OSPF Master slave selection

[mahesh18]

Hi,

In OSPF  between two Routers  Point to Point connection one is masters and other is slave.

Need to confirm for MAster slave election does it decide by highest Router ID or which ever comes online first becomes the master?

Regards

MAhesh

[Masoud Pourshabanian]

Hello,

The criteria for the election, per adjacency, is the router-id of both nodes. The node with the highest router-id is the master. This is why OSPF needs a router-id generated prior to the OSPF application being turned on

"Once the master/slave relationship has been negotiated (the router with the highest Router-ID becomes the master), the neighboring routers transition into the exchange state"

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13684-12.html

Hope it helps,

Masoud

[mahesh18]

Hi Masoud,

If router with lowest router id  is powered up then it will become master right?

Regards

MAhesh

[Masoud Pourshabanian · Accepted solution]

Hello,

Master and slave are defined in a segment( can be said point to point or per neighbor basis). When a router goes down and comes back, all OSPF stages must be repeated again. Master will be chosen in exstart, so always based on the higher router-id master will be selected. It is diffrent from DR selection because DR is chosen for a group of routers. Even if a router with higher router-id comes, it can not replace the previous DR. 

Hope it helps

Masoud

[mahesh18]

Many thanks Masoud.

MAhesh

[paul driver]

Hello

NOTE;

if none of the criteria I specified above hasn't been applied  then election is by router I'd which also has its own election criteria

res

paul

[paul driver · Accepted solution]

Hello

OSPF DR/BDR election don’t take place on point-point networks - They are only valid on ospf network types:

Non-Broadcast
Broadcast

All routers in a DR/BDR election by default have an opsf priority of 1 thus by virtue of this the router with the highest RID will be preferred - (highest loopback or if non specified, then highest ip address of router)

Obviously when you have DR/BDR in your network you will want to specify manually what router will become the DR/BDR and you do this by manually applying an opsf priority to a given interface with highest ospf priority taking preffence ( ip ospf priority xx) --- So to clarify the opsf priority take precedence over the router id

Non DR/BDR routers can be made to sit out the election by giving them a opsf priority value of 0 which will make the Drothers

DR's are responsible for receiving/sending routing updates to every spoke in the broadcast/non broadcast network via two multicast address of 224.0.0.6 ( sent by spokes)
224.0.0.5 ( sent by DR)

Once a DR has been elected they cannot be substituted by another router with even an higher opsf priority - this will only happen by manually forcing the clearing of the opsf process ( clear ip ospf process), reload or interface disconnects

res
Paul

[mahesh18]

Many thanks Paul!

Regards

MAhesh