What is a traffic LOOP Ethereal?

Hi all

I am stable and by analyzing the packet captured in ethereal.

but I can't know the traffic of LOOP.

I connect two mutually routers with fast ethernet.

R1 ------------------------------------- R2

fa0/1                                    fa0/1

then R1 and R2 send traffic LOOP each other by 60 dry.

Card fast ethernet R1 MAC: 00: 1a: 6 c: fe: 49:80

Card fast ethernet R2 MAC: 00: 1a: 6 c: 70:7 has: 66

Where R1 and R2 are connecting between them and no configuration is not applied in this interface.

R1 sends LOOP frame in the media as below

source_MAC: 00: 1a: 6 c: fe: 49:80

destination_MAC: 00: 1a: 6 c: fe: 49:80

Finally, source_MAC and destination_MAC is the same.

What is this image?

is this keepalive framework?

Why R1 sends frame LOOP which use the same MAC (source and destination MAC)

If R1 sends the frame above LOOP,

I think that R2 will ignore it as destination MAC is not Mac of R2

is for this reason that R1 sends LOOP frame just to test the State of the media?

I have attached the file CAP captured.

You can find this number in strange package number (1,3, 5...)

Hello Peter,.

This is my current understanding of the keepalive ethernet.

If I take an Rj-45 ethernet port for example ethernet to a 4th PA and I leave it unplugged the interface is down/down

If the port unplugged I disable keepalive (no keepalive under config interface) ports rises and it's even pings (because when ping interface ethernet, that the packet is not sent on the wire as it happens with serial number or ATM interfaces).

Ethernet interfaces have no carrier detect and no Management TDM to be detected.

My first idea was that keepalive frames were sent with SA MAC router = NIC MAC and MAC DA = broadcast.

I did some research and most of the links point to the following:

http://www.mit.edu/~jhawk/CTP.PDF

http://LWN.NET/articles/330797/

ECTP packets can be sent to unicast, broadcast, or the ECTP reserved
+       cf:00:00:00:00:00 multicast address

> If the frame of the LOOP was supposed to be received in return, what action would be if it wasn't back?

See above, to consider the port down

receving save his own keepalive frames is that a sign of a loop for me is linked to a famous bug on C3750 and other switches to fibers according to ports where workaround suggested was to disable keepalive on these ports.

This bug has been through several discussions in the forums.

It is reasonable that if the goal is to test the ability to send and receive ethernet frames a test with a frame sent the wire and then received back would be the most significant.

Sebastin:

http://wiki.Wireshark.org/loop

We are talking about something so fundamental, that the received frames could be simply ignored and not transmitted to the upper layers as suggested above documents. I'll watch again how you did your tests.

I'm willing to change my mind again about it as I did many times in the past.

Hope to help

Giuseppe

Tags: Cisco Network

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