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How routers determine where to forward packets

Dear friends

I know about how the routers distinguish between classful and classless address. But I could not find how the router calculates the path when forwarding packets.

I know fact like

  • Routers use longest match for address in their address table (but why, Why cant we use other one)Cla

  • Classful protocol dont include masks when sending updates

  • Classless protocol include mask when sending updates

  • and so on

Consider scenario like

LAN -> Router-A <------> Router-B -> LAN

(172.16.0.0/16) (192.168.1.0/30) (172.20.0.0/24)

Please specify how routers calculate/find route when forwarding packets between lans in Router-A's LAN interface and Router-B's LAN interface.

Assuming router is being configured with RIP.

What about when we configure the same with OSPF and please mention how router in both cases use routing table to forward those packets.

In short, What going inside routers brain when forwarding packets and how routers behave when there is RIP and when there is OSPF.

Frankly I know the rules but I could not understand fundamentals like

  • If the router has a directly connected interface belonging to the same major network, it will apply the same subnet mask as that interface.

  • If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route.

Sometimes even forwarding to default gateway fails when configured with classful behavior. Why?

Please help My total fundamentals is shaken with this wierd.

I WANT TO BE CONFIDENT IN HOW AND WHEN ROUTERS COULD AND COULD NOT FORWARD PACKETS.

PLEASE SOME HELP WITH SOME PERFECT EXPLANATIONS AND EXAMPLES WITH ALL CLASS OF NETWORK IN ONE EXAMPLE IF POSSIBLE.

Everyone's tags (1)
5 REPLIES
Green

Re: How routers determine where to forward packets

Vishal,

Have a look through this cisco pdf

Regards,
Alex.
Please rate useful posts.

Regards, Alex. Please rate useful posts.
Community Member

How routers determine where to forward packets

Hi Mr A Campbell

Thanks for good reference manual. But can you please provide me with some good example and probably with route summarization and classful protocols behaviour thereby.

Hope one example is lot better than reading 100 books or so..

Thanks

Hall of Fame Super Gold

Re: How routers determine where to forward packets

Hope one example is lot better than reading 100 books or so..

LOL!

Alex is not recommending you read 100 books.  He's recommending you to read just ONE file.

You might as well just tell Alex "Can you please summarize it in 200 words or less so my professor/instructor can give me an A+".

Community Member

How routers determine where to forward packets

Dear Leo Laohoo

I need one example which shows the classful and classless behavior of routing protocols.

1. But probably one with discontiguous network in between two same class of network (This is normal example of this type of case)

2. Another one with example containing same class network but with several different masks but two WANS connected must also be of same class of network.

Can someone please provide a solid example

Hall of Fame Super Gold

How routers determine where to forward packets

Vishal,

Take the time to read Alex's PDF file. 

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