between PC1 and PC2 is not interrupted.

The interface connecting to the information socket

When a large number of terminals go online frequently, TCS are generated. As a result, the switch frequently deletes its MAC address and ARP entries. As a result, a large number of unknown unicast packets are generated, which causes the network to flood and affects network performance (as described in the TC mechanism of RSTP). When a TC occurs on the network, the MAC address and ARP entries corresponding to the interfaces that are not configured with edge ports are also deleted, and unknown unicast packets are flooded.

When a user accesses a switch or intentionally constructs illegal BPDUs to attack the network; When a user is connected to a switch or a BPDU attack is launched on the network, an edge port can work with BPDUs for defense. In this case, the edge port shuts down after receiving a BPDU.

The administrator needs to manually enable the shutdown interface or run the error-down auto-recovery causes bpdu-protection interval XX command to enable the shutdown interface automatically after the timeout period.

💬 summary

1️ This article shares edge port features in RSTP

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