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OSPF area concept

In small networks, there not too much routers, there are not too much updates, the LSDB is small so the Djisktra calculation is quick and it doesn’t consume much resources. However in huge networks we can face precise opposite situation: Frequent SPF algorithm calculations – in large networks, changes are inevitable. So the OSPF consume  Full Article…

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Structure of the OSPF packet

OSPF packet is solely encapsulated by IP and data-link headers. OSPF does not define a way to fragment its protocol headers, IP fragmentation is used when large DBD needs to be send over medium. OSPF can use multicast. OSPF is IP protocol number 89. Two distinct IP addresses are used: ALLSPFRouters – 224.0.0.5, All OSPF routers  Full Article…

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LSA types

Into any given OSPF area, a router will originate several LSAs. Each router originates a router-LSA. If the router is also the Designated Router for any of the areas networks, it will originate network-LSAs for those networks. Area border routers originate a single summary-LSA for each known inter-area destination. AS boundary routers originate a single AS-external-LSA for each  Full Article…

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OSPF over NBMA an overview

OSPF over NBMA is weird. You will see 😉 We must know some foundation concept to know how this weirdness works. So lets hop into it OSPF supports many network types like: Broadcast, multi access networks Ethernet, Token ring single operation mode DR/BDR election, 10 sec hellos Point-to-Point networks T1, ISDN BRI/PRI single operation mode  Full Article…

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OSPF Metric Calculation

OSPF use cost as a metric. On Cisco routers cost is nothing more than calculated cumulative bandwidth on the path from source to destination. However we are just using bandwidth, there is a formula for calculating the cost. The lower the metric the better the path. More bandwidth means better right? Thats why the formula  Full Article…

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Router ID & Establishing OSPF neighbor adjacency

Router ID uniquely identifies each OSPF router in the network, preciselly said OSPF database uses router ID to uniquely identify each router. By default OSPF uses highest IP address of physical interface as router ID. If Loopback is present it chooses the highest loopback interface instead. If you configure router-id by command under OSPF mode  Full Article…

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passive-interface in OSPF

Passive-interface command can be used with all routing protocols except BGP. In general, it prevents from sending the routing updates but it behaves differently in every routing protocol. In OSPF the specified interface appears as a STUB network in OSPF domain and OSPF information is neigher sent nor received through the specified interface. When you  Full Article…

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Propagating default route and route summarization in OSPF

Default route To redistribute default route int ospf you have two choices: Use command default-information originate Use command default-information originate always Difference between them is that in case you put „alway“ after the command you will advertise default route also in case you dont have any configured. The metric for default network is 1 and  Full Article…

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Virtual links

OSPF two layer hierarchy requires that every non backbone area must connect to backbone area to be able to communicate with other areas. Virtual link is a link that allows discontiguous areas 0 to connect together. Or to to connect non backbone area to backbone area via transit area. The OSPF virtual link feature should  Full Article…

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