Notes

This section covers discussions and some background work that went into this project.

Overview

The things covered here are:

• DHCP relay agents
• VLANS
• Trunking

VLANs

VLANs are a way to split up broadcast domains at L2. They can be statically or dynamically set, with dynamic VLANs sort of working in a similar way to DHCP.

DHCP Relay agents

Relay agents are virtual layer 3 devices residing on a switch with VLANs. In this case, they allow DHCP broadcasts to be relayed across 2 VLANs on the same switch. This is not necessary for us because the DHCP server lives beyond the ASA, and the ASA not only acts as a router between the switch and the server, but also serves to manipulate the VLAN tags (masquerades as hosts sending out DHCP requests).

Trunking

Trunking is done by making a "backbone" for all the different VLANs on a local switch to converge to. Frames from different VLANs are identified bythe means of VLAN tags, a four-byte addition to the Ethernet frame identifying which VLAN it came from. The tagging is also known as 802.1q.

Native VLANs

Normally the 4 byte addition to the Ethernet frame would cause the tagged frame to be rejected on a VLAN, and regular frames would be rejected if found in the trunk due to the lack of a tag. This is unless the "regular" frame is from a native VLAN , whose frames are intended not to have tags for devices that don't understand tagging.

A site explaining what a native vlan is: ​http://networkers-online.com/blog/2008/06/native-vlan-explained/

In our case this matters because DHCP coming from the trunk needs to assign IP addresses to both the hosts connected to the switch on different VLANs (VLAN 27) and to the switch itsself. To do this, The virtual interface to which the IP address is assigned needs to be on the trunked port(s).

some thoughts regarding th virtual switches (6/9)

• They are overlay devices that "run" on top of VLANs. VLANs need to be set up before virtual switches can be created.
• They are identified by the VLAN number of the VLAN they overlay, so one virtual switch can't encompass more than 1 VLAN.
• Conversely, they can overlay part of a VLAN. In that case, the whole VLAN takes on the virtual switch's behavior.

The last point is somewhat annoying, because the default behavior of a controllerless virtual switch is to "wait till the flow table times out." That would mean "become nonfunctional" in our case, since the virtual switches start off controller-less, and hence, flow-less. The whole switch would become a virtual brick if the trunk is specified in setvsi. This was demonstrated by overlaying a virtual switch over a single port on VLAN 27. It stopped forwarding DHCP messages all together when it used to be the only VLAN with DHCP.

Questions:

• can a virtual switch be loaded with a default flow table that would allow it to function, at least until its table times out?