Saturday, March 20, 2010

Last nights problem...

So my post last night was concerned with the fragment size noted in the show ppp multilink command.  Well, I figured it out after about an hour of thinking; I then verified this morning.  If you calculate the fragment size at 160 bytes, LFI is automatically going to halve that for a multilink with 2 attached links (because each additional link comprises that percentage of the overall bandwidth).  Now the 72 number is funny, because it should be 80 right?  Well, turns out LFI is smart enough to know 80bytes is the maximum packet size on that link in order to get the 10ms serialization time.  It is also smart enough to know that using ppp you are going to have 6 bytes of ppp header overhead added to each fragment, as well as another 2 bytes to keep track of the fragments.  So all in all, you have 72 + 6 + 2 = 80...and voila. 

So check it out.  I killed serial 0/1, and the bandwidth changes, but not the fragment size...proving that it calculated the fragment size, and then halved it because there were two equal bandwidth links.  Nice.  I still need to find out what the weight number is there...little research after this to find that out. 

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