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OSPF DR and BDR Election

ospf

Participating routers in an OSPF network have varying roles to play in ensuring that the processes which route the data around the network maintain a true and accurate picture of the network topology. The two primary roles in this structure are the Designated Router (DR) and the Backup Designated Router (BDR). Their roles, whilst primarily aimed at maintaining network function from a routing perspective, are equally focused on ensuring that network bandwidth used to accomplish this is used judiciously.

An Election


Consider a multi-access environment (such as LAN or MAN), where three or more routers are connected together. If all the routers in the OSPF network had to form adjacencies with every other OSPF router present, forming a fully meshed OSPF adjacency network, the resultant conceptualised routing mesh would be overly chatty flooding Link State Advertisements (LSAs) with each and every other OSPF router. As a direct result, router CPU load and network bandwidth would be consumed wastefully. OSPF is designed to be far more efficient in its use of these resources, and, in order to prevent this from happening, OSPF holds an election process to determine and fill roles named Designated Router (DR) and a Backup Designated Router (BDR) so that the workload of propagating routing information around the network is more effectively managed. Election for the DR and BDR is determined primarily on the Router Priority (which by default is 1) and the Router ID. If the value of the router interface priority is changed to 0, it prevents that router from becoming the DR or the BDR.

Router priority can be adjusted on Cisco routers on a per interface basis. The Router ID however is a 32-bit number that uniquely identifies the router in the Autonomous System. One algorithm for Router ID assignment is to choose the largest or smallest IP address assigned to the router. If a router's OSPF Router ID is changed, the router's OSPF software should be restarted before the new Router ID takes effect. Before restarting in order to change its Router ID, the router should flush its self-originated LSAs from the routing domain or they will persist for up to MaxAge minutes. Cisco uses a method that some other vendors choose to follow, but it is not a requirement. If you have a loopback interface, since that's the most stable interface on your router, that will be used. If there is no loopback interface, the highest IP address on the router is used. If there is more than one loopback then the highest of them is used. In many elections in OSPF, the higher RID wins. this is the logic for choosing higher over lower. It should be noted however that you can manually specify an RID that isn't even a valid ip address such as 224.1.1.1.

Drothers


The DR is the router which receives LSAs and other updates when there is a change in the inter-neighbor communications. These LSAs are sent out by the DROTHERS routers (all non-DR/BDR routers), and consequently, any further updates are propagated by the DR to the rest of the DROTHERS routers. The show ip ospf neighbor command, when executed, indicates the non-DR/BDR routers as DROTHERS. Every network segment in OSPF has a DR and a BDR.

The Process


What actually happens is that whenever there is a change in network routing status, instead of flooding each and every path with LSAs advertising new information about network topology, the update is only sent to the DR. The DR then takes on this job and floods the routers in its network segment with the update. If the DR fails or is not functioning, the BDR takes over. When this happens, the BDR replaces the existing DR as the new Designated Router, and a new BDR is elected.

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