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📢 Edge port features in RSTP

  • 💬 Functions of edge ports
  • 💬 Problems encountered when using edge ports
  • 💬 summary

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💬 Functions of edge ports

  • The edge port enters the forwarding state immediately after the port is UP. Traffic can be forwarded without waiting for twice the forwarding delay
  • In the P/A mechanism, edge ports are not blocked and can forward traffic continuously
  • When the edge port is UP, the TC mechanism is not enabled

💬 Problems encountered when using edge ports

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Environment description

SW1’s G0/0/1 and G0/0/2 are configured with an Edge port connected to a non-network management switch

Description of loop formation

When the link is first connected, the port configured with the Edge Port enters the forwarding state immediately. When no BPDU is received, e0/0/0 and E0/0/1 of the hub and G0/0/1 and G0/0/2 of SW1 enter the forwarding state, and a loop occurs.

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

Application scenarios of edge ports

Connect to a DHCP client to quickly obtain an IP address

If the Windows DHCP client does not obtain the IP address assigned by the DHCP server within a certain period of time, the DHCP client automatically uses the 169.254.x.X/16 address (this address is used to communicate with other devices when no IP address is allocated). 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 quickly obtain a valid IP address

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.

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The interface connecting the information socket

If there are a large number of terminals on the network, frequent access of terminals will generate TCS. As a result, the switch frequently deletes MAC and ARP entries, and a large number of unknown unicast packets are generated, causing network flooding and affecting network performance (described in THE TC mechanism in RSTP). When TC occurs on the network, MAC and ARP entries corresponding to interfaces without edge ports are deleted, and unknown unicast packets are generated for flooding.

When a user connects to a switch privately or intentionally constructs an illegal BPDU to attack the network. When a user connects to a switch or attacks bPDUs on the network, an edge port can be used to defend against BPDUs. After receiving BPDUs, the edge port shuts down the port.

The administrator needs to manually enable or configure the error-down auto-recovery causes bpdu-protection interval XX command for an interface after shutdown. The command is automatically enabled when the timeout period expires.

💬 summary

1️ This article shares edge port characteristics in RSTP

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