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OSPF

OSPF Virtual-links vs GRE tunnels

Everyone who works in networking knows that every area in the OSPF domain must be connected to the backbone area (Area0). The reason behind this constrain is explained here. However it may be difficult for some reason to physically connect an area to the backbone; in such cases you will …

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distribute-list gateway with OSPF

The following example is showing how to use distribute-list with the gateway option for inbound route filtering  in OSPF. The diagram below shows R1 directly connected to R2 with OSPF configured between them. Initial Configuration: !-- R1 OSPF configuration router ospf 1 router-id 1.1.1.1 log-adjacency-changes network 172.16.12.1 0.0.0.0 area 0 …

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OSPF DR/BDR Election Myth

OSPF DR/BDR election process is something that is full of many details, i hope that in the following thread i can cover its main aspects. Each multi-access segment (ex: Ethernet Segment), will have 1 DR and 1 BDR. Each router on the segment forms a Full adjacency with the DR/BDR. …

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OSPF RID Story

Understanding the need for OSPF RID and how to control it is indeed a very important aspect to take care of when dealing with OSPF. The RID is a dotted decimal value used by OSPF routers to identify the other OSPF routers. Beside being used in OSPF operations, and to …

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How OSPF selects best routes

It is known for all of us that when two routes are received from the same routing protocol, the route with the lowest metric will be selected and installed in the routing table. In OSPF this is only true when the two routes are of the same type. OSPF has …

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Passive-interface command behavior in RIP, EIGRP & OSPF

Passive-interface command is used in all routing protocols to disable sending updates out from a specific interface. However the command behavior varies from o­ne protocol to another. In RIP this command will disable sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP …

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