Yu Cheng (Jade)
ICS 351
Prelab Report 6
December 04, 2008 (late)
[Question 1]
Describe the difference between a LAN switch/bridge and a router
The LAN switch is a box like device that connect multiple ethernet segments.
The router is a more sophisticated network device. It is designed to join
muliple area networks. It is a combination of ethernet switch and Network
Address Translator (NAT). They usually include a Dynamic Host Configuration
Prototol (DHCP) server, Domain Name Service (DNS), and a hardware firewall to
protect the LAN from mallicious intrusion from the internet. It runs routing
protocols. It routes TCP/IP packets between multiple PCs on LAN and a WAN,
and much more.
References: http://www.duxcw.com/faq/ics/diffrout.htm
http://compnetworking.about.com/od/homenetworkhardware/f
/routervsswitch.htm
[Question 2]
What is the difference between an Ethernet switch and an Ethernet hub? Which is
more suitable for a network with a high traffic load, a switch or a hub? Why?
A hub is less expensive, less intelligent, and less complicated of the two. Its
job is simple: anything that comes in one port is sent out to the others. PCs
connected with a hub can "see" everything transmitted through the hub.
A switch does the what a hub does. It "learns" where particular addresses are.
It remembers the port that is used for transmission to a certain PC. Later, it
knows that this PC is connected to this port and the traffic to this PC nneds to
only ben sent to that port and not to any of the others. The broadcasting
happens at the beginning. Most of the traffic only goes where it needs to go
rather than to every port.
So a switch is faster. When there's high traffic load, we want to use a switch.
References: http://ask-leo.com/whats_the_difference_between_a_hub_a_switch_and
_a_router.html
http://compnetworking.about.com/od/homenetworkhardware/f
/routervsswitch.htm
[Question 3]
What motivates the use of the term transparent in transparent bridges?
Transparent bridge is a paticular case of network bridge. The network bridge
simply enables local networks to communicate with each other, but forwards the
tranfic to all ports. The transparent bridge is capable of redirecting the
packets to the proper port, hence it can isolate the networks from broadcast
traffic. The transparent bridge keeps a forwarding table that associates
addresses to ports. The table is built by learning the network topology from
the analysis of the incoming traffic.
Transparent bridge is named this way because its presence and operation are
transparent to network hosts.
Reference: http://en.wikipedia.org/wiki/Transparent_bridge
http://www.pulsewan.com/data101/transparent_bridging_basics.htm
[Question 4]
Which role does the spanning tree protocol play when interconnecting LAN
switches/bridges?
STP is a link management protocol that provides path redundancy while preventing
undesireable loops in the network. When loops occur, some switches see sending
and receiving stations appear on both sides of the switch. This confusion make
the forwarding algorithm to send duplicate frames.
STP prevent this confusion from happening. It defines a tree and forces certain
redundant data paths into a standby state. If one network segment in the STP
becomes unreachable, or if STP costs changes, the STP algorithm reconfigures the
spanning tree topoloty and re-establishes the link by activating standby paths.
Reference: http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman
/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm
[Question 5]
In the context of the IEEE 802.Id specificatoin of the spanning tree protocol,
define the following terms:
(a) Root bridge:
In the spanning tree, each bridge has a unique ID and a configurable priority
number. The root bridge of the spanning tree is the bridge with the smallest
bridge ID. It continuously transmits network topology information to other
bridges using STP, in order to notify all other bridges on the network when
topoloty changes are required.
The root bridges is where the paths that frames take through the network they
are assigned. It should be located centrally on the network to provide the
shortest path to other links on the network. Unlike other bridges, the root
bridge always forwards frames out over all of its ports.
(b) Root port:
After the root bridge has been chosen, each bridge determines the cost of each
possible path from itself to the root. The bridge picks the one with the
smallest cost. The port connecting to that path becomes the root port of this
bridge.
(c) Designated bridge
After the root bridge has been chosen, the bridges on a network segment
collectively determine which bridge has the least-cost path from the network
segment to the root. This bridge is called the designated bridge. So, for a
bridge on a network or a LAN, its designated bridge is the bridge that provides
the minimum root path cost. The designated bridge is the only bridge allowed to
forward frames to and from the this bridge or the LAN.
(d) Designated port
The port that connects the designated bridge to the network segment is the
designated port for the segment.
(e) Blocked port
Any active port that is not a root port or a designated port is a blocked port.
A blocked port is the port that will not forward frames, in order to prevent
loops. However, a blocked port will alwyas listen to frames. All ports are in
blocking state by default when the switch is powered up.
The decisions of which port is to be blocked and which port is to be put in
forwarding mode are made from the perspective of the root bridge.
References: http://en.wikipedia.org/wiki/Spanning_tree_protocol
http://encyclopedia2.thefreedictionary.com/root+bridge
http://harrychanputra.wordpress.com/2007/05/06/spanning-tree-
protocol-stp/
[Question 6]
In the spanning tree protocol, how does a LAN switch/bridge decide which ports
are in a blocking state
This is the same procedure of selecting the designated port and root port
because when those two are decided, the rest of the them are set to blocking
state.
The bridge ID is used to elect the root bridge in the STP domain as well as to
determine the root port. The priority of each bridge is combined with its MAC
address. If two switches or bridges happen to have the same priority value, the
MAC address becomes the tie breaker for figuring out which one has the lowest
ID.
If only one link is connected to the root bridge, then it needs to be a
designated port. If otherwise, port cost becomes the factor used to determine
the root port/designated port. The path that has a smaller total cost wins.
Other than the root port and the designated port, the rest of them are set to
blocking state.
