FYI: this is not a homework question. I was reading an article (http://packetlife.net/blog/2008/jun/19/ospf-network-types/) in OSPF and couldn't understand this point.

"a segment containing ten routers would require 45 adjacencies to form a mesh, but only 17 when a DR and BDR are in place."

This was specifically for the broadcast type of network in OSPF.

closed as off-topic by Ron Maupin Aug 7 '18 at 0:19

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 7 '17 at 20:09

8 routers form adjacencies to both the DR and BDR (8*2 = 16) plus an adjacency formed between the DR and BDR equals 17.

If a full mesh were required it would be 10 routers each with 9 connections. Counted as adjacencies this is (9*10)/2 = 45.

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