Rebuild the topology based on OSPF LSDB - Series of OSPF Episode 5 Highlighted

Latest reply: Sep 25, 2019 10:26:17 953 18 2 4

In the previous four episodes, we introduced how the OSPF establish adjacency relationship and exchange its own LSDB with other routers, finally, every routers in the same area maintains the same LSDB. Routers can rebuild the topology and caculated the route path based on LSAs kept in the LSDB. 

First of all, let's talk about how the router rebuild the topology by the router-LSAs kept in the LSDB.

Topology as below:

topology

check LSDB on the router ar2 

lsdb_router


as the LSDB describe, two areas contained on router ar2.
check router-LSA in area 0 one by one


router-lsa-1-a0

picture above shows that a router-LSA with link-state ID 2.2.2.2, it is the RID of the router which generates this router-LSA. link count means how many links has connected to this router. As we talked about in episode 4, in router-LSA, if the type of the link is TransNet, link ID is the interface IP address of the DR, Data is the interface IP address which connects to this TransNet. In the picture above, a router with RID 2.2.2.2 connects three network segment, two TransNets and one Virtual. Link ID: 10.1.12.2, Data: 10.1.12.2, link type: TransNet, these three data points indicate that a router connects to transport network through an interface with IP address 10.1.12.2, in this network, the IP address of the interface which belongs to the DR is 10.1.12.2, as shown below.
PIC1

In this way, we can draw the rest of the router based on the router-LSAs as below.

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Combine the above topology results, we can get a topology rebuilt by the LSDB on the router shows as below.

rebuilt topology based on the LSDB on the router

when all the OSPF routers rebuild the topogoly based on their own LSDB, the topology of the entire OSPF network is completely reproduced.


In the next episode, we will introduce the SPF algorithm and how the OSPF generate the route path based on the LSDB 


 





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jomana
Created Dec 13, 2018 15:56:53 Helpful(0) Helpful(0)

Its hard
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Hawasli
Created Dec 15, 2018 07:26:31 Helpful(0) Helpful(0)

Its hard
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jomos
Created Dec 15, 2018 07:33:10 Helpful(0) Helpful(0)

Its hard
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jomos
Created Dec 15, 2018 15:25:09 Helpful(0) Helpful(0)

Thanks you
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Hawasli
Created Dec 15, 2018 20:56:36 Helpful(0) Helpful(0)

Its hard
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MR.HTC
Created Dec 16, 2018 20:12:19 Helpful(0) Helpful(0)

Its hard
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Finn92
Created Dec 18, 2018 07:02:47 Helpful(0) Helpful(0)

i have to say , author , it's a good sharing for OSPF LSDB , thank you , learn a lot knowledge from it .
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No.9527
Created Dec 18, 2018 07:28:56 Helpful(0) Helpful(0)

OSPF Cost
The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. There is more overhead (higher cost) and time delays involved in crossing a 56k serial line than crossing a 10M ethernet line. The formula used to calculate the cost is:

cost= 10000 0000/bandwith in bps

For example, it will cost 10 EXP8/10 EXP7 = 10 to cross a 10M Ethernet line and will cost 10 EXP8/1544000 = 64 to cross a T1 line.

By default, the cost of an interface is calculated based on the bandwidth; you can force the cost of an interface with the ip ospf cost <value> interface subconfiguration mode command.

Shortest Path Tree
Assume we have the following network diagram with the indicated interface costs. In order to build the shortest path tree for RTA, we would have to make RTA the root of the tree and calculate the smallest cost for each destination.
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yogijain
MVE Created Sep 25, 2019 10:26:17 Helpful(0) Helpful(0)

very helpful
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