This post assumes that you have a basic understanding of OSPF… if not, I suggest jumping over http://en.wikipedia.org/wiki/OSPF for a first quick read. However for the sake of this post, I will go over some basic reminders.
The “hello” packet
The OSPF routers sends a periodic packet referred to as the hello packet ‘multicast 126.96.36.199’ which is composed of the OSPF header + different fields ID necessary for routers to neighbor and become adjacent. The hello packet is by default sent at a 10 seconds interval on a multi-access network and each 30 second on a point to point network.
The HELLO PACKET (roughly 50 bytes) looks as following
[ OSPF HEADER ] | Network Mask | Hello Interval | Options | Router Priority | Router Dead Interval | DR | BDR | Neighbor
The OSPF HEADER (20 bytes) looks as following
Version number | Type | Packet Length | Router ID | Area ID | Checksum
The neighboring and adjacent process
Like I explained earlier, OSPF uses the hello packet not only to discover another peer router, but also to neighbor with this router. For 2 OSPF routers to neighbor, they must belong to the same AREA (Area ID), use the same Authentication schema, have the same hello and dead intervals. Past the agreement phase, the routers becomes “neighbors”.
Only when they are neighbors, OSPF routers will start exchanging their database… this process is referred as Adjacency.
Now let’s imagine a singular segment on which we have 10 OSPF routers… in theory, each router would peer with each other and start exchanging their database with each others. The number of adjacency is then calculated as followed
(n (n - 1) ) / 2
So 10 routers, will give us 45 Adjacency
To minimize the amount of information shared, OSPF will elect a Designated Router (DR) and a Backup Designated Router (BDR). Once the DR and BDR are elected, every other OSPF router will start exchanging database only with the DR and BDR and no longer with each other.
Now keep in mind, as we said earlier OSPF routers use multicast IP 188.8.131.52 to send their hello packets but also exchange their databases… in presence of a DR/BDR, the other routers will send their updates on multicast 184.108.40.206, which in return the DR/BDR will resend on multicast 220.127.116.11
So how does the DR and BDR election takes place?
It is quite simple, if you are used to the switch root bridge election, this will not look much different. The BDR and DR takes place through the HELLO PACKET by comparing the Priority ID (which if you recall is located in the hello packet as shown earlier).
The router with the highest Priority ID is elected the Designated Router (DR), the next router with second highest Priority ID will become the BDR. Now keep in mind, by default all router interfaces have a priority ID of 1… if on a particular segment, all the Priority ID of all routers match, the Router ID (OSPF header) will then be the next ID to compare in order to elect the DR/BDR. Again in the same mind set, the OSPF router with the highest Router ID will be elected the DR or BDR.
Keep in mind that once the DR/BDR are elected, if a new OSPF router is added with the highest priority of all, the DR/BDR will not change… to start the election process, you will have to clear up the OSPF process
Once the DR and BDR are elected, the BDR will only listen to the exchange between the peers and the DR and elects itself as the DR if the current DR was to fail.
As a last thing to remember, without DR/BDR, we calculated 45 Adjacency for 10 routers on a multi-access segment. Now how many adjacency do we have with a DR and BDR? Simple!
2*n - 1 –> 2x10 - 1 = 19 Adjacency … so from 45 Adjacency, we dropped down to 19 Adjacency with a DR and a BDR.
If you were to only elect a DR without BDR, then you would naturally obtain 9 Adjacency.
To keep in mind
If you do not want a router to participate in the DR/BDR election, sets its Priority ID to 0, it will then be shown as DROTHER.
You can override the RID of the OSPF router by creating a loopback interface with a different IP than the one used on the router’s interface
The BDR and DR election only take place on broadcast and non-broadcast multi-access… That is to say routers on serial WAN would not have a BDR/DR election