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The SPF algorithm places each router at the root of the tree and calculates the shortest path to each node utilizing the Dijkstra’s algorithm. You can also Statically assign the Router ID in the OSPF router configuration mode:ĭo NOT use same loopback address on different routers Overridden by Loopback interface if present The method of identification that OSPF uses is Router IDs (RID).ģ2 bits that uniquely identifies an OSPF router A priority of 0 causes a router to not be eligible to be a DR.Įach router that is participating in OSPF needs to be uniquely identified.
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Change the priority value of the Fa0/0 interface of R1 and R3 to 0.Configure a loopback interface on the R2 with an IP address higher than any IP address on the other routers.Configure the priority value of the Fa0/0 interface of the R2 router to a higher value than any other interface on the Ethernet network.There are three options that will ensure that R2 will be the DR for the LAN segment 172.16.1.0/24: DRs and BDRs listen to multicast traffic on both multicast address 224.0.0.5 and 224.0.0.6 224.0.0.6 is exclusively listed to by DRsįirst, what is the RID of each router? Which router is the default DR for the 172.16.1.0 LAN?.Existing DR will not be overthrown if “better” router is turned on after initial election.If DR fails, BDR promoted to DR and a new BDR is elected.RouterID breaks tie, Router ID is either the Highest Loopback or Highest Configured IP address on any given active interface.Router with highest priority wins BDR (1 – 255), if no other router has a higher priority the BDR will then become the DR.Priority of zero (0) means router can not be elected as a DR.All routers set by default to priority 1 (0-255).The following outlines the process OSPF takes and rules that are followed when electing a Designated Router: ONLY Broadcast and Non-Broadcast Network types have Designated and Backup Designated Routers.It is generally not a good idea to select the same router to be designated router on many LANs simultaneously.BDR assumes DR role if DR fails but performs no DR functions if DR remains available.The DR informs all other clients of changes in topology.Clients send updates to the DR and BDR only.All routers on a sub-net form adjacencies with the DR and BDR.Elected routers that coordinate the flow of route updates within a subnet.
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Only Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!! Usually discovered by hello’s but can also be configured administrativelyĪdjacency – Relationship formed between selected neighbors in which routing information is exchanged. Neighbor – Two routers that have an interface on a common network Routing Table – Generated when the SPF algorithm is run on the link-state database Routers in the same area have identical link state databases Link State/Topological Database – List of information about all other routers in the same area Neighborship Database – Listing of all the neighbors to which a router has established bi-directional communication Includes a router’s interface and its relationship to its neighboring routersĬost – Indication of the overhead required to send packets across an interfaceĪrea – A contiguous grouping of networks and routers sharing the same Area ID and topology databaseĭR (Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routersīDR (Backup Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routers that will promote it self if the Designated Router is no longer available Link-state – Status of a link between two routers