Friday, August 31, 2012

Sorting out System MAC addresses with VPC and VSS – Part 1

A number of multichassis aggregation technologies are deployed in the data center today, for example, Cisco’s Multichassis EtherChannel (MEC) on catalyst 6500 VSS, and Virtual Port Channel (vPC) on Nexus platforms. Inter-chassis aggregation greatly increases link utilization, while simplifying design by eliminating topology dependence on spanning tree protocol. STP becomes passive as most links are forwarding, and most failure scenarios no longer require STP re-convergence, thus minimizing disruptions. Furthermore, a more elegant data center design can be achieved, with lower operational complexity, and higher return on investment.  

System MAC address exists on individual devices, often used for device level negotiation, for example, bridge ID field in STP BPDU, or as part of LACP LAGID.

When multiple chassis operate in unison, software simulates the behavior of a common logical system, with the use of common virtual identifiers. Differentiating and sorting out the use of virtual system identifier and various MAC addresses is helpful for understanding, designing and deploying such systems.

It can be illustrated with a simple topology such as the one shown in the diagram, in which a pair of Nexus (in VPC domain 100) is connected to another pair (in VPC domain 101) on back to back VPCs.