ospf topology and process

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Sep 11th, 2008
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Hi all, just a quick question on ospf,When people say ospf routers know about the whole topology ofthe network, what does this mean?

I would of thought that they would only know about the neighbouring routers?

also am i right in saying ospf does not have an autonomous system number like eigrp? if this is the case how would you join 2 totally seperate ospf domains, i.e 2 companies with there own backbone areas.

can anyone clarify this

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satish_zanjurne Thu, 09/11/2008 - 07:43
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OSPF exchange the database , in that they exchange link state information with neighbors.

Database is not the routing table..it is information of each link in the domain.







***Here assume R5 is connected to R2

R3 : will have information of links from R4

R2 : will have link information from R5,R3(R4)

R5 : will have link information from R2(R3(R4))

R1 : will have information from R2(R3(R5(R4)))

Although they are not directly connected, they will have link information all links in the domain.

To connect 2 seprate OSPF domain, use eBGP & redistribution..

coolest option!!!

HTH..rate if helpful...

carl_townshend Thu, 09/11/2008 - 08:22
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can you possibly give me a show output of what this looks like on a router?

Doesnt every router know about all other routers in the domain then, or only directly connected , im a little confused ?

Giuseppe Larosa Thu, 09/11/2008 - 08:44
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Hello Carl,

to make an example let's consider the following:

a router R5 is connected via a serial interface to R4 that is connected with another serial interface to R3.

R3 in its lan interface exchange ospf hellos with other 6 routers.

the router R5 can get a complete picture of all links and of the situation on the remote lan segment:

R5 will receive OSPF data structures originated in all routers:

LSA type 1 router LSA describes links, link state, link type

the OSPF DR for the lan segment will generate a type 2 LSA network LSA that will list all the routers that are in the segment with their router-ids.

Very important: all this is true only within a single OSPF area.

When using multiple ospf areas the info about other areas is less detailed and is limited to prefix information like in distance vector protocols.

in a single area all routers have the link state DB in sync and make their own calculations of SPF using themselves as the root of the tree: the calculation is done one for each IP prefix and is loop free because the info is detailed and first hand : no router is allowed to modify the LSA generated by another router.

The LSAs are passed unchanged out all the links in the same area (flooding)

So a router receives the LSA also of unconnected routers this is the difference.

the OSPF router-id identifies the single nodes that have at least one link in the area and must be unique.

Hope to help


vishal_bhugra Thu, 09/11/2008 - 08:28
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Hi Carl

For explaining the answer let me start with distance vector protocols.

Distance vector protocols exchange routing tables. Every router keeps the copy of routing table and modify it before forwarding to others (like link cost.etc). Which means receiving router will have the knowledge of network as given by previous router. that is known as routing by rumor (it believes that if 1st router can reach network, then obviously me too).

But In case of Link state protocol (like ospf) router exchange LSAs. Every router sends the information of its directly connected neighbours. Every router keeps the copy of received LSA and forward it without altering it. Which means every router has a database of same LSA (i.e. same information about the network). Now they will form a tree keeping them selves as root and find the shortest path for every destination.(unlike distance vector, where every router add its metric before forwarding to other).

For joining two backbones areas you can use anything, IGP,BGP,static or redistribution.If you want to filter some network you can use filter-list



carl_townshend Thu, 09/11/2008 - 08:59
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hi, thanks for that,

could you possibly give me an output of a router to show me what this looks like?

also, can you not redistribute one ospf domain into another, or would you have to use another protocol between them ?


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