Reading - Portland

Question 1

Briefly describe the functions of the logically centralized Fabric Manager used in PortLand.
Where does this Fabric Manager reside?

Answer 1

The Fabric Manager is primarily responsible for maintaining network configuration soft state.
Using this soft state, the Fabric Manager performs ARP resolution, provides multicast capability
to the network, and achieves fault tolerance goals. The Fabric Manager is a user process,
running on a dedicated machine. This machine may be located on the network itself, or it can
reside on a separate control network.

Question 2

Explain how PMAC (Pseudo MAC) addresses are generated for end hosts in a PortLand
network, i.e. What are the four components of a PMAC, and what does each component
encode? How does this design improve forwarding table sizes on large scale data center
networks?

Answer 2

A PMAC encodes the position of an end host in a fat-tree network. This encoding consists of
four components in the format pod.position.port.vmid . The first component encodes the pod
number the end host and the edge switch reside in, and the position number encodes the end
host’s position in the pod. The port component encodes the switch’s physical port number the
end host is attached to.
The vmid component encodes a unique ID for each virtual machine that is present on the end
host. The edge switch maintains a mapping for each VM, which uses its own AMAC (actual
MAC) address. This permits multiplexing of virtual hosts resident on a single physical host.
The use of PMACs greatly simplify layer 2 forwarding due to their hierarchical nature. Switches
no longer need a forwarding table entry per virtual host. A single forwarding table entry can
be used to aggregate hosts, enabling forwarding behavior that exploits longest prefix match.
Using AMACs, switch state size is O(n), where n is the number of virtual hosts in the data
center, whereas state size is O(k) for PMACs, where k is the number of ports on switches used
to construct the fat tree network.