Not sure, as never have

zafar_118 · ‎03-18-2016

Hello,

I have couple of questions regarding OSPF LSA flooding process.

I have four routers connected on a broadcast link (R1,R2,R3,R4). R1 and R2 are DR and BDR respectively and R3, R4 are nonDR/BDR routers.

1. If there is a change in a network connected to R3, it will send LSUpdate to allDRouter address (224.0.0.6). When DR receives it, it is supposed to flood the change (LSUpdate) to allSPF router mcast address (224.0.0.5). Does BDR also flood it? If not, any condition in which BDR will also send

2. Given that a router doesnt "own" a LSA, when LSA expires (maxAge of 3600sec reaches), will it send LSRequest or silently drop it from its database?

3. R3 has a LSA (which it doesn't own) with sequence number B. R1, which is the DR, sends a LSUpdate for same LSA with sequence number A such that A<B.When R3 receives that LSA, it will ignore it and send back its own LSA (with sequence number A). What destination IP will R3 use? The unicast ip address of R1 or allDRouter address (224.0.0.6)?

4. R3 has a LSA (which it doesn't own) with sequence number B. R1, which is the DR, sends a LSUpdate for same LSA with sequence number A such that A<B and when R3 received this LSA, LSA in its database is expiring (maxAge reached 3600) . At this point, will R3 send back LSUpdate?

Thanks

Vasilii Mikhailovskii · ‎03-19-2016

Hello.

Assuming the questions are about v2 (rfc 2328):

A1: BDR usually does not flood anything back, but may need to take over if DR fails during the process.

A2: no extra communication - silently purged.

A3: not sure here, if peers are in FULL state, per 13.0(8), goes unicast

    (8) Else, the database copy is more recent.  If the database copy
        has LS age equal to MaxAge and LS sequence number equal to
        MaxSequenceNumber, simply discard the received LSA without
        acknowledging it. (In this case, the LSA's LS sequence number is
        wrapping, and the MaxSequenceNumber LSA must be completely
        flushed before any new LSA instance can be introduced).
        Otherwise, as long as the database copy has not been sent in a
        Link State Update within the last MinLSArrival seconds, send the
        database copy back to the sending neighbor, encapsulated within
        a Link State Update Packet. The Link State Update Packet should
        be sent directly to the neighbor. In so doing, do not put the
        database copy of the LSA on the neighbor's link state
        retransmission list, and do not acknowledge the received (less
        recent) LSA instance.

A4: if router has LSA "B" in any retransmition list, it won't be purged until successfully received/acknowledged by R1.

zafar_118 · ‎03-19-2016

Hi Vasili,

thanks for the responses.

"A3: not sure here, if peers are in FULL state, per 13.0(8), goes unicast",==> yes they are in FULL state .

Regarding A4, what if "LSA B" is not in retransmission list i.e. at exactly maxage 3600, R3 receives LSA from A, will it send unicast to R1 with LSA age set to 3600?

For the retransmission, how long will R3 keep retransmitting? is there any timer?

And when R1 is acknowledging, will it send to R3 unicast address or allSPF mcast address?

thanks

Vasilii Mikhailovskii · ‎03-19-2016

Not sure, as never have thought of such details.

If you dig this deep, I would suggest to read RFC.