Subject: RE: [Hubmib] Point-to-point ethernet interfaces

From: "Logg, Connie A." <[email-masked]>
Date: Wed, 04 Apr 2001 11:17:37 -0700

We have thousands of point to point links and used to (years ago) have a shared ethernet environment.  There are several advantages of point to point.  You can run full duplex (10 mbit, 100 mbit, gigabit) and thereby eliminate collisions and the touchiness of nic to nic compatibility issues (the half duplex spec is not rigorously implemented by many nic vendors).
You can get much higher throughput. In a properly implemented switched environment each node effectively has its own collision (and error) domain.  One thing you have to be careful of although, is a server attached to the network on a gigabit link talking to a server(s) on 100mbit or 10 mbit links. Depending upon the protocol (tcp is more tolerant), you may actually suffer degradation in performance because the packets coming from the gigabit linked node cannot fit in the 100mbit or 10 mbit pipe and get "lost" (discarded). Detection of these types of problems can be challenging.

I do not know of any pointers to documentation. These are just some of my experiences.

Connie Logg - Network Analyst - 650-926-2879
Stanford Linear Accelerator Center
MS 97; 2575 SandHill Road; Menlo Park CA 94025
"Happiness is found along the way, not at the end of the road"

-----Original Message-----
From: Francis Arts [mailto:[email-masked]]
Sent: Tuesday, April 03, 2001 10:11 PM
To: [email-masked]
Subject: [Hubmib] Point-to-point ethernet interfaces

Hello,

I am looking for all the differences between ethernet interfaces used as
shared media and ethernet interfaces used as point-to-point links. Could
someone give me a pointer where I can find more information about these
differences and about ethernet interfaces used as point-to-point links
in particular?

Thanks in advance for any help,

    Francis.

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From: Francis Arts <[email-masked]>
Date: Wed, 04 Apr 2001 20:46:33 +0200

Hello Connie,

Thank you for your fast reply.

I have some additional questions I am struggling with:

1) MAC address used.
If a unicast IP packet needs to be sent to the neighboring station on a pt-to-pt ethernet link, is then still a unicast DA MAC address used? Or also other practices common since there is only 1 neighboring station (e.g. when sending a unicast IP packet on a pt-to-pt ethernet link, using the broadcast MAC as the DA MAC since there is only 1 neighboring station on the ethernet link?)?

2) ARP.
Assuming that a unicast MAC address is used as the DA MAC: How is for pt-to-pt ethernet links this remote MAC address learned? Is it configured (using CLI, SNMP, ...) or is it also learned using ARP, just as on an ethernet used as a shared medium (and therefore connecting more than 2 stations)?

Could you please provide me with your experience / opinion on these questions? This would be a great help.

Kind regards,

    Francis.

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From: "Geoff Thompson" <[email-masked]>
Date: Wed, 04 Apr 2001 13:59:09 -0700

Francis

At 08:46 PM 4/4/01 +0200, Francis Arts wrote:

>If a unicast IP packet needs to be sent to the neighboring station on a 
>pt-to-pt ethernet link, is then still a unicast DA MAC address used?

YES!

>Or also other practices common since there is only 1 neighboring station 
>(e.g. when sending a unicast IP packet on a pt-to-pt ethernet link, using 
>the broadcast MAC as the DA MAC since there is only 1 neighboring station 
>on the ethernet link?)?

No! An Ethernet station is not supposed to have to know how many other 
stations are hooked to the network or what the network looks like.

>2) ARP.
>Assuming that a unicast MAC address is used as the DA MAC: How is for 
>pt-to-pt ethernet links this remote MAC address learned?

It has nothing to do with ARP. See IEEE Std 802.1D, Clause 7.8. A switch 
(i.e. 802.1D bridge) looks at the SA of the packets that comes into its 
port and loads them into its filtering database.

>Is it configured (using CLI, SNMP, ...) or is it also learned using ARP, 
>just as on an ethernet used as a shared medium (and therefore connecting 
>more than 2 stations)?
>

None of this has anything to do with whether or not there is a 
point-to-point link or a shared media LAN attached to the Ethernet port on 
a device. It all works the same way in either case.

Geoff