Wendell Odom's ICND1 book query.

I didn't understand something i read in ODOM's CCNA book . It was written in the book :

"For example, if a network had 30 access layer switches, and the network
engineer decided that each access layer switch should be cabled directly to every other
access layer switch, the LAN would need 435 cables between switches! "

Why 435 cables would be used ? If there are 30 switches to be interconnected then 30 cables should be needed.
Kindly explain .

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Comments

Iristheangel

Interconnected = mesh network. He's talking about a fully mesh network where EVERY switch is directly connected to the remaining 29 switches. The formula to determine this is (N x (N-1))/2

(30 x 29)/2 = 435

hassantalal785

Oh yes . Now i understand thanks a lot

hassantalal785

I have another query if you m mind. Suppose i have a switch with only layer 2 functionality and not a layer 3 switch .Its port 1 is configured as vlan 1 and its port 2 as vlan 2 . If port1's PC send a packet to Port 2's PC , would a Router have to be involved as both ports are on separate Vlans or not ?

Diggs

Yes, if a PC connected to a switchport which is assigned to vlan 1 wanted to send a packet to a PC assigned to a different vlan it would need to send the packet to its default gateway (router). The PC in a different vlan would be in a separate subnet

Ivanjam

Diggs wrote: »

Yes, if a PC connected to a switchport which is assigned to vlan 1 wanted to send a packet to a PC assigned to a different vlan it would need to send the packet to its default gateway (router). The PC in a different vlan would be in a separate subnet

Someone correct me here if I am wrong but, offhand, from my limited perusal of the CCENT material, the two PC's can very well be in the same subnet and still not be able to communicate, as vlan segregation occurs at layer 2 of the OSI model.

MrXpert

Ivanjam wrote: »

Someone correct me here if I am wrong but, offhand, from my limited perusal of the CCENT material, the two PC's can very well be on the same subnet and still not be able to communicate, as vlan segregation occurs at layer 2 of the OSI model.

Yes VLANS are verymuch a layer 2 implementation.A VLAN=a subnet=broadcast domain

Ivanjam

MrXpert wrote: »

Yes VLANS are verymuch a layer 2 implementation.A VLAN=a subnet=broadcast domain

So, I take it that you must first create the vlans that you need, then assign different subnets to each vlan?

MrXpert

Yes this is how I have done it.