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About a table in CCNP official guide.

I added an image of a table present in the "CCNP Official Guide" by David Hucaby.

Is it possible that the author reports so many fields with wrong lenghts?

For example, protocol(16) or ICMP type(4).

Or have I lost some concepts?

8 REPLIES
Hall of Fame Super Blue

Re: About a table in CCNP official guide.

speculor_cisco wrote:

I added an image of a table present in the "CCNP Official Guide" by David Hucaby.

Is it possible that the author reports so many fields with wrong lenghts?

For example, protocol(16) or ICMP type(4).

Or have I lost some concepts?

Not sure what you are querying. Is it because in the ICMP header for example the code and type are both 8 bits whereas in the above table the ICMP type is only 4 bits ?

If so this is because the table you are looking at is not about the number of bits in the headers ie. IP or ICMP, it is to do with how the Ternary CAM stores these values. The Ternary CAM is what is used on switches to store acl entries. There is not necessarily a direct correlation between the number of bits in the relevant header (IP/ICMP etc.) and how many bits the TCAM uses for the same entry.

Jon

New Member

Re: About a table in CCNP official guide.

Hello Jon.

You have understood correctly my question.

Thanks for the answer.

Re: About a table in CCNP official guide.

No, you're right. The chart is wrong about the header length of the protocol field for IP and the type field for ICMP.

Hall of Fame Super Blue

Re: About a table in CCNP official guide.

Jason

Do you mean the the chart is wrong for the TCAM values ?

Jon

Re: About a table in CCNP official guide.

No, on second glance the book is correct. I found the chart in my copy, where it is chart 3-2. I thought the chart posted was re: the header fields only and didn't realize it was talking about the TCAM field lengths.  Your response above is correct. The field lengths in the TCAM for these fields does not necessarily match the actual length in the protocol specification. Good catch!

Hall of Fame Super Blue

Re: About a table in CCNP official guide.

Jason

Thanks for that. Just wanted to make sure i wasn't giving bad info to the OP.

Jon

New Member

Re: About a table in CCNP official guide.

Someone knows why it make sense to have not the same number of bit?

Hall of Fame Super Blue

Re: About a table in CCNP official guide.

speculor_cisco wrote:

Someone knows why it make sense to have not the same number of bit?

Yes Cisco

I suspect is is purely to do with efficiency. TCAM is a finite resource and it needs to be used as efficiently as possible because that is partly what allows switches to do some much in hardware.

Jon

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