Trigger update of ospf (Tencent interview)

OSPF triggered update means that in an OSPF area, when the interface down of one of the routers falls down or the link state information changes, it will send a multicast packet to declare its new state. This process is called triggered update. The process of triggering the update amount is:

1. When the router discovers that the link state has changed, it uses the multicast address 224.0.0.6 to send LSU packets to DR and BDR in the OSPF area. The LSU packet contains the update information of LSA.

2. DR uses the LSACK packet to reply to the changed router and sends LSU packet flooding to other routers using the multicast address 224.0.0.5, and each router that receives the flooding uses the LASCK packet to reply to DR.

3. If a router is connected to another network, it forwards LSU flooding packets to the DR,DR of that network and then to other routers in the network.

4. The router that receives the LSU update packet updates its own LSDB and updates the shortest path to the routing table through the shortest spanning tree algorithm.

It is important to note that routers in OSPF periodically send DBD packets, with a default period of 30s. If a link has not been updated after 60 minutes, the router will remove it from the LSDB.