isl header and mac addresses in isl header.

Answered Question
Jul 8th, 2009

Hi every body!

ISl header has source mac and destination mac address feilds among others.

sw1 f0/1-----------------f0/2 sw2

Sw1 and sw2 are connected by trunk .

What would be src mac address and destination mac address in ISL header when sw1 sends a frame to sw2? Is it the mac address associated with f0/1 on sw1 and mac address associated with f0/2 on sw2 ? or it is the base mac address switches uses ?

Thanks a lot andhave a nice day!

Correct Answer by Giuseppe Larosa about 7 years 7 months ago

Hello Sarah,

as Jon has noted the ISL frame uses MAC addresses in the ISL header but in a way different from the ethernet frame carried inside it.

It could be said that ISL designers had thought also of some forms of multipoint connections or about the presence of hubs or non Cisco switches in the middle.

the following document is very useful because it provides detailed information of all L2 protocols

http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#pre4

Hope to help

Giuseppe

Correct Answer by Jon Marshall about 7 years 7 months ago

Sarah

The source address should be the mac-address of the fa0/1 interface.

The destination address however is not the mac-address of fa0/2 on sw2. The destination address in the ISL header is a multicast destination address, either

0x01-00-0C-00-00 or 0x03-00-0c-00-00

Jon

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Correct Answer
Jon Marshall Wed, 07/08/2009 - 11:12

Sarah

The source address should be the mac-address of the fa0/1 interface.

The destination address however is not the mac-address of fa0/2 on sw2. The destination address in the ISL header is a multicast destination address, either

0x01-00-0C-00-00 or 0x03-00-0c-00-00

Jon

sarahr202 Fri, 07/10/2009 - 19:22

Hi Jon

"The source address should be the mac-address of the fa0/1 interface."

According to another cisco book, SA source mac adress of the switch.

thanks and have a nice weekend !

Correct Answer
Giuseppe Larosa Wed, 07/08/2009 - 11:34

Hello Sarah,

as Jon has noted the ISL frame uses MAC addresses in the ISL header but in a way different from the ethernet frame carried inside it.

It could be said that ISL designers had thought also of some forms of multipoint connections or about the presence of hubs or non Cisco switches in the middle.

the following document is very useful because it provides detailed information of all L2 protocols

http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#pre4

Hope to help

Giuseppe

sarahr202 Wed, 07/08/2009 - 11:54

thanks Giuseepe and Jon.

Do we need mac addresses in isl header? If yes then why dot1q header does not use such mac addesses.

Thanks

Jon Marshall Wed, 07/08/2009 - 11:58

Sarah

Yes you do because ISL encapsulates the entire frame which includes the src/dst mac-addresses.

802.1q does not encapsulate the entire frame, it inserts a tag after the src/dst mac-addresses in the ethernet frame.

Jon

Giuseppe Larosa Wed, 07/08/2009 - 12:16

Hello Sarah,

802.1Q tagged frame is still an ethernet frame an ISL frame is formally a different matter.

Hope to help

Giuseppe

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