OSPF in Shared Nework
OSPF in Shared Nework
If I understand OSPF in shared network uses the concept of DR and BDR
Let's say we have 7 Routers connected to L2 Switch
case1 :
the first time 7 routers boot up at the same time, what are the steps they go through before they elect DR and BDR
Case 2:
2 More new routers have been brought to the network what is the process of exchanging information with the existing routers should they go through, assuming DR and BDR have been already elected in case 1
Thank you
If I understand OSPF in shared network uses the concept of DR and BDR
Let's say we have 7 Routers connected to L2 Switch
case1 :
the first time 7 routers boot up at the same time, what are the steps they go through before they elect DR and BDR
Case 2:
2 More new routers have been brought to the network what is the process of exchanging information with the existing routers should they go through, assuming DR and BDR have been already elected in case 1
Thank you
ASKER
** The first time they boot up, are they going to send Hellos to 224.0.0.5 ,even when DR has not been determined yet ?
OR
each router has to send Hellos to all other routers and from there, the DR will be elected ?
You see where I am going ? before the DR is elected , if we have 5 Routers , then the Hellos will go 5(5-1)/2 , which is 10 Hellos
** now after the DR and BDR have been elected , will each router send updates to the DR and BDR or they send updates to the DR and the DR will update the BDR ?
*** if we have multiple shared Segments in one OSPF area , can we have multiples DRs and BDRs ?
Thank you
OR
each router has to send Hellos to all other routers and from there, the DR will be elected ?
You see where I am going ? before the DR is elected , if we have 5 Routers , then the Hellos will go 5(5-1)/2 , which is 10 Hellos
** now after the DR and BDR have been elected , will each router send updates to the DR and BDR or they send updates to the DR and the DR will update the BDR ?
*** if we have multiple shared Segments in one OSPF area , can we have multiples DRs and BDRs ?
Thank you
IF There is NO DR and there is a BDR the BDR will be promoted to DR and a BDR will be elected.
IF there is NO BDR a BDR will be elected.....
If there are no DR & BDR obviously both need to be elected.
If BOTH are available then all others will only mention the are there..., and an election will take place when either the BDR gets lost or gets promoted (kind of lost).
Maybe this helps explaining aspects of OSPF: https://www.juniper.net/documentation/en_US/junos/topics/concept/ospf-routing-understanding-ospf-areas-overview.html
(the whoe pupose of having a DR is to Not need broadcasts...., without DR broadcasts are needed. ... for anything.
IF there is NO BDR a BDR will be elected.....
If there are no DR & BDR obviously both need to be elected.
If BOTH are available then all others will only mention the are there..., and an election will take place when either the BDR gets lost or gets promoted (kind of lost).
Maybe this helps explaining aspects of OSPF: https://www.juniper.net/documentation/en_US/junos/topics/concept/ospf-routing-understanding-ospf-areas-overview.html
(the whoe pupose of having a DR is to Not need broadcasts...., without DR broadcasts are needed. ... for anything.
OSPF election process:
- when OSPF interface is up send hello packet (hello packets are send to multicast address 224.0.0.5 - all routers)
- on just powered device (or OSPF interface that just came up) DR and BDR address are configured to 0.0.0.0
- if DR/BDR are not found in other router hello packets wait for WAIT timer to expire (wait timer = dead timer) and elect DR/BDR
- criteria to elect DR/BDR - highest priority
- highest routerID
- interface with priority 0 can't be elected as DR/BDR (
Second case when DR and BDR are elected in broadcast network segment.
- on just powered device (or OSPF interface that just came up) DR and BDR address are configured to 0.0.0.0
- device will receive DR/DBR from other OSPF routers and configure itself in that network segment accordingly (OSPF has no concept of preemption)
OSPF hello packet always has destination address of 224.0.0.5
DR/BDR are elected per broadcast network segment (area must match - otherwise neighbor relationship will not be created), but ignore area to simplify concept ... per segment
- when OSPF interface is up send hello packet (hello packets are send to multicast address 224.0.0.5 - all routers)
- on just powered device (or OSPF interface that just came up) DR and BDR address are configured to 0.0.0.0
- if DR/BDR are not found in other router hello packets wait for WAIT timer to expire (wait timer = dead timer) and elect DR/BDR
- criteria to elect DR/BDR - highest priority
- highest routerID
- interface with priority 0 can't be elected as DR/BDR (
Second case when DR and BDR are elected in broadcast network segment.
- on just powered device (or OSPF interface that just came up) DR and BDR address are configured to 0.0.0.0
- device will receive DR/DBR from other OSPF routers and configure itself in that network segment accordingly (OSPF has no concept of preemption)
OSPF hello packet always has destination address of 224.0.0.5
if we have 5 Routersrouters need to wait for WAIT time to expire if DR/BDR are not received in any hello packet - 40 seconds by default Hello 10, Dead 40, Wait 40, Retransmit 5 wait time need to expire before DR/BDR election - so it is not 10 packets it is 4 x 5 = up to 20 packets can be exchanged - each router will receive up to 16 hello messages before DR/BDR are elected (all router will never come up at the same time).
DR/BDR are elected per broadcast network segment (area must match - otherwise neighbor relationship will not be created), but ignore area to simplify concept ... per segment
ASKER
JustInCase
*** So regardless if DR/BDR already exist or does not exist ,
OSPF routers when they come up they will send Hello to 224.0.0.5
Correct ?
***Regarding BDR , it gets the Updates from DR only , or from other OSPF routers ?
I thought when there is an update on OSPF router it will send it to the DR only, and the DR will send it to all OSPF Routers in the shared Segment including the BDR.
*** Mentioning the Shared Segment, if we have more than one Shared Segment in an OSPF area then each shared segment will have its own DR and BDR.. Correct ?
Thank you
*** So regardless if DR/BDR already exist or does not exist ,
OSPF routers when they come up they will send Hello to 224.0.0.5
Correct ?
***Regarding BDR , it gets the Updates from DR only , or from other OSPF routers ?
I thought when there is an update on OSPF router it will send it to the DR only, and the DR will send it to all OSPF Routers in the shared Segment including the BDR.
*** Mentioning the Shared Segment, if we have more than one Shared Segment in an OSPF area then each shared segment will have its own DR and BDR.. Correct ?
Thank you
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ASKER
Thank you Guys
http://notthenetwork.me/blog/2014/01/06/ccnp-route-study-ospf-drbdr-elections-and-how-to-rig-them/