Hub or no hub

[Robert M]

Can someone comment on this?
A bridge contains two LAN cards and is used to isolate or divide collision domains in an Ethernet network.

Therefore, each LAN card in a bridge must connect to a seperate hub, and the two hubs must not be interconnected directly.

A router is used to shift traffic based on a logical network address, therefore can both Ethernet LAN adapters of the router be connected to the same hub..... or does each LAN adapter in a router require its own hub?

[jstanek7]

If you are connected through a router then why do you need a hub?

[Robert M]

Routers contain network cards;
A router must be a "good citizen" of a LAN, and connect like any other "citizen" of the LAN. If the existing LAN uses twisted pair wiring and a hub, that would be the way to connect a router also.

The same is true of a bridge.

[Train]

I have 6 computers hooked to a gigabit switch for my gigabit lan and to allow all of them to connect to the modem. I use one port of a router which also assigns the ip numbers to each computer.

No need for the hubs.

[Tuttle]

I guess the answer to this depends on what you mean by the LAN ports on the router.

If you're talking a SOHO device, those 4 LAN ports are actually a switch. If you connect two of them into an existing hub or switch you'll set up a nice loop and kill your network (SOHO stuff doesn't come with STP to guard against mis-wiring).

If you mean separate interfaces on a proper router, you could probably get away with hooking them into the same hub or switch if you want to -- the router won't forward Ethernet frames between its interfaces, so there's no loop. I can't think of any situation where you would want to actually do this though. The only one which springs to mind (having two subnets in use on the same hub/switch and wanting to route between them) can be handled by just giving the router multiple IP addresses on one interface which is connected to the hub/switch.

[Robert M]

Tuttle,
No, I'm not talking about SOHO equipment that may contain a built-in hub or switching.

For example if just purchased a Cisco router with two network cards, and the pre-existing network was using a hub and I decided to create a new network address.

Would I need another hub to create two logical networks?
For Example:
Network 1 had a network address of 130.215
Network 2 had a network address of 140.222

Would a packet from a workstation on network one arrive on network two if a single hub was used? Would it be processed correctly by the workstation on network 2?

[Tuttle]

You wouldn't need to use both interfaces on the router for this. You'd just configure both networks on the one interface:

interface FastEthernet0
 ip address 130.215.1.1 255.255.0.0
 ip address 140.222.1.1 255.255.0.0 secondary

It doesn't matter which one you call secondary (and you can have more than one secondary IP address if you want). Then you just plug that one interface into the existing network.

Traffic between two 130.215.x.x machines or two 140.222.y.y machines will go directly from one machine to the other.

For packets between the two subnets, the sending PC will forward the data to the router (since as far as the PC is concerned, the destination is on a different subnet). The router will take the packet, work out it belongs back out the same interface, and forward it to the destination PC.

That works great as a transitional phase when you're renumbering a network, but there are serious performance issues (from both the shared bandwidth and router CPU load) if you send a lot of traffic between the two subnets. It's best as an interim measure, not a permanent solution.

If you did need something like this permanently for some reason, a better (but far more expensive) option would be to get a layer 3 switch which can route between the subnets in the switch itself.

[Robert M]

Yes, I'm aware of the possible performance problem.
My only interest at this point is in what is possible.
So traffic between the two logical IP networks is possible over the same hub, even using both network adapters? How about with a Novell IPX based network?

[Robert M]

Layer 3 Switches should not be called Layer 3 Switches at all in my opinion, they should be called what they are....routers.

I don't know who started this trend but it sure is annoying.

Any device that can perform both bridging/switching and routing functions should be called a brouter....as they have always been called.

[Tuttle]

Yes, I'm aware of the possible performance problem.
My only interest at this point is in what is possible.
So traffic between the two logical IP networks is possible over the same hub, even using both network adapters?

Should be. But given what Ethernet interfaces cost, I can't think why anyone would.

How about with a Novell IPX based network?

I'm uber-rusty on IPX, but it should be the same -- you still have network and host parts to an IPX address.

Layer 3 Switches should not be called Layer 3 Switches at all in my opinion, they should be called what they are....routers.

I don't know who started this trend but it sure is annoying.

So then how do you quickly differentiate between something like a Cisco 2851 and a 3750? Vastly different feature sets, vastly different purposes.

[Robert M]

Not familiar with those Cisco products.
My old school thinking goes like this:
A hub is a hub.
A hub that can also perform bridging is called a switch, or switching hub.
A router is used to access different networks.
A router that can perform bridging functions or router functions is a combination Bridge/Router.

So if the product is capable of performing bridging or routing, it could be called a "brouter".

If a brouter also performs additional functions above layer three for network traffic, I would still call it a "brouter" with additional capability and name the additional capability.

If a brouter contained a built-in hub, I would call it a brouter with a built-in hub.

To me, the phrase "layer three switch" is kind of an oxymoron.