In OSPF, what is the difference between LSA and LSU ?

[news2010a]

I have been reading OSPF documentation and so far I couldn't distinguish difference between LSA(Link State Advertisement) and LSU(Link State Update) ?

Are LSA and LSU terms used interchangeably, and perhaps mean the same thing ?

[gpulos]

in a nutshell: (with link for full explanation from the RFC which defines OSPF v2)

an LSA is a linkStateAdvertisement.

each router in an AS generates one or more LSAs.

the collection of LSAs from within an OSPF AS forms the link state database.

there are 5 different LSA types, each with its own function.

an LSU is a linkStateUpdate.

LSUs implement the flooding of LSAs. an LSU is a collection of LSAs one hop further from their origin.

responsible for providing updated LSAs to the rest of the OSPF AS.

for the best definitions, as well as a wealth of other info, please see the following link for RFC 2328 - OSPF v2:

http://www.faqs.org/rfcs/rfc2328.html

[Richard Burts]

I think that Greg has provided a good answer and it is frequently good to go to the RFCs for information like this, so it is good that he provided a link to the RFC.

I would offer a slightly more simple explanation: an LSA is the advertisement that contains information about the state of a particular link in the OSPF network. An LSU is an IP packet whose payload is one or more LSAs.

HTH

Rick