what's the difference between ospf lsa "copy" and "instance"

[sunrise1916]

Hi Guys,

I've been reading rfc 2328 now  and getting a very confused point that what's the difference between ospf lsa "copy" and "instance"?

I know it's very obscure ,is anyone konw about this?

Regards,

sunrise

[Rolf Fischer · Accepted solution]

Hi,

the term "instance" is normally used when more than one "version" of the same LSA exists and the algorithm needs to determine which one is more up-to-date (or ignore the last received if it is not newer than the local copy).

I think "copy" is a term taken from link-state protocol principals: Each router describes its own local piece of topology information in LSA(s) and, by means of database synchronization, other routers store copies of  those LSAs and use their content for path-calculations. (Only the originating router is allowed to change a LSA, it generates a new instance then.)

HTH
Rolf

[sunrise1916]

Hi,

so,if I could say that the most difference between instance and copy is not the meaning but the title used in different  scenes?

by the way,waht's the title of the LSA Header putted in DBD packets during Exchange state?

Regards,

sunrise

[Rolf Fischer]

so,if I could say that the most difference between instance and copy is not the meaning but the title used in different  Scenes?

Well, yes; I'd say which term is more adequate depends on the particular context.

what's the title of the LSA Header putted in DBD packets during Exchange state?

Not sure what you mean by "title of the LSA header". Perhaps you'll find the answer in appendix A.3.3 "The Database Description packet" (page 194) and A.3.1 "The OSPF packet header" (page 198).

Regards,
Rolf

[sunrise1916]

Hi ,

yeah,I just mean when the LSA Header putted in DBD packets then we should call it instance or copy?

Regards,

sunrise

[Rolf Fischer]

Ah, I see. Routers use DBDs to inform their adjacent neighbors about the current instances of their self-originated LSAs; DBDs do not contain the complete LSAs but merely compact descriptions of their content. The receiving neighbors compare their decisive fields with those of [the instances of] their respective local copies. If an LSA instance in the received DBD is more recent than [the instance of] the local copy, an update for this LSA will be requested. Otherwise the DBD is ignored because this part of the LSDB is still up-to-date. The most important field to compare two instances of a LSA is the LS sequence number.

Hope this make sense.

Regards,
Rolf