OSPF LSU, LSA, and LSR

[peterjames]

Good day!

In the neighborship process of OSPF, I've known in my studies that LSU's are sent in response to LSR during Loading state. LSA's are sent inside an LSU packet. My question is, are LSU's only sent or being generated in response to LSR's?

How about when there's a change in cost or maybe a link went down? It will send an LSA for that, right? But LSA is not a packet but only a piece of information inside an LSU packet. Can LSU independently be sent without LSR?

[balaji.bandi]

Nice there is a good deep dive to understand :

https://networkwalk.blog/2019/10/09/deep-dive-into-ospf-and-troubleshooting/

[peterjames]

Hi Balaji. Thanks for the link. It was very detailed, but I still did not see the answer to my question. I want to know if LSU can be sent independently without LSR.

Taken from the link that you have given:

"The Link State Update packet is used in the flooding of LSAs and to send LSAs in response to Link State Requests"

Does "flooding of LSA'' pertains to when a link went down and has to inform adjacent neighbors/DR?

[Giuseppe Larosa · Accepted solution]

Hello Peter,

yes the flooding of an LSA is the process of sending it to all routers in the same OSPF area and as you have noted an LSA is a data structure not a packet and it is sent inside a LSU packet that is not in this case sent as an answer to a LSR request packet.

So the answer to your question is yes LSU can be sent "unsolicited" not as an answer to an LSR.

As noted by others each LSA needs to be refreshed even if no topology change happened. Max age is 3600 seconds and Cisco routers refresh info after 30 minutes.

This does not create a lot of traffic. The new version of the LSA will have a newer sequence number (higher).

Hope to help

Giuseppe

Giuseppe

[MHM Cisco World · Accepted solution]

If link down then the router that origin this LSA flood LSA to all neighbor without waiting neighbor LSR.
LSR and LSU is use when :-
1- the first time the two neighbor OSPF establish 
2- the time that LSA aging in one neighbor then it send LSR for this LSA and the router origin this LSA respond with LSU

that what i understand about the LSR/LSU/LSA