Connecting Routers via Ethernet Ports

jezg76

Greetings all. I have a question that I hope can be answered here. I simply would like to know under what circumstances you would want to connect routers together by their Ethernet interfaces. I do understand that routers break up collision and broadcast domains, but if they are on the same subnet, why not just use a switch? Why have separate broadcast domains in the same subnet (wait, is that possible?! Oh, my head is spinning )

I am reading up on OSPF and it is discussing Designated Routers and what not and a sentence reads as follows:

"For instance, if ten routers were attached to the same LAN subnet, and they were. . ."

Why the f would you have 10 routers on the same LAN subnet? If I'm not mistaken from all of my Lammle/Odom readings before now, routers behave as follows in regards to IP addressing:

* All IP addresses in the same group must not be seperated by a router.
* IP addresses separated by a router must be in different groups. - Odom

Someone please help me get my boat floating in the right direction again.

Thanks a ton!!1

networker050184

If you have a large switched network with routers placed in different locations that need to be connected you can connect them to the switched network on a common VLAN and viola they can all connect with out having to have a line ran between each router and every other router. There is a little more that goes into connecting ospf over broadcast networks, but that is the basics. It is a lot more practical to just have one ethernet line connecting to the switched network.

rossonieri#1

jezg76 wrote:

Greetings all. I have a question that I hope can be answered here. I simply would like to know under what circumstances you would want to connect routers together by their Ethernet interfaces. I do understand that routers break up collision and broadcast domains, but if they are on the same subnet, why not just use a switch? Why have separate broadcast domains in the same subnet (wait, is that possible?! Oh, my head is spinning )

I am reading up on OSPF and it is discussing Designated Routers and what not and a sentence reads as follows:

"For instance, if ten routers were attached to the same LAN subnet, and they were. . ."

Why the f would you have 10 routers on the same LAN subnet? If I'm not mistaken from all of my Lammle/Odom readings before now, routers behave as follows in regards to IP addressing:

* All IP addresses in the same group must not be seperated by a router.
* IP addresses separated by a router must be in different groups. - Odom

Someone please help me get my boat floating in the right direction again.

Thanks a ton!!1

hi,

your first Q :
I would connect my router interface/port to be exactly the same interface/port as my CPE devices provides me. For example : if its metro-ether than usually the CPE device would provides me an ethernet interface.

Or - if i cant get direct control over my ISP router then i will force their edge router to be connected to my own router via its ethernet using bridge connection.

but, again : routing is L3 (both router or switch) - switching is L2.

your second Q:
if you have a large campus network consisting multiple subnet then you probably dont need a serial port right?

i cant type any further but in case you are wondering why ethernet? try to search differences between serial and paralel communication (between conventional router and switch, that is).

HTH.