OSPF forwarding address

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For S series switches supporting OSPF, the functions and setting rules of the OSPF forwarding address are as follows:
The forwarding address (FA) field in an OSPF Type 5 LSA, similar to that of RIPv2 or BGPv4, specifies information about the next hop so that devices in a routing area can forward data to the next-hop address outside the AS through the imported route described in the Type 5 LSA. This prevents a device on a broadcast network from sending data to itself as the next hop and then forwarding the data to a device in an external routing area on the same broadcast network.

For a Type 5 LSA generated by the ASBR for external routes, the FA value can be 0.0.0.0 or non-0.0.0.0.
FA setting rules are as follows:
If OSPF is not enabled on the next-hop interfaces of the external routes imported by an ASBR, the FA is 0.0.0.0.
If the FA of a Type 5 LSA is 0.0.0.0, the device that receives the LSA calculates the route using Adv Rtr (the ASBR address) as the next-hop address.
If all of the following conditions are met, the FA is a non-0.0.0.0 address.
The ASBR has a routable next-hop interface.
The next-hop interface on the ASBR is not configured as a silent interface.
The next-hop interface on the ASBR is not an OSPF P2P or P2MP interface.
The IP address of the next-hop interface on the ASBR is within the OSPF network range.
If any condition is not met, the FA is 0.0.0.0.
After the ASBR fills a non-0.0.0.0 address in the FA field of a Type 5 LSA, the device that receives the LSA calculates the route using the non-0.0.0.0 address as the next-hop address.

The FA of an NSSA LSA cannot be 0.0.0.0.
If a loopback address is configured for an interface in the NSSA, the loopback address is automatically selected as the FA.
If no loopback interface exists in the NSSA, the IP address of the first Up interface is selected as the FA.


Before calculating a route using an LSA, OSPF checks whether there are intra-area or inter-area routes to the ASBR. If not, OSPF does not use the LSA in route calculation and no route can be calculated.

Other related questions:
Relationship between a secondary IP address and an OSPF IP address
They are actually the same address. A secondary address is usually referred to as an OSFP IP address on the NMS.

Difference between forwarding modes of Layer 2 multicast on an S series switch
After Layer 2 multicast is enabled on an S series switch, the switch maintains a Layer 2 multicast forwarding table. When receiving a multicast packet, the switch matches the multicast address of the packet with multicast forwarding entries to determine the outbound interface. Whether the switch determines the outbound interface based on the multicast IP address or multicast MAC address depends on the configured Layer 2 multicast forwarding mode. Multiple multicast IP addresses may be mapped to one MAC address. If multicast data is forwarded based on the MAC address, multicast data may be sent to the users who do not order the multicast data. MAC address-based multicast forwarding can improve specifications in certain scenarios. Different models support different Layer 2 multicast forwarding modes: - The S1720, S2700SI, S2710SI, S2700EI, S2720, S2750EI, S5700S-LI, and S5700LI support only MAC address-based multicast forwarding. - The S3700SI and S5710-C-LI support only IP address-based multicast forwarding. - The S5700SI supports only IP address-based multicast forwarding in V200R002 and earlier versions, and supports MAC address-based and IP address-based forwarding modes starting from V200R003. - The S3700EI, S3700HI, S5710-X-LI, S5720S-SI, S5720SI, S5700EI, S5710EI, S5720EI, S5700HI, S5710HI, S5720HI, S6700EI, S6720EI, S7700, S9700, S12700, S9300, and E600 support MAC address-based and IP address-based forwarding modes.

How to display the source and destination addresses of forwarded packets on S series switches
For S series switches (except the S1700), no command is available to display the source or destination addresses of the forwarded packets. If you want to obtain the addresses, use a tool to capture packets.

Data forwarding mode
Data forwarding mode: - Centralized forwarding (also called tunnel forwarding): APs set up control tunnels and data tunnels with an AC. Control data between the AP and AC and service data from WLAN users are encapsulated in the control tunnel and data tunnel, respectively. After the service data reaches the APs, it must be sent to the AC through the data tunnel first and then be forwarded at L2 and L3. - Direct forwarding (also called local forwarding): APs only set up control tunnels with an AC. After the service data reaches the APs, it directly enters the wired network through the switch that connects to the APs and then be forwarded at L2 and L3. In summary, tunnel forwarding facilitates centralized management and control, and service data must be forwarded through an AC, reducing packet forwarding efficiency and burdening the AC. For direct forwarding, packet forwarding efficiency is high. You can select either of the forwarding mode as required. AC deployment - Bypass mode ACs are directly connected to aggregation switches or core switches in bypass mode. If two ACs provide 1+1 backup, they connect to the aggregation switch or core switch. - Establish an independent AC management area that connects to the core switch in bypass mode. In actual networking, an aggregation switch is connected to the core switch in bypass mode, and all the ACs are connected to the aggregation switch. Verify the forwarding mode by checking that an AC serves as the gateway or running the display forward-type service xx command.

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