OSPF LSA  Updates and Hellos

jskfan
jskfan used Ask the Experts™
on
OSPF LSA  Updates and Hellos

on the Screenshot below , we have DR and BDR

I want to know if other routers will have to send an update(LSA type 1) to DR and BDR, or they will send an update (LSA type 1) just to the DR, and the DR will send (LSA type2) to all the routers including the BDR

OR

Other Routers  will  send an update(LSA type 1) to 224.0.0.6 then DR and BDR will get the Update but only the DR that will send (LSA type 2) to 224.0.0.5 (where all other routers are listening to)

in case of the second case , I wonder how does BDR get LSA type 2 from DR

Thank you

DR
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Distinguished Expert 2018

Commented:
In multi access network topology change is sent to all DR address 224.0.0.6. DR then informs all OSPF routers about topology change (224.0.0.5).

To simplify function of DR and BDR. DR is pseudo node - it represents whole network segment (that's why it is advertising LSA type 2). BDR is generally irrelevant until DR is lost (BDR it is not important to understand OSPF network functioning).


R7(config-router)#do sh ip ospf neigh

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:39    1.1.1.1         FastEthernet0/0
1.1.1.2           1   FULL/BDR        00:00:39    1.1.1.2         FastEthernet0/0

To understand packet capture:
1.1.1.1 was running and promote itself as DR already when 1.1.1.2 and 1.1.1.3 are starting ospf protocol. (In first captured hello packet you can see 1.1.1.1 is advertising itself as DR). You can see 1.1.1.2 and 1.1.1.3 initial hellos (DR and BDR are set to 0.0.0.0). Topology change after initial updates starts with packet number 64.

64      149.758423      1.1.1.3      224.0.0.6      OSPF      110      LS Update   <= update to all DR (advertising loopback network)

65      149.768472      1.1.1.1      224.0.0.5      OSPF      110      LS Update   <= DR informs all OSPF routers regarding topology change

68      152.286933      1.1.1.2      224.0.0.5      OSPF      78      LS Acknowledge <= BDR ACK
OSPF-DR-BDR-election---update_110_2.zip

Author

Commented:
In multi access network topology change is sent to all DR address 224.0.0.6. DR then informs all OSPF routers about topology change (224.0.0.5).

who listens to 224.0.0.6 ? Both DR and BDR or only DR ?

when DR sends LSA type 2 to 224.0.0.5 all other Routers will get the Update.. Now if BDR does not listen to 224.0.0.5 then how does it synchronize with DR.
Distinguished Expert 2018

Commented:
224.0.0.6 are all DR (means DR and BDR)

224.0.0.5 are all OSPF routers listen  (DR, BDR and DRother). Whatever is sent to 224.0.0.5 will receive all OSFP routers.
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Author

Commented:
OK..

DR/BDR Listen on 224.0.0.6 and 224.0.0.5
DROTHER  listen on 224.0.0.5  only

Author

Commented:
to make  it clear:

OSPF Routers , except DR/BDR, will send LSA type 1  to 224.0.0.5
DR/BDR will get the Updates (LSA type 1),  DR will send LSA Type 2 to 224.0.0.5 , all OSPF routers including BDR will get the LSA type 2 from the DR.

Correct ?
Distinguished Expert 2018
Commented:
I don't think it is correct. I need to verify it, but should be stated like:

OSPF Routers , except DR , will send LSA type 1  to 224.0.0.6
Generally, BDR can be ignored as long as DR is present (it will receive updates, but only DR can propagate received topology change.

But again, need to verify it.

Other part is correct.

Author

Commented:
I guess I should have said:
OSPF Routers , except DR/BDR, will send LSA type 1  to 224.0.0.6

DR/BDR will get the Updates (LSA type 1),  DR will send LSA Type 2 to 224.0.0.5 , all OSPF routers including BDR will get the LSA type 2 from the DR.

Author

Commented:
Thanks

Author

Commented:
Thanks
Distinguished Expert 2018

Commented:
You're welcome.

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