Layer 2 Loops Occur in Hot-Standby S9700 Networking

Issue Description

Two S9700s are deployed at the core layer in a new building of the customer. Two S5700s are deployed in the server area and connected to S9700-1 and S9700-2 through two links. The networking is as follows.

QQ图片20170817093359

After devices are commissioned based on the network plan, terminals cannot access services on servers.

Alarm Information

None

Handling Process

1. Replan the network, as shown in the following figure.

QQ图片20170817093501

  1. Enable MSTP, and configure instance 10 for VLAN 10 and instance 11 for VLAN 20. The default instance is 0.s
    3. Configure the links between S9700-1 and S5700-1 and between S9700-2 and S5700-2 to block packets of VLAN 20. Configure the links between S9700-2 and S5700-1 and between S9700-1 and S5700-2 to block packets of VLAN 10.
    4. Configure MSTP multi-instance to implement HSB and load balancing of core switches.
    (1) Configure S9700-1.
    stp instance 0 root primary
    stp instance 10 root primary
    stp instance 11 root secondary
    stp pathcost-standard legacy
    stp bpdu-protection
    stp tc-protection ea
    #
    stp region-configuration
    region-name SZPOST
    instance 10 vlan 10
    instance 11 vlan 20
    active region-configuration

    (2) Configure S9700-2.
    stp instance 0 root secondary
    stp instance 10 root secondary
    stp instance 11 root primary
    stp pathcost-standard legacy
    stp bpdu-protection enable
    stp tc-protection
    #
    stp region-configuration
    region-name SZPOST
    instance 10 vlan 10
    instance 11 vlan 20
    active region-configuration

    (3) Configure S5700-1.
    Stp enable
    stp instance 0 priority 8192
    stp instance 10 priority 8192
    stp instance 11 priority 8192
    stp bpdu-protection
    stp pathcost-standard legacy
    stp tc-protection
    #
    stp region-configuration
    region-name SZPOST
    instance 10 vlan 10
    instance 11 vlan 20
    active region-configuration
    #
    interface G3/0/0
    stp instance 11 cost 20000
    #
    interface G3/0/1
    stp instance 0 cost 20000
    stp instance 10 cost 20000
    #

    (4) Configure S5700-2.
    Stp enable
    stp instance 0 priority 8192
    stp instance 10 priority 8192
    stp instance 11 priority 8192
    stp bpdu-protection
    stp pathcost-standard legacy
    stp tc-protection
    #
    stp region-configuration
    region-name SZPOST
    instance 10 vlan 10
    instance 11 vlan 20
    active region-configuration
    #
    interface G3/0/0
    stp instance 11 cost 20000
    #
    interface G3/0/1
    stp instance 0 cost 20000
    stp instance 10 cost 20000

Root Cause

  1. Interfaces connecting S9700-1 and S9700-2 to the S5700s block packets.
    2. MSTP is enabled on the S9700s to prevent loops by default.
    3. Two S9700s and two S5700s form two Layer 2 loops in VLANs 10 and 20.

Suggestions

During switching network planning, you can use MSTP together with the multi-instance function to implement load balancing and prevent Layer 2 loops.

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