host ,router on a lan and packet processing

Answered Question
Aug 4th, 2009

Hi every body.

Please consider the following scenario.

h1 host1,

h2 host2

R router

All are in same lan with h1 and h2 are using R 'f0 as their default gateway.

Let say h1 sends a packet outside the subnet it is on.

According to my book, following will occur:

1) router receives the frame and it then check the frame for error by checking Fcs

2) if the destination mac is the broadcast address or mac of its f0 , it will process the frame.

Now let say h1 sends frame to h2. Based on the above book logic, then following will occur:

router recives the frame and checks for error by checking fcs.

router then find destination address is not broadcast or its f0 int so it discards the frame.

Here the router is wasting cpu cyles for checking error in the frame first before even determining if the frame is destined for the router.

would it not be better, if router first determines if the frame is destined for router and if the frame is indeed destined for the router, then check the frame for error.

thanks and havea nice day

I have this problem too.
0 votes
Correct Answer by Istvan_Rabai about 7 years 4 months ago

Hi Sarah,

Frame processing is done by the interface processor itself, not by the router's central processor.

So it will not put any burden on the central processor.

Cheers:

Istvan

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Correct Answer
Istvan_Rabai Tue, 08/04/2009 - 08:26

Hi Sarah,

Frame processing is done by the interface processor itself, not by the router's central processor.

So it will not put any burden on the central processor.

Cheers:

Istvan

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