If all routers in a broadcast network (such as ethernet), required a neighbour relationship, the number of neighbour relationships would quickly grow out of proportion. For example, in a network of only 8 routers, 28 neighbour relationships could potentially be formed (the formula to calculate this is n(n-1)/2 in case you are wondering). Clearly this does not scale well.
Instead a more optimal way is to designate representative routers, which all other routers become neighbours with. These routers are labelled the Designated Router (DR) and the Backup Designated Router (BDR). The remaining routers are assigned (DROther) – i.e. not the DR or the BDR.
If the DR or BDR needs to send updates to the other routers they simply multicast this information to the 224.0.0.5 address – known as the “All OSPF Routers” group.
Similarly if any non DR router (DROther) needs to send updates to the DRs they multicast this information to the 224.0.0.6 address – known as the “All OSPF DR Routers” group. The DR then floods this update to the other routers using the 224.0.0.5 multicast address. As well as informing the other routers, this serves as an acknowledgement to the originating router.
Consider the following simple OSPF network:
Let’s say Router 1 is elected the DR and Router 2 is elected the BDR.
If R3 were to learn a new route, it would multicast a LSU to the DR (R1) on multicast address 224.0.06 (all DRs), which would then flood the same update to 224.0.0.5 (All Routers).
To illustrate this R3 is configured with a new loopback of 33.33.33.33/32
R3(config-if)#ip address 33.33.33.33 255.255.255.255 R3(config-if)#ip ospf 1 area 0
A Wireshark capture is shown below:
- R3 (DROther) sends a LSU update to the DR multicast group 224.0.0.6 containing the new information (in the form of a type 1 LSA).
- R1 (DR) then responds by flooding this to the other routers on multicast group 224.0.0.5
- R4 (DROther) Acknowledges the LSU on 224.0.0.5
- R2 (BDR) Acknowledges the LSU on 224.0.0.6
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