OSPF intra-area route calculation

jason_hu · Created Sep 3, 2021 00:52:08

Hi friend!

The OSPF route calculation process is as follows. The OSPF route calculation process in the same area can be described as follows:

each OSPF router generates Link State Advertisement (LSA) based on its surrounding network topology and sends the LSA to other OSPF routers through Update packets;
each OSPF router collects LSAs advertised by other routers. All LSAs form a Link State Database (LSDB). An LSA describes the network topology around a router, and an LSDB describes the network topology of the entire AS;
an OSPF router converts the LSDB into a weighted directed graph, which is a true reflection of the entire network topology. Each router gets the same directed graph;
according to the directed graph, each router uses the SPF algorithm to calculate a shortest-path tree with itself as the root. The tree provides routes to each node in the AS.

Hope this helps!

ariase88 · Created Sep 2, 2021 15:51:25

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BAZ · Created Sep 2, 2021 17:26:33

For each case, I used interface costs and states to illustrate OSPF selection rules in action.

Order of preference and criteria

Rules

1. Intra-area (O)

Lowest cost
Multipath

– Intra-area routes are always preferred over inter-area ones.

– Intra-area routing to a destination inside a non-backbone area will take the shortest path without traversing the backbone area.- Intra-area routing to a destination inside a backbone area will take the shortest path without traversing a non-backbone area.

– ABR’s advertise only intra-area routes from non-backbone area to the backbone area and advertise intra-area and inter-area routes from backbone area to a non-backbone area.
– ABRs do not take into account in SPF calculations LSAs received from non-backbone areas.

2. Inter-area (IA)

– Inter-area route between two non-backbone areas must pass through the backbone area.
– Inter-area route will take the path with the shortest total cost.

rfc2328

…Routing in the Autonomous System takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing is used) or different areas (inter-area routing is used). In intra-area routing, the packet is routed solely on information obtained within the area; no routing information obtained from outside the area can be used. This protects intra-area routing from the injection of bad routing information.…

user_3134129 · Created Sep 2, 2021 18:00:21

Dijkstra's algorithm

hamza11 · Created Sep 2, 2021 19:53:17

Hey there;
To calculate the best path to a route, first the router builds an LSDB that is then run against the SPF Algorithm, which then comes up with its best paths to subnets.

Ultimately what makes it into the IP Route Table is decided by CEF, which views all protocols different tables to decide the best path to a destination, but for the OSPF Route Table it uses the SPF Algorithm and its LSDB.

Cost is OSPF’s Metric, and it is calculated by the available Bandwidth on a link, which the higher the bandwidth speed the lower the cost will be – Lowest cost is always preferred.

Cost is determined by the “egress” interface or the interface the data will be taking outbound from a router.

The formula used to calculate the cost is: Cost = 100M/Interface bandwidth, 100 Mbit/s is the OSPF reference bandwidth.

The
cost of an OSPF route is the sum of the cost values of all incoming
interfaces from the source router to the destination router.

jason_hu · Created Sep 3, 2021 00:52:08

Hi friend!

The OSPF route calculation process is as follows. The OSPF route calculation process in the same area can be described as follows:

each OSPF router generates Link State Advertisement (LSA) based on its surrounding network topology and sends the LSA to other OSPF routers through Update packets;
each OSPF router collects LSAs advertised by other routers. All LSAs form a Link State Database (LSDB). An LSA describes the network topology around a router, and an LSDB describes the network topology of the entire AS;
an OSPF router converts the LSDB into a weighted directed graph, which is a true reflection of the entire network topology. Each router gets the same directed graph;
according to the directed graph, each router uses the SPF algorithm to calculate a shortest-path tree with itself as the root. The tree provides routes to each node in the AS.

Hope this helps!

chantha · Created Sep 3, 2021 04:06:31

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