My thoughts on how can 802.1q can save on network cablings and potential problems.

IEEE 802.1Q or commonly known as VLAN tagging is a networking standard for sharing of physical Ethernet network link by multiple independent logical networks.

The protocol works with the MAC layer and Spanning Tree Protocol (802.1D) to allow nodes / hosts on different VLAN to communicate with each other through network switch or router on Network Layer.

VLAN tagging

If i have 2 different environment where servers are members of different network segments, in order to allow the different hosts which are in the p750 server to share the same physical ethernet link, the switch or router need to understand and route the network traffic for both the 10.10.10.* and 10.10.20.* to the p750 machine.

Within the p750 machine, the NIC is capable of deciphering the VLAN ID and route the traffic to the designated hosts or LPARs for inward traffic. For outward traffic, the NIC would tag the VLAN ID to the traffic and route it out to the gateway.

This is also similar to the VTP or ISL protocol that is proprietary to Cisco.

Why is it good.

With 2 VIO servers, the different LPARs in the same IBM p750 machine would traditionally need more than 20 UTP cables. With 802.1Q, we need only 2 cables for all the LPARs and 2 cables for HMC. This is more than 80% savings!

Why it may be bad

In the event that there are lots of host sharing the same UTP link, the nightmare of link failure will be catastrophic. It may be mitigated by having 2 NIC controllers with 2 links each, and distributed connection to 2 different switches.

i guess you cant have your cake and eat it!