CAN ANYBODY PLEASE HELP-DIFFERENCE B/W ROOTSWITCH & DESIGNATEDSWITCH IN STP

[shahul-hameed]

Hi ALL,

In STP, what does root switch mean and designated switch mean?Iam not able to know the exact meaning of designated switch with an example.Can anybody plz help me in providing the exact meaning for designated bridge with the ref. to the cisco link.

designated switch - This is the switch closest to the root switch through which frames will be forwarded to the root.I hope this is the exact definition.If this is the exact definition i need to know how stp is configured with root switch and designated root switch with an example. can anybody plz help?

Regards,

Shahul

[rais]

STP undergoes three elections:

1. Elect the Root bridge

2. Elect Root ports on all switches other than root bridge

3. Elect deisgnated ports on each segment.

In step 1, root bridge selected is the center of universe. All switches hang off of root switch like a star with root in the center.

In step 3, switch with designated port is the designated switch for that segment. Designated switch is per segment. A switch that is closest to the root switch. Simple as that.

For designated switch also refer to the following link:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

which says:

STP Rule Two: Once a switch determines its root port, that port must be set to forwarding mode.

In addition, for each LAN segment, the switches communicate with each other to determine which switch on that LAN segment is best to use for moving data from that segment to the root bridge. This switch is called the designated switch.

Hope this helps.

[shahul-hameed]

Hi rais,

Thankyou for your immediate response.Please let me know what does this mean - switch with designated port is the designated switch for that segment.How to find the designated port.I already had gone through this article before but i was not able to get the exact thing as they haven't explained about the designated switch in detail and no explanation is there with figure for the designated switch.

If you could, plz help me.

Regards,

Shahul

[rais]

Each segment, e.g. switch1-----switch2----switch3 has two segments, has a designated switch.

Designated port (hence designated switch) on a segment are selected based on root path costs. Port closest to the root becomes designated port. In case of a tie, Lowest bridge id wins.

Hope this helps.