ISl, vtp , destination mac

Answered Question
Jun 12th, 2010
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Hi every body.


I have few questions ;


1)  Let say we have isl trunk between two switches:


sw1 -----trunk-----sw2


sw1 has host1 in vlan 2 and sw2 has host2 in vlan2.


host2 sends a frame to hosts2.


sw1 receives the frame and encapsulates that frame in isl.

sw1 will use 40 bit multicast address 01:00:0c:00:00  0r 03:00:0c:00:00

1)what source address is used in source address field of isl?  will it be the address associated with port acting as a trunk?


2) sw1-----trunk----sw2


trunk is isl.


sw1 is vtp server, sw2 is vtp client.


Let say sw1 wants to send vtp message to sw2.


Will this message be encapsulated by ethernet frame, which in turn is encapsulated by isl? or  vtp message will be encapsulated by only isl?



thanks  and have a nice weekend

Correct Answer by Giuseppe Larosa about 7 years 1 month ago

Hello Sarah,


>> But what about source address in ethernet header? what will it be set to?


as Reza has noted the only reasonable source MAC address is that associated  to the switch port and it can appear both in ISL and inner ethernet headers.


To be noted an ISL frame is not an ethernet frame and carries one ethernet in its payload.


Hope to help

Giuseppe

Correct Answer by Reza Sharifi about 7 years 1 month ago

Hi Sarah,


1-It uses the Mac address of the switch port that sends the frame

2-The VTP message is send using ISL


Here is nore info:


http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml


HTH

Reza

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sarahr202 Sat, 06/12/2010 - 17:04
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Thanks Reza.



I just want to confirm my understanding.



Vtp messages are encapsulated by ethernet which is then encapsulated by isl.

ISl sets its destination address in isl header as 01:00:00:00:00

Destination mac address in ethernet header is set to 01:00:0c:cc:cc ( multicast address of vtp)

Am i correct?

But what about source address in ethernet header? what will it be set to?



thanks and have a nice weekend.

Correct Answer
Giuseppe Larosa Sun, 06/13/2010 - 02:50
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Hello Sarah,


>> But what about source address in ethernet header? what will it be set to?


as Reza has noted the only reasonable source MAC address is that associated  to the switch port and it can appear both in ISL and inner ethernet headers.


To be noted an ISL frame is not an ethernet frame and carries one ethernet in its payload.


Hope to help

Giuseppe

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