MTU value of OSPF DD packets sent by S series switches

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According to RFC 2328, the value of the Interface MTU field in a DD packet is the MTU value of the link (if the link is a virtual link, the value is 0). When a device receives a DD packet with an Interface MTU value larger than the MTU value in DD packets sent by the device, the device drops the DD packet and the neighbor status stays Exchange Start.

On an S series switch supporting OSPF, you can run the ospf mtu-enable command in an interface view to check the MTU value in received DD packets. By default, OSPF does not check the MTU value in a DD packet. If the MTU match check is not performed, LSDBs of the two ends can be synchronized and the OSPF neighbor relationship can enter the Full state with the MTU values configured on the two ends varying slightly.

Other related questions:
Are the MTUs in the DD packets exchanged to establish an OSPF neighbor relationship checked
When the interfaces are configured with different MTUs, OSPF can normally set up the neighbor relationship. By default, OSPF does not check the MTUs in DD packets. When the ospf mtu-enable command is run on an interface, the OSPF neighbor relationship cannot be set up when the MTUs in the DD packets are inconsistent with the one set on the interface. In this case, the MTUs in the DD packets need to be checked. If the MTU in a DD packet is greater than the MTU set on the interface, the interface discards the DD packet.

Destination IP addresses of packets of different network types sent by OSPF interfaces on S series switches
Destination IP addresses of packets of different network types sent by S series switches supporting OSPF are as follows: Hello packets On a broadcast, P2MP, or P2P network, Hello packets are sent to the manycast IP address 224.0.0.5. On a virtual link, Hello packets are sent to the other end of the virtual link. On an NBMA network, Hello packets are sent to neighbors in unicast mode. DD, LSR, and LSU packets On broadcast, P2MP, and NBMA networks, DD, LSR, and LSU packets are sent to the IP addresses of neighbors in unicast mode. On P2P networks, DD, LSR, and LSU packets are sent to the IP address 224.0.0.5. LSAck packets On a P2P network, LSAck packets are sent to the IP address 224.0.0.5. On P2MP and NBMA networks, LSAck packets are sent to neighbors in unicast mode. On a broadcast network, LSAck packets are transmitted in multicast mode. If sent by the DR or BDR, the packets are transmitted to 224.0.0.5; otherwise, the packets are transmitted to 224.0.0.6.

OSPF cost of S series switches
The ospf cost command can be executed on an S series switch that supports OSPF to configure the cost required for running the OSPF protocol on an interface. Application scenario: The interface cost can be automatically calculated by OSPF based on the interface bandwidth or manually configured by using the ospf cost command. If the costs and destination IP addresses of multiple routes discovered by one routing protocol are the same, load balancing can be implemented among the routes. Modify interface costs to implement load balancing as required. Configuration impact: If no interface cost is configured by using the ospf cost command, OSPF automatically calculates the interface cost based on the interface bandwidth. The calculation formula is as follows: Cost of the interface = Bandwidth reference value/Interface bandwidth. The integer of the calculated result is the cost of the interface. If the calculated result is smaller than 1, the cost value is 1. Changing the bandwidth reference value can change the cost of an interface. By default, the bandwidth reference value is 100 Mbit/s. Therefore, the default cost value of an Ethernet (100 Mbit/s) interface is 1 (100,000,000/the interface bandwidth). Example: Set the cost required by VLANIF 100 for running OSPF to 65. [HUAWEI] interface vlanif 100 [HUAWEI-Vlanif100] ospf cost 65 Set the cost required by GE0/0/1 for running OSPF to 65. [HUAWEI] interface gigabitethernet 0/0/1 [HUAWEI-GigabitEthernet0/0/1] undo portswitch [HUAWEI-GigabitEthernet0/0/1] ospf cost 65 Note: The cost calculated by OSPF is the cost of the route to the outbound interface of the destination network.

How are VRRP packets of an S series switch sent in a super-VLAN
For S series switches (except the S1700), you are not advised to create a VRRP group on the corresponding VLANIF interface for a super-VLAN. If such a VRRP group has been created, the total number of VLANs (including common VLANs, super-VLANs, and sub-VLANs) in the VRRP group cannot exceed 256 to minimize impact on host performance. By default, if a VRRP group is configured for a super-VLAN, the master switch sends VRRP Advertisement packets to the super-VLAN's sub-VLAN that is in Up state and has the smallest VLAN ID among all the sub-VLANs. You can run a related command to configure VRRP Advertisement packets to be sent in a specified sub-VLAN. This prevents VRRP Advertisement packets from being broadcast in all sub-VLANs, reducing network bandwidth consumption. The configuration procedure is as follows: 1. Run the system-view command to enter the system view. 2. Run the interface vlanif vlan-id command to enter the VLANIF interface view. 3. Run the vrrp advertise send-mode { sub-vlan-id | all } command to configure a mode in which the master switch sends VRRP Advertisement packets in a super-VLAN. - sub-vlan-id: specifies the ID of a sub-VLAN where the master switch sends VRRP Advertisement packets. - all: indicates that the master switch sends VRRP Advertisement packets to all sub-VLANs in a super-VLAN. Note: The parameter all is not recommended. If all is configured, a super-VLAN broadcasts a VRRP Advertisement packet to all its sub-VLANs, which increases bandwidth usage.

How do I set the MTU value for the MCU?
By default, the MTU value of the MCU is 1500 bytes, which can be changed and be valid for only signaling packets. Generally, you are advised not to change the MTU value. Instead, change the MTU value of the network device to the same as the MTU value of the MCU or endpoint. The method for changing the MTU value is as follows: Run the sys setMTU command, press Enter, enter the new value, press Enter, save the setting, and restart the network device.

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