[LARTC] Why multiple NICs in a multiple route situation?

[LARTC] Why multiple NICs in a multiple route situation?

[Greg Scott]

In a situation with multiple routers to the Internet and a
Linux firewall/router that either makes a choice about which
route to use or load balances among the routes:

> I don't think you *need* to have a separate NIC for each 
> router, but if I were doing it, I'd want each router on a 
> separate network.

This has been bugging me - if a single NIC will work then what 
value does another NIC add?  Let's say the circuits are both T1.
With two possible circuits, that's just a little more than 3mb
per second.  At 100 mbit per NIC, it would take more than 50
T1s to swamp it. 

So why a NIC per T1?  Why not just give a single NIC an 
IP address in all the networks for each T1?  Or am I missing
something important?

thanks

- Greg Scott
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Re: [LARTC] Why multiple NICs in a multiple route situation?

[William L. Thomson Jr.]

Greg,

On Fri, 2002-08-02 at 00:46, Greg Scott wrote:
> In a situation with multiple routers to the Internet and a
> Linux firewall/router that either makes a choice about which
> route to use or load balances among the routes:.
> 
> This has been bugging me - if a single NIC will work then what 
> value does another NIC add?  Let's say the circuits are both T1.
> With two possible circuits, that's just a little more than 3mb
> per second.  At 100 mbit per NIC, it would take more than 50
> T1s to swamp it. 
>
> So why a NIC per T1?  Why not just give a single NIC an 
> IP address in all the networks for each T1?  Or am I missing
> something important?

Please someone correct me if I am wrong.

To my knowledge ethernet does mostly broadcasting. So the traffic from
one router will be bounced off both.

If one says hello I am this ip, the other will say it's ip, until the
proper path is determined.

It's kind of like the difference between a switch and a hub, sort of.

I would recommend trying to isolate the traffic from each to the Linux
router if possible, using a nic per connection/router. With dual or
multiple port nic cards it's not so much of an issue.

Even though the nic is on the Linux side will be 100mbit, most routers
are only 10mbit, so the broadcast flooding could become an issue.

Also in order to use a single nic with multiple connections another
device like a switch or a hub will have to be inline. Which will create
another hop.

So for that reason alone it's worth having two nics, or at least a port
per connection/router.

So for example in my case I do

Router1 --|              |            |-- Server
          | Linux Router |-- Switch --|-- Server
Router2 --|              |            |-- Server

Otherwise you will have to have two switches or hubs. I mean how can you
connect two devices to one ethernet port?

So two reasons I can think of are:

1. To isolate broadcasts from each router
2. To reduce hops to the Linux Router

I really can't think of any other reason.

Maybe someone else can?

-- 
Sincerely,
William L. Thomson Jr.
Support Group
Obsidian-Studios Inc.
439 Amber Way
Petaluma, Ca. 94952
Phone  707.766.9509
Fax    707.766.8989
http://www.obsidian-studios.com

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Re: [LARTC] Why multiple NICs in a multiple route situation?

[Michael T. Babcock]

Greg Scott wrote:

>So why a NIC per T1?  Why not just give a single NIC an 
>IP address in all the networks for each T1?  Or am I missing
>something important?
>  
>
I don't like the fact that one ISP can 'see' the other ISP if they 
decide to actually 'look'.
-- 
Michael T. Babcock


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