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isl trunk and destination mac

sarahr202
Level 5
Level 5

Hi everybody

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml

According to this link, ISL uses 40 bit multicast mac addresses shown below:

The DA field of the ISL packet is a 40-bit destination address. This address is a multicast address and is set at "0x01-00-0C-00-00" or "0x03-00-0c-00-00". The first 40 bits of the DA field signal the receiver that the packet is in ISL format.

1)Mac address consists of 48 bits, so what does switch use for the last 8 bits in ISL destination mac ?

2) How does switch determine which  destination mac multicast address to use in ISL  as we have two option i.e "0x01-00-0C-00-00  and 0x03-00-0c-00-00".?

thanks and have a great day.

2 Accepted Solutions

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Sarah,

the use of a multicast MAC address on ISL trunks was likely thought because ISL by design can accomodate multipoint to multipoint trunk ports like having multiple LAN switches connected with an hub in the middle. But that would mean half duplex operation in a single collision domain.

Clearly all real world trunks are point to point and full duplex making this ISL feature useless.

My personal guess is that the exact value of the less significant 8 bits are don't care, that is they can have any value and the ISL peer device will process the frames.

Between 0x01-00-0c-00-00-xx and 0x03-00-0c-00-00-xx there is only a slight difference of second bit set in most meaningful octect.

They are both multicast MAC address and the bit set means local meaning.

As explained in the link below, the less significant bit of most significant octect is the I/G individual/group address and means multicast when set (or broadcast). The near bit stays for universal or local meaning of the MAC addres

see

http://packetsdropped.wordpress.com/2011/01/13/mac-address-universally-or-locally-administered-bit-and-individualgroup-bit/

Hope to help

Giuseppe

View solution in original post

Hi Sarah,

Good luck with Tshoot exam.

Thanks

Mahesh

View solution in original post

6 Replies 6

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Sarah,

the use of a multicast MAC address on ISL trunks was likely thought because ISL by design can accomodate multipoint to multipoint trunk ports like having multiple LAN switches connected with an hub in the middle. But that would mean half duplex operation in a single collision domain.

Clearly all real world trunks are point to point and full duplex making this ISL feature useless.

My personal guess is that the exact value of the less significant 8 bits are don't care, that is they can have any value and the ISL peer device will process the frames.

Between 0x01-00-0c-00-00-xx and 0x03-00-0c-00-00-xx there is only a slight difference of second bit set in most meaningful octect.

They are both multicast MAC address and the bit set means local meaning.

As explained in the link below, the less significant bit of most significant octect is the I/G individual/group address and means multicast when set (or broadcast). The near bit stays for universal or local meaning of the MAC addres

see

http://packetsdropped.wordpress.com/2011/01/13/mac-address-universally-or-locally-administered-bit-and-individualgroup-bit/

Hope to help

Giuseppe

Thanks Giuseppe.

Just brushing up on some switch topics, I am taking my last exam Tshoot on next mon. Hopefully I will pass it first time unlike Route and switch exams which I have failed once.

Nice to hear from you after a while.

Have a great day.

Hi Sarah,

Good luck with Tshoot exam.

Thanks

Mahesh

Thanks Mahesh

Hello Sarah,

good luck for your exam.

Have you checked the cisco learning network ?

We are all happy to see your progress.

Best Regards

Giuseppe

Hi Giuseppe.

Yes, I did. Luckily Cisco put the topology diagram on its web site for CCNP Tshoot. It saves a lot of time that could otherwise be wasted understanding the topology.

Once I get this exam done, plan is to actively participate on this forum answering questions by researching the question first.

Thanks for your kind words.

Have a great day.

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