GS GBIC using both ports to alternate core switches

[jcowtan]

I am designing a redundant access to a server switches. My plan is to use a single GS GBIC on each the 3548 server switches and connect port-1 to core-switch-1 (3508- containing a GS GBIC), and port-2 to core-swtich-2 (3550 - containing a GS GBIC. These core switch also have redundant links to other access servers and as well as between them. I expect one of the server uplinks to be blocked (STP).

Questions:

1 - If one of the GS GBIC portds is blocked, will the other port run at full-duplex?

2 - If it runs at half-duplex, what do I need to change to run at full-duplex?

[konigl · Accepted solution]

Answer to question 1:

If you have two cables plugged into the GigaStack GBIC, it will run at half-duplex. Two cables means the GigaStack backbone media is being shared, which requires the Collision Detection (CD) part of CSMA/CD, which limits communications to half duplex.

Even if one port gets blocked by STP. Because even though STP will not forward normal traffic through the blocked port, it will still need to send or receive Spanning Tree Bridge Protocol Data Units (BPDUs) at regular intervals. These BPDUs are issued at two-second intervals by default, per VLAN.

Answer to question 2:

To change the GS GBIC connection to run at full-duplex you must connect one, and only one, cable between each pair of GS GBICs. That makes it a point-to-point connection, not shared media, so no need for CD, therefore you can run full-duplex.

Hope this helps.