UNDERSTANDING OSPF LSA DATA-STRUCTURE

Each OSPF protocol LSA has its own aging timer that its link state age field carries.

Default of 30 minutes expressed in seconds i.e. 1800 seconds is the Link State Age. What this means is that router that originated the LSA resends the LSA now with a higher sequence number in a Link State update to verify the link is still active.

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LSA also has a max age timer field with default value of 60 minutes. If a LSA isn’t before this period the Max age timer will expire and the LSA would be discarded in that case.

A link state update can contain more than one LSA.LSA validation method saves on bandwidth compared to distance vector method which sends the entire routing table at short periodic intervals.

Let us understand what a router does when it receives an LSU using the below flowchart:understanding-ospf-lsa-data-structure

 

  • If the LSA entry doesn’t exist the router adds the entry to LSDB, acknowledges the receipt of LSA with LSAck, floods the new information to other routers, runs SPF and updates its routing table.
  • If the entry already exists with the same sequence number the router must ignore the LSA.
  • If the LSA already exists but contains newer information i.e. it has higher sequence number the router adds the entry to LSDB, acknowledges the receipt of LSA with LSAck, floods the new information to other routers, runs SPF and updates its routing table.
  • If the entry already exists but the LSA includes older information it sends an LSU to sender with its newer information.
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