Nov 12, 2009

What is Multicast Designated Router (DR) ?

晚上閒來無事檢查一下e-mule的成果,看到了一份古早的multicast course slide + content的PDF,真的有點衝動想要把它整本印出來,裏面的內容真的很詳細很棒,但是現在完全沒有教這些來龍去脈,所以我想現在的學生就算是考過CCNP/CCIP對於multicast也只是一知半解。

在文章裏面我剛好看到了一個也出現在BSCI課文中的一個專有名詞 Designated Router/Querier,更重要的是…Cisco BSCI 教科書中那段文章還是錯的,它把highest IP寫成了lowest IP...(心中OS:習慣就好 習慣就好 教書教了三年還真的仍不習慣~)

• Designated Router (DR)
– For multi-access networks, a Designated Router (DR) is elected. In PIM Sparse mode networks, the DR is responsible for sending Joins to the RP for members on the multi-access network and for sending Registers to the RP for sources on the multi-access network. For Dense mode, the DR has no meaning. The exception to this is when IGMPv1 is in use. In this case, the DR also functions as the IGMP Querier for the Multi-Access network.

• Designated Router (DR) Election
– To elect the DR, each PIM node on a multi-access network examines the received PIM Hello messages from its neighbors and compares the IP Address of its interface with the IP Address of its PIM Neighbors. The PIM Neighbor with the highest IP Address is elected the DR.

– If no PIM Hellos have been received from the elected DR after some period (configurable), the DR Election mechanism is run again to elect a new DR.
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