The reason of S series switches TUNNEL port MTU does not take effect

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S series switches (except S1700) MTU of the TUNNEL port is valid only for the uplink control plane and the packets sent from the control plane. The MTU does not take effect for the data packets directly forwarded at the hardware level. The hardware level MTU need to configure the MTU value of the physical interface.

Other related questions:
Why does not the MTU configuration take effect on tunnel interfaces of a switch
The maximum transmission unit (MTU) configuration is valid for packets sent to and from the control plane, but invalid for packets directly forwarded by hardware. To limit the size of packets forwarded by hardware, configure the MTU on physical interfaces.

Can the MTU of the GRE tunnel interface take effect
If you set an MTU value on a GRE tunnel interface, forwarding of data packets through the GRE tunnel will be affected. If the packet length exceeds the MTU value on the tunnel interface, the device fragments the packet.

DHCP configuration on S series switch does not take effect
DHCP configuration may not take effect on an S series switch due to the following reasons: 1. DHCP server is disabled on the switch. 2. The DHCP address pool is configured incorrectly. 3. No IP network segment is specified on the interface connected to DHCP clients, or the specified IP network segment is on a different network segment from the address pool. 4. If ACL resources are exhausted, the DHCP commands run globally or on an interface will not take effect.

IPSG does not take effect on an S series switch. What are the possible causes
If IPSG does not take effect on an S series switch (except the S1700), possible causes include the following: 1. A binding entry is incorrect. a. A static binding table is created using the user-bind static command. If the binding entry of a valid host is not in the binding table, add the host's binding entry to the binding table. If the host's entry exists in the binding table, check whether the MAC address in the entry is the same as the host's MAC address. If the network card of the host is replaced, the MAC address in the entry may not be updated. Check whether the host's entry contains VLAN information. Only when the interface connected to this host has been added to the correct VLAN, the switch allows the packets from the host to pass. b. A dynamic binding table is generated only when DHCP snooping is enabled, the interface connected to the DHCP server is configured as a trusted interface, and then the PC obtains a new IP address. 2. IPSG is not enabled in the specified interface or VLAN view. After a binding table is generated, the IPSG function must be enabled in the interface or VLAN view using the ip source check user-bind enable command. IPSG takes effect only on the interface or VLAN where it is enabled, and IPSG check is not performed on the interfaces or VLANs with IPSG disabled. Therefore, if IPSG does not take effect on an interface or in a VLAN, the IPSG function may not be enabled on this interface or in this VLAN. 3. IPSG is enabled in the VLAN to which the uplink interface belongs. IPSG is enabled on the user-side interface, namely, the downlink interface. If IPSG is enabled on the uplink interface, the packets returned by the gateway may be discarded. As a result, user service is interrupted. Solution: Disable IPSG in the VLAN to which the uplink interface belongs. 4. DHCP snooping is disabled or a DHCP snooping trusted interface is configured on the uplink interface or in the VLAN to which the uplink interface belongs. If DHCP snooping if disabled on an interface using the dhcp snooping disable command, or if a DHCP snooping trusted interface is configured on the interface using the dhcp snooping trusted command, the IPSG function on the interface or in the VLAN to which the interface belongs does not take effect. 5. Hardware ACL resources are insufficient. The hardware ACL resources are used by IPSG and other services. If the ACL resources are insufficient, IPSG cannot take effect. For example, you can run the display dhcp static user-bind all verbose command to view the IPSG status corresponding to static binding entries. If the value of IPSG Status is ineffective, IPSG of this entry does not take effect. The possible cause is that hardware ACL resources are insufficient. 6. A QoS traffic policy conflicts with IPSG. This situation may only occur in V1R6C05. When a QoS traffic policy conflicts with IPSG, the traffic behavior in the QoS traffic policy takes effect. In this situation, you need to modify service configurations.

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