Q and A for Ch. 17 CS 332, Spring 2014

Fiber Modems Q: Why use fiber modem extensions? Is it to save money on not having to use as much fiber (otherwise wouldn't it be easier to use all fiber)? A: I think that is correct: you use (cheap) twisted pair in the building, and then fiber between them. What I don’t know about is what happens to LAN distance requirements when you insert an extension in the middle like that.

Fiber Modems (2) Q: At what layer does a fiber modem operate? A: I don’t know… Layer 1 or layer 2? I suspect it is layer 2. I’ll have to investigate more.

LAN Segment Q: The book says that a bridge is a mechanism that connects two LAN segments. What is meant by "LAN segments"? Are they together part of one LAN or are they both different LANS? A: LAN segment is collision domain. That is, it is all machines that when they send a packet, no other packet can send at the same time, or there will be a collision. E.g., a bus or hub with many computers connected.

Bridge with > 2 ports? Q: Could there exist a bridge that forwards between more than just two hubs? A: Yes! It is called a “switch”. It can have connected to it as many LAN segments as it has ports.

No more bridges? Q: Comer makes a point in mentioning that bridges are no longer sold as stand alone devices, but are integrated into other interfaces. What are some examples of interfaces that use bridging technology? A: Switches do bridging, and routers can be configured to do bridging.

Computers are ignorant Q: How can computers not tell that they are using a bridged LAN instead of a continuous LAN? A: A computer just knows it is connected to an ethernet and it can send and receive packets on it. It doesn’t know what it is connected to, and doesn’t have to know. (It does have to know if what it is sending to is on the local LAN or has to be routed, but we’ll learn about that in due course.)

Learning Bridges / Switches Q: Is the forwarding table in a switch formed the same way a bridge adaptively learns? A: Yes. It is the same thing.

Advantage of bridging Q: What is the advantage of bridging instead of using a hub? A: A bridge/switch splits up the collision domain, allowing more than one computer to talk at once. However, it maintains the broadcast domain – which means it maintains the illusion that the whole LAN is a bus. This means multiple pairs of conversations can happen through a switch – i.e., more than one machine can talk at once.

Bridging Disadvantage Q: Are there any limitations to bridging outside of its distance limitations? A: A bridge/switch does not repeat packets on every interface, which means you can't snoop packets -- which you might want/need to do.

Switches have buffers Q: The book talks about switches having buffers and that if two computers sent packets to a third computer simultaneously, the buffer would store one of those packets while the other went ahead and sent. What would happen if the switch were a hub instead? A: If it were a hub, you’d have a collision. A hub is a layer 1 device: it repeats voltages on all its ports. A switch is a layer 2 device: it knows about ethernet packets, can store them, and forward them.

What about quiet computers? Q: What would happen in this scenario: a device on a LAN is quiet for a while (not sending or receiving packets). In this time, the bridge eventually thinks it knows all the devices on the LAN. If a packet were finally sent to the quiet device, what would stop the bridge from filtering out that packet? A: A learning bridge/switch times out its entries, so when the packet comes for the unknown computer, it would not know where to send it, and would send it on all ports.

Other advantages to switches? Q: Besides the ability to provide VLAN technology and to buffer packets, what other advantages do switches provide over hubs? A: Can support multiple speed interfaces (I think). Can support fiber interfaces. Split up the collision domain (as I said above).

Figure 17.4 Q: I don't understand Figure Could we go over this in class? A: There is an error in the table. The heading should be “Frame Seen”, not “Frame Sent”.

Switch fabric Q: How does the fabric of the switch work? What does broadcasting on a switch look like? A: I don’t know the hardware details of a switch fabric. The simplest architecture I can imagine is having a hardware device listening on each Rx line, and putting received packets into a shared queue in memory. The processor reads packets out of the queue and repeats them onto queues for the Tx lines. Separate hardware is then responsible for transmitting them and retransmitting, if necessary.

VLAN configuration Q: How does the manager in the VLAN work? How do you set up a VLAN? A: VLANs configuration is often configured on a switch’s ports. On a switch, you can assign different interfaces to different VLANs. In this case, the NICs know nothing about it – and send out untagged packets.

VLAN configuration (2) (continued) VLAN interfaces can be configured on NICs. You configure an interface to be on a VLAN (say, VLAN 10). The packets from there have 802.1q tagging (the extra fields in the ethernet header). Then, switches can be configured to forward packets from VLANs to other ports (including multiple VLANs forwarded over certain ports). There is also “default” VLAN configuration – if a packet is untagged, it gets tagged with that VLAN # before being forwarded.

VLAN switch #s Q: How many VLAN switches can you possibly have in a network? A: Most switches are VLAN switches. So, you can have as many as you need.

VLAN routing Q: Would there ever be a circumstance where one computer on a VLAN need to talk to another computer on a separate VLAN within the same physical topology? If so, how would that work? A: If you need to communicate with another machine on another VLAN, the packets must be routed. If the machine is actually on the same physical LAN segment, then you have to have a “one-armed router”.

Q: Also, is there any functional difference between a MAC address and an Ethernet address? A: No. A MAC address is an Ethernet address. Same thing (for our purposes).