Q and A for Ch. 17
CS 332, Spring 2016
Intro: What is a LAN?
Q: What is a LAN?
A: A local area network is a layer-2 network,
where any machine can send data directly to any
other machine on the LAN. I.e., each packet can
be addressed with the destination’s address and
sent to it without any changes to the packet’s
addressing.
Different layered devices
Identify which layer the following devices
operate at:
• switch
• repeater
• modem
• hub
• bridge
• VLAN switch
Frame vs. Packet
Q: What is a “frame”? Is it different than a
”packet”?
A: A frame is just a layer-2 packet. A generic
term for frames/packets/messages/etc. is PDU:
protocol data unit.
What is a bridge?
Q: How many network ports are there?
A: 2
Q: How many machines does it assume are
reachable on each port?
A: Multiple.
Q: Does it connect 2 LANs?
A: It connects 2 LAN segments.
Q: What is the difference between a bridge and a
switch?
A: A bridge has only 2 ports. A switch has more.
Figure 17.4 (p. 297)
• Error: “Frame Sent” should be “Frame Seen”.
• Q: How does the bridge find out about
computer C?
• A: Error: it doesn’t!
• Q: Do the computers (A, B, C, X, Y, Z) know the
bridge is there?
• A: No!
Distributed Spanning Tree
Q: Can you explain it more technically?
A: Sure!
• Bridges/switches run this protocol automatically.
• Each has a unique identifier from the factory.
• They all send to a prescribed multicast MAC address.
• They somehow elect a leader.
• They agree to turn off some ports so that there are not loops.
Those ports are not used for bcasts or flooded unicasts (when the
dest MAC is not known).
• The spanning tree is computed using Prim’s or Kruskal’s algorithm.
• You don’t have to know any of this…
Addressing and switches
Q: How does addressing work with switches? Do
they get IP addresses from DHCP?
A: Switches don’t necessarily get IP addresses.
In fact, switches don’t even need MAC
addresses, as they are “never” sent packets –
they exist “covertly”.
Switch vs. Managed Switch
• A regular switch just does its work – no way to
manually intervene or configure anything, or
get stats from it, etc.
• A managed switch runs a TCP/IP stack, so that
you can log in to it and configure it (assign
VLANs, e.g.), get stats, etc.
• The management network will typically be on
VLAN 1 (the default VLAN).
Broadcast Domain vs. Collision Domain
Q: What is a broadcast domain? (p 301)
A: A set of computers such that a layer-2
broadcast message from any machine in that set
reaches all the others. (This is perhaps another
definition of a LAN.)
Q: What is a collision domain?
A: A set of computers where if any two
computers try to use the network at the same
time, there is a collision.
VLANs and Physical vs. Logical
• A Virtual LAN really separates the physical
topology from the logical topology.
• Can put machines on one physical LAN (wire)
into multiple VLANs.
• Can put machines on multiple physical LANs
into one VLAN.
– These physical LANs could even be very far apart
geographically.
Configuring a VLAN
• Assigning a machine’s interface to a VLAN
depends on physical topology.
– Bus: have to configure the machine itself.
– Switched (star):
• Port-based VLANs: assign ports to VLANs and forward
only to those in the same VLAN.
• Or, MAC-based VLANs: assign MACs to VLANs.
(Compare adv/disadv. of MAC-based vs. Port-based.)
• And/or can alter the Ethernet frame, inserting the
802.1q header with the assigned VLAN before
forwarding it. (trunking).
Extra: Submarine cables, repeaters,
etc.
• https://en.wikipedia.org/wiki/Submarine_com
munications_cable
• https://www.youtube.com/watch?v=XQVzU_Y
Q3IQ
• http://www.submarinecablemap.com/
Old Slides
What is a hub?
Q: Can you draw a picture of the connectivity
within and outside of a hub?
A: Yes! A hub is a “bus in a box”. It converts
what appears to be a star topology into a bus
topology. (section 15.10)
Hub modifications?
Q: Can the network hub be modified to improve
the bandwidth that is received by computers in
the network, in the case where there are
multiple computers connected to that hub?
A: Not sure about this. I’d like to hear more.
Extensions and repeaters
Q: Are technologies such as fiber modem
extensions and repeaters being built in to
computers?
A: Not desktop/laptop computers. These are less
sophisticated devices the repeat signals.
Q: Is there any advantage to using fiber modem
extensions as opposed to a repeater or vice-versa?
A: Converting to fiber extends the reach of a LAN.
I’m not sure how “smart” a fiber modem is,
though…
Going over the top
Q: What exactly would happen if you tried to
connect more than four repeaters (above the limit)?
Would it be possible to implement other tools to
offset or work around the delay?
A: Did the text say there was a limit on repeaters?
Anyway, there must be a limit, theoretical or
practical. If you go over the limit, my guess would
be that you’d get very bad/flaky connectivity – lost
packets, corrupted packets, timeouts, etc.
Repeaters at Calvin?
Q: Does Calvin have repeaters on campus?
A: I don’t know… I doubt it.
Bridges at Calvin
Q: Does Calvin use bridges for its separate
networks?
A: No. No one uses bridges anymore. People
don’t even use hubs anymore. Everyone uses
switches.
Learning Bridges
Q: When the text talks about simultaneous
activity on attached segments, is it saying that
the computers attached to hub 1 (figure 17.3)
can all talk without being interrupted by
computers attached to hub 2?
A: Yes! That is correct. Note: typo in figure
17.4.
Bridge vs. switch
Q: Why use a bridge over a switch?
A: No good reason, really. Cost, maybe? I don’t
think bridges are made anymore.
Info kept after disconnection
Q: One of the questions that came up in lab
yesterday was whether or not there are devices,
such as bridges, which continue to store
computer locations even after network cables
have been disconnected.
A: I don’t know *for sure*, but I think when a
cable is disconnected, the device will clear any
info it has learned about machines on that port.
Switch Fabric
Q: What is a switching fabric?
A: http://etherealmind.com/what-is-thedefinition-of-switch-fabric/
This helps, but does not explain everything to
me. It is (mostly, I think) a hardware thing,
which is not my forte…
Switch: what layer?
Q: What does it mean for a switch to operate at
layer 2?
A: It means it knows about Ethernet packets – not
just voltages. It reads source MAC addresses, stores
packets in queues, etc. It is much smarter than a
hub.
NOTE: It *still* maintains the broadcast domain
that Ethernet requires.
NOTE: computers connected by a
switch/hub/bridge do NOT know that the devices
are there.
Switch is stupid or smart?
Q: During the predictive activity, I remember
you saying ""assume the switch is stupid;""
however, from the book, the switch is actually
smart.
A: Everything is relative. The switch is stupid
compared to a router, but smart compared to a
hub. The switch does *not* generate any
packets on its own. It is “passive”.
N/2 transfers max?
Q: Why can only up to N/2 transfers occur at the
same time with a switch that has N ports
connected to N computers?
A: I would have preferred that the textbook said
“up to N/2 conversations can occur at the same
time.”, because for every packet it, it must go
out, so 2 ports are used.
VLAN switches
Q: Can you please shed some more light on how the
VLAN switch works?
A: A VLAN can be made by assigning multiple ports
in a switch to a LAN – i.e., to participate in one
broadcast domain. So, when a bcast packet comes
from one port, or when a packet to an unknown
dest MAC comes in on that port, the packet is
duplicated and sent on only the other ports in the
VLAN. This is port-based VLANs.
Another way to think about it: the switch acts like
multiple switches, one per LAN.
Types of VLANs
• Port-based VLANs.
• MAC-based VLANs. Have to specify exactly which
MAC addresses belong to which VLANs.
– Very good for moving a workstation to a new part of
the company/institution.
• Protocol-based VLAN: switch looks at layer 3 type
(in the layer 2 header) to see what protocol is
used and switches based on that…
• IP-subnet-based VLAN: switch looks at layer 3
source IP address subnet (!) and switches based
on that.
VLANs as firewalls
Q: Can firewalls be installed only when there is a
VLAN? (pg. 301)
A: I think a VLAN is a simple kind of firewall –
isolating some traffic from other traffic. There
are *very* sophisticated and better firewalls,
too.
ISPs
Q: What connected ISPs?
A: ISPs connected to each other via fiber/copper
between routers, not switches. This is because
each ISP has its own LAN, and you cannot
connect LANs together with layer 2 devices.
Even Older Slides
Fiber modems
Q: Do fiber modems convert/translate Ethernet
packets to optical fiber packets via
encapsulation?
A: I don’t think so. I think they just forward
them over the fiber using the fiber’s physical
layer 1 specs. (But, a fiber modem might be a
layer 2 device – I’m not sure!)
Common LAN extension
Q: What is the most common form of LAN
extension. Is there a most common form or is it
based on circumstances?
A: I think the most common form is a switch
with twisted pair ports and one fiber port. So
you have twisted pair in a building and fiber
between buildings.
Repeaters
Q: So do the repeater make it so that the signal
can stay stronger for a longer distance? Does it
give the signal as extra push?
A: Yes, that is correct. It boosts the signal.
Q: How many repeaters can you have before
errors can occur?
A: I don’t know the answer to that… Hardware
stuff…
Analog?
Q: What exactly is “analog” (what kinds of
devices use it) and how does it amplify in
repeater devices for long distances? And why
do repeaters now use infrared receivers?
A: Analog is voltages on a wire – not interpreted
into bits or frames. It means it is layer 1.
Repeaters simply operate at layer 1.
(I don’t think Ethernet repeaters use infrared.
The book just uses that as an example.)
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.
Do we have bridges/switches?
Q: In our lab, where are these "bridging" devices
seen?
A: Over in the corner. All the yellow cables go to
one switch and all the green/blue cables go to
another. Each forms a separate LAN.
(We also have a drawer full of hubs/switches, for
this Friday’s lab.)
Offline computers?
Q: A bridge sounds kind of like what we did for
the activity we did in class with the switch. How
then does a bridge know if a computer is
offline? Or is it exactly what we talked about in
class since a bridge is associated with a switch?
A: A bridge/switch does not know if a computer
is offline. But, if it does not hear from a
computer for a while, it removes its entry from
its forwarding table.
Figure 17.4
Q: Can you explain the process in Figure 17.4
(pg.297)? (Why is the bridge involved when
packets/frames are being sent within each
segment?)
A: There is a mistake in the table. The heading
“Frame Sent” should be “Frame Seen”.
Advantages of a hub?
Q: The chapter also seems to paint switches in a
very good light. Are there any advantages to
using a hub instead of a switch?
A: The only advantage I know of is that a hub
repeats everything everywhere, so you can use
it to snoop packets between other computers.
Building connectivity
Q: And how are buildings connected by bridges
(Is there a third building that hosts the bridging
device?)?
A: First, “no one” uses bridges anymore (bridges
have only 2 ports). Buildings are connected by
having a switch in each building and a
switch/router in one main building (CIT) to
forward packets between all. Or they may have
multiple connections, and then use DST.
Broadcasting with switches
Q: Can you explain how broadcasting with
bridges works a little bit more?
A: When a bridge/switch receives a broadcast
packet, it sends to all (other) ports.
(A switch also sends to all other ports when it
receives a packet for a host it does not know
about.)
This behavior allows the switch to emulate the
same broadcast domain as a bus/hub.
Simultaneous transmission
Q: Could you please elaborate on this "A bridge
permits simultaneous transmission on each
segment" ?
A: On a bus/hub topology, only one computer
can talk at a time, or you have a collision. With
a switch, you only have collision domains on
each segment connected to the switch’s ports.
Queue overflow
Q: Can the queue within a switch grow to be too
long and therefore drop packets?
A: Yes! This is how you get congestion,
increased latency (delay), and dropped packets.
Switch vs. VLAN switch
Q: When would you want to use a switch rather
than a VLAN switch?
A: A regular old plain-Jane switch is going to be
cheaper than a VLAN switch. A VLAN switch has
to be a “managed” switch – something you can
log in to and configure. Many cheapo switches
aren’t manageable.
Repeater speeds
Q: It says in the book that a repeater can
actually speed up a network by allowing
bidirectional communication. However, since
the repeater is a another node that a given
message needs to pass through, doesn't it slow
down each direction it transmits?
A: Theoretically, I suppose it does. But, a
repeater, at layer 1, is very fast… I might be able
to keep up (“operate at line speed”).
DST/STP
Q: Could you explain the Distributed Spanning Tree?
A: Spanning Tree Protocol (STP) is 802.1d. A
spanning tree is the set of links that connect all
devices, without any loops. Each switch is
configured (at the factory) to belong to a special
multicast MAC group. The switches send many
messages to each other to determine who is
connected to whom, and which ports should be put
in “backup” state so that a loop is broken.