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Community Member

## Need explanation on adjacencies in fullmesh and DR/BDR

How is the adjacencies in full mesh is n(n-1)/2 and with DR/BDR is 2n-2 ?I have attached a sample diagram , in which there are 5 routers, for full mesh there needs 10 adjacencies ( which equals to n(n-1)/2, 5*4 /2 =10) and in the other case, for DR/DBR election there needs 7 adjacencies (which is not equal to ( 2n-2 , 2*5 -2 = 8).Kindly clear me on this.

Thanks,

Vijay

2 REPLIES
Community Member

## Re: Need explanation on adjacencies in fullmesh and DR/BDR

Hi,

Iam attaching the same diagram.

Thanks,

Vijay

Cisco Employee

## Re: Need explanation on adjacencies in fullmesh and DR/BDR

The proper formula to calculate the number of adjacencies with DR/BDR is 2n-3 not 2n-2.

Hope this helps,

Harold Ritter
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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