From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Wed Oct 06 2004 - 08:02:12 GMT-3
Eric,
I don't think there's an RFC that forces an implementation to use any
method to do layer 3 to layer 2 mapping in any particular way.
I can see reasons for a simple implementation to use ARP for all
traffic. Also, I can think of why would someone try to get simetrical
traffic by using the same MAC for a flow, which requires a more
elaborated implementatioon. (Simetrical traffic flows are considered
good most of the time).
You need some notion of "flow" for doing that, actually a bidirectional
flow! (Or it could be just auto learning L2 addresses like a switch, w/o
using ARP for that, much the way frame relay learns mapping just by
receiving a INARP request).
-Carlos
Eric Cables wrote:
> Hey all, I ran into an issue that I was hoping someone here could help with.
>
> The basic question is this:
>
> If a frame is received from a device on a local subnet, is the
> response sent to the layer 2 address the frame was received from, or
> is the layer 2 address re-written based on the local ARP table.
>
> Let's say two systems are on a subnet 192.168.1.0/24:
> - System A = 192.168.1.10
> - System B = 192.168.1.20
>
> Let's also say that system B has two NICs connecting to a common
> switch, and is performing TLB (an HP form of "teaming" to allow
> outbound load balancing).
>
> Server B load balances between its two NICs based on MAC address or IP
> address, and depending on the algorithm transmits data out Adapter 1
> or Adapter 2. I could get into it more, but assume Adapter 1 is the
> "primary" adapter, and all inbound traffic is to be sent to there.
> Let's assign Adapter 1 and Adapter 2 MAC addresses:
> - Adapter 1 = 1111.1111.1111
> - Adapter 2 = 2222.2222.2222
>
> Now, the data transmission! System B sends traffic, ICMP echo-request
> for example, to System A. The load balancing mechanism determines
> that Adapater 2 should be used, so an ICMP echo-request is sent to
> System A using the layer 2 address of 2222.2222.2222. When System A
> receives the frame it sends an ICMP echo-response, now the question
> is, which layer 2 address should the response be sent to? System A's
> ARP table lists 1111.1111.1111 for 192.168.1.20, but the frame was
> received from 192.168.1.20 (2222.2222.2222).
>
> I would love to know which RFC this is defined in, and hear some
> information on what you all have to say. Just to let you know, a
> Windows PC rewrites the response to 1111.1111.1111, but the system I
> am working on (an EMC Solara) is responding to 2222.2222.2222, even
> though its ARP table has 1111.1111.1111 for the destination IP
> address.
>
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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