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Identifying Ethernet Multicast.doc

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Identifying Ethernet Multicast Filed in CCIE, Switching by Brad Hedlund on November 21, 200 ! 1 Comment# $u#t li%e there are di&&erent Ethernet header ty'e#, there are al#o di&&erent ty'e# o& Ethernet addre##e#( ã )nica#t ã Broadca#t ã *ultica#t + unica#t &rame contain# the uni,ue *+C addre## o& the de#tination receiver- + broadca#t &rame contain# all binary 1.# a# the de#tination addre## /FFFF-FFFF-FFFF0- + multica#t &rame contain# the uni,ue multica#t *+C addre## o& an a''lication, 'r
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  Identifying Ethernet Multicast Filed in CCIE, Switching by Brad Hedlund on November 21, 200 ! 1 Comment#  $u#t li%e there are di&&erent Ethernet header ty'e#, there are al#o di&&erent ty'e# o& Ethernet addre##e#( ã )nica#t ã Broadca#t ã *ultica#t+ unica#t &rame contain# the uniue *+C addre## o& the de#tination receiver- + broadca#t&rame contain# all binary 1.# a# the de#tination addre## /FFFF-FFFF-FFFF- + multica#t &rame contain# the uniue multica#t *+C addre## o& an a''lication, 'rotocol, or data#tream-hy i# it im'ortant to be able to di#tingui#h between the ty'e# o& Ethernet addre## ty'e In an Ethernet #witch, each o& the three are treated di&&erently-+ unica#t addre##ed &rame i# only #ent out the #'eci&ic 'ort leading to the receiver- +  broadca#t &rame i# &looded out all 'ort#- + multica#t addre##ed &rame i# either &looded out all 'ort# /i& no multica#t o'timi3ation i# con&igured or #ent out only the 'ort# intere#ted in receiving the tra&&ic-It.# ea#y &or the ethernet #witch to identi&y a broadca#t &rame becau#e there i# only one univer#ally %nown broadca#t addre##, FFFF-FFFF-FFFF /all binary one#- 4here&ore it i# ea#y &or the #witch to %now the#e &rame# need to be &looded out all 'ort#-However, given there i# #uch a wide variety o& 'o##ible unica#t and multica#t Ethernet addre##e#, how doe# the #witch di#tingui#h between the two It i# im'ortant to 'ro'erly ma%e the di#tinction becau#e the the two are handled #o di&&erently within the #witch /a unica#t &rame# goe# to only one 'ort, a multica#t &rame goe# to #ome or all 'ort#-5oe# the #witch have a databa#e o& all 'o##ible multica#t *+C addre##e# it re&erence# &or each &rame No, that would be ine&&icient-How thi# i# done e&&iciently i# there i# one #'eci&ic bit in a Ethernet *+C addre## that #igni&ie# i& the &rame i# unica#t or multica#t- 4he #witch need only loo% at thi# one bit to ma%e the di#tinction-4he IEEE ha# #'eci&ied that the mo#t #igni&icant bit o& the mo#t #igni&icant byte be u#ed &or thi# 'ur'o#e- I& it# a 1, that mean# multica#t, 0 mean# unica#t- 4he mo#t #igni&icant  byte i# the le&t mo#t byte in the addre##, and the mo#t #igni&icant bit i# the right mo#t bit  o& the byte /thi# i# counter intuitive to mo#t binary im'lementation# where the le&t mo#t  bit u#ually labeled mo#t #igni&icant- Some uic% e6am'le# o& mulica#t *+C addre##e#( ã 0 1 00-CCCC-5555 ã 0 9 00-++++-BBBBSome uic% e6am'le# o& unica#t *+C addre##e#( ã 0 0 01-7788-99 ã 0 8 00-22 -7788Each o& the bolded number# re're#ent# a 1 or 0 're#ent in the mo#t #igni&icant bit o& the mo#t #igni&icant byte-4hi# bit i# al#o re&erred to a# the Individual:;rou' bit-From the 'er#'ective o& an Ethernet hub device, none o& thi# matter#, a# all &rame# are &looded out all 'ort# regardle## o& their addre## being unica#t, broadca#t, or multica#t- It ma%e# no #en#e &or an Ethernet hub to di#tingui#h between the three-<<<  References:CCIE Routing & Switching Exam Guide Vol 3
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