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whats a LOOP traffic in ethereal?

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whats a LOOP traffic in ethereal?

Postby Guest » Sun Jan 02, 2011 9:06 am

Hi all,

 

Im testing and analyzing the captured packet in ethereal.

but i can know about LOOP traffic.

 

I connect two routers each other with fast-ethernet.

 

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

      fa0/1                                    fa0/1

   

then R1 and R2 send the LOOP traffic each other per 60 secs.

 

R1s fast ethernet MAC : 00:1a:6c:fe:49:80

R2s fast ethernet MAC : 00:1a:6c:70:7a:66

 

when R1 and R2 connect each other and any configuration is not applyed in that interface,

R1 sends LOOP frame into the media as the below

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

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

 

finally, source_MAC and destination_MAC is the same.

  ScreenHunter_001.jpg

 

what is this frame??

 

is it keepalive frame?

 

why R1 sends LOOP frame that has use the same MACsource and destination MAC

if R1 sends the above LOOP frame,

I think R2 will ignore it because desination MAC is not R2s MAC.

 

is the reason that R1 sends LOOP frame just to test media state?

 

 

i attache the captured cap file.

you can find this issue in odd packet number(1,3,5....)

Guest
 

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Re:whats a LOOP traffic in ethereal?

Postby Guest » Sun Jan 02, 2011 9:16 am

Hi Sanghee,

 

Configuration testing protocol is a part of the original Ethernet specification that doesn appear in IEEE 802.*, or any other specification.Your logs is basically a type of Ethernet Keepalives messages type what i can predict.With the code Ethernet type code of 0x9000 i can say it be a ethernet keep alive messages.

 

On broadcast media like an Ethernet, keepalives are slightly different. Since there are many possible neighbors on the Ethernet, the keepalive is not designed to determine if the path to any one particular neighbor on the wire is available. It is only designed to check that the local system has read and write access to the Ethernet wire itself. The router produces an Ethernet packet with itself as the source and destination MAC-address and a special Ethernet type code of 0x9000. The Ethernet hardware sends this packet onto the Ethernet wire and then immediately receives this packet back again. This checks the sending and receiving hardware on the Ethernet adapter and the basic integrity of the wire.

 

Hope that clear out your query !!

 

If helpful do rate the valauable post.

 

Regards

Ganesh.H

Guest
 

Re:whats a LOOP traffic in ethereal?

Postby Guest » Sun Jan 02, 2011 10:13 am

thanks for your help.

 

but i have a question about your comment.

 

in your saying,

 

The Ethernet hardware sends this packet onto the Ethernet wire and then immediately receives this packet back again.

 

 

how can the router receive keepalive frame as soon as it sent the frame?

 

does the router wait the frame from other device?

or does it determines by itself without receiving the frame on the wire?

 

could you explain this more detail for me?

Guest
 

Re:whats a LOOP traffic in ethereal?

Postby Guest » Sun Jan 02, 2011 10:51 am

Hello Sanghee,

 

>> the frame is sent out on the wire, whoever is on other side of the cable by seeing ethertype 0x9000 sends back the frame.

 

that is the ethertype 0x9000 obliges the other party to send back the frame to help original sender to perform its link integrity test

 

It  should be part of ethernet conformance to behave in this way when receiving a loop frame.

 

Hope to help

Giuseppe

Guest
 

Re:whats a LOOP traffic in ethereal?

Postby Guest » Sun Jan 02, 2011 11:46 am

Hello Sanghee,

your understanding is correct they are ethernet keepalive frames used to check the link.

They can be recognized because MAC SA = MAC DA = Router NIC MAC address and ethertype = 0x9000

 

the frame is sent out on the wire, whoever is on other side of the cable by seeing ethertype 0x9000 sends back the frame.

The frame original sender receives its own frames back and knows that the ethernet port can be considered in operational state.

 

 

see

 

http://www.groupstudy.com/archives/cisco/200112/msg01021.html

 

Hope to help

Giuseppe

Guest
 

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