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  • OSPF Link State Advertisements (LSAs)

    When OSPF routers become neighbours, they synchronise their Link State Databases between them. If a OSPF router adds or removes a directly connected route from its database, it floods a Link State Advertisment (LSA) out of the OSPF enabled interfaces. There are six types of LSAs that are used in an OSPF network: Type LSA…

  • OSPF Route Types

    Intra-Area Routes A network route learned from another router within the same area is known as a intra-area route. In show ip route ospf , any routes learned this way are displayed with the key symbol O These routes are advertised using a Type 1 LSA, and are preferred over Type 3 LSAs. If multiple…

  • OSPF Areas

    An OSPF area is a logical grouping of OSPF enabled interfaces across multiple routers. Area IDs The area is identified by a number or dotted decimal that is assigned at an interface level. The interface can only belong to one area. The area number is included in OSPF Hello packets that are sent by the…

  • OSPF Network Type: Loopback Networks

    The OSPF network type loopback is enabled by default for loopback interfaces, it can also only be used on loopback interfaces. The network type of loopback will always advertise the host address of the loopback into the network, and not the network itself, even if not configured with a 255.255.255.255 subnet. Changing the loopback to…

  • OSPF Network Type: Point to Point

    The network circuit that only has two devices on it that can communicate is considered a point to point circuit. Point to point circuits do not require the use of ARP or broadcast traffic. OSPF sets the network type of point to point by default for serial, GRE, and point-to-point frame relay interfaces. The Hello…

  • OSPF Network Type: Broadcast

    Ethernet is a broadcast network type for OSPF by default. Broadcast networks can be multi-access with the possibility of there being more than two OSPF enabled routers on the subnet, so the election of a DR/BDR is required. The Hello timer on a broadcast network is 10 seconds, and defined in RFC 2328

  • OSPF Network Types

    The default network type in an OSPF network is set by the physical media used. It can be overridden in interface configuration mode with the command ip ospf network Type Description DR/BDR field used Timers Broadcast Default in Ethernet networks Yes Hello: 10Wait: 40Dead: 40 Non-broadcast Used in Frame Relay main or multipoint subinterfaces Yes…

  • OSPF Failure Detection

    The OSPF hello packet has a secondary function: it can detect when a neighbour is unreachable or in a poor state. OSPF has a second timer called the dead interval timer, which by default is 4 times the length of the hello timer. When a hello packet is received, the dead timer resets. If a…

  • OSPF Link Costs

    Interface costs are part of the the shortest path first calculation that OSPF performs – the metric that helps OSPF decide what route to take is based on how much it will cost to reach the destination. OSPF can assign a dynamic interface cost based on a formula with a reference bandwidth of 100Mbps. Cost…

  • Advertising default route via OSPF

    OSPF can advertise a default router on a router into the rest of the OSPF domain. The command default-information originate in the router configuration mode will advertise the default route. The command on its own will only advertise a default route if the local router has a default route of its own in the routing…