Prelabs
1.)
mtu <64-18000> (size in bytes)
2.)
The router sends an ARP request to the destination host, if
there is an ARP reply then the host is available. If not, continue to
forward according to routing table.
3.)
Routers.
4.)
30 hops which is the same default used for TCP connections.
5.)
The role of the default gateway is to provide the next-hop IP
address and interface for all destinations that are not located on its
subnet. Without a default gateway, communication with remote
destination
is not possible unless additional routes are added to the IP routing
table.
6.)
192.110.50.0
7.)
Network IP address is the (host ip && subnet mask), and the
network prefix is the first N bits of (host ip && subnet mask), where N
is given by xxx.xxx.xxx.xxx/N.
8.)
1092 networks. The maximum number of hosts for the whole network
number is 2^(32-16) = 2^16 = 65536. And there needs to be 60 hosts on
each network so you divide 65536/60 = 1092.
Prelab 4
1.) The command that configures a Linux PC as an IP router is:
echo "1" > /proc/sys/net/ipv4/ip_forward
2.) The main differences between a distance vector routing protocol and a link state routing protocol
are:
--link-state algorithms send small updates everywhere, while distance vector algorithms send larger
updates only to neighboring routers.
--Because they converge more quickly, link-state algorithms are somewhat less prone to routing loops
than distance vector algorithms.
--link-state algorithms require more CPU power and memory than distance vector algorithms.
--Link-state protocols are generally more scalable than distance vector protocols.
3.) The differences between an intradomain routing protocol (interior gateway protocol IGP) and in
interdomain routing protocol (exterior gateway protocol EGP) are:
--Intradomain Routing Protocols work only within domains.
--Interdomian Routing Protocols work within and between domains.
An example of an intradomain protocol is RIP and OSPF.
An example of an interdomain protocol is BGP.
4.) Zebra supports RIP, GDP, and OSPF routing protocols.
5.) The process zebra updates the routing tables and exchanges routes between different routing
protocols.
6.) The user starts zebra and then the specific routing protocol using for example ‘zebra start’ and then
‘ripd start’. Then he or she telnets into the localhost on the 2602 port using this command ‘telnet
localhost 2602’ and then logs in and it emulates a real router OS such as the Cisco IOS.
7.) RIP 2 enabled RIP messages to carry more information, which permitted the use of a simple
authentication mechanism to secure table updates. More importantly, RIP 2 supported subnet masks, a
critical feature that was not available in RIP.
8.) Passive mode means that the host receives and processes incoming routing messages but does not
transmit routing messages. Active routers advertise their routes (reachability information) to others;
passive routers listen and update their routes based on advertisements but do not advertise (flood).
Typically, routers run RIP in active mode, while hosts use passive mode.
9.) When the RIP sends routing-update when the network topology changes. When a router receives a
routing update that includes changes to an entry, it updates its routing table to reflect the new route.
After updating its routing table, the router immediately begins transmitting routing updates to inform
other network routers of the change. These updates are sent independently of the regularly scheduled
updates that RIP routers send. These independently sent updates are called triggered updates.
10.) Split-horizon is a mechanism that prevents incorrect routing information from being propagated.
The split horizon rule prohibits a router from advertising a route through an interface that the router
itself is using to reach the destination. In other words, its routing technique prevents information from
exiting the router interface from which that information was received. Split-horizon updates are useful
in preventing routing loops. In general, split horizon with poisoned reverse is safer than simple split
horizon. If two gateways have routes pointing at each other, advertising reverse routes with a metric of
16 will break the loop immediately. If the reverse routes are simply not advertised, the erroneous routes
will have to be eliminated by waiting for a timeout. However, poisoned reverse does have a
disadvantage: it increases the size of the routing messages.
11.) Network areas usually are connected to other network areas via routers, making up a single
autonomous system. An autonomous system is a collection of networks under a common
administration sharing a common routing strategy. Autonomous systems are subdivided by areas. In
other words, an autonomous systerm (AS) is a collection of IP networks under control of a single
entity, typically an Internet Serivce Provider or a very large organization with redundant connections to
the rest of the internet. A unique AS number is allocated to each AS for use in BGP routing.
numbers are assigned by the same authorities that allocate IP addresses.
AS, Stub AS, and Transit AS.
12.) UCI’s AS # is Number 2. NACS.uci.edu has AS Number 1.
The
Types of AS are Multihomed
13.) A Stub AS is only connected to one other AS. For routing purposes, it could be regarded as a
simple extension of the other AS. In fact, most networks with a single Internet connection don't have a
unique AS number assigned, and their network addresses are treated as part of the parent AS.
A Transit AS has connections to more than one other AS and allows itself to be used as a
conduit for traffic (transit traffic) between other AS's. Most large Internet Service Providers are transit
AS's.
A Multihomed AS has connections to more than one other AS, but does not allow transit traffic to pass,
though its interior hosts may route traffic through multiple AS's. This is the typical configuration for a
large corporate network with multiple redundant Internet connections, but which does not wish to pass
traffic for others.
Prelab 6
1. Routers - Each host's IP address must be configured. If network is reconfigured, IP addresses may
need to be reassigned. Routing done via RIP or OSPF. Each router manipulates packet header. Routers
operate on the network layer.
Bridges - MAC addresses are hardwired. No network configuration needed. No routing protocol
needed, but learning bridge and spanning tree algorithm. Bridges do not manipulate frames. Bridges
operate on the data link layer.
2. When a hub receives a packet (chunk) of data (a frame in Ethernet lingo) at one of its ports from a
PC on the network, it transmits (repeats) the packet to all of its ports and, thus, to all of the other PCs
on the network. If two or more PCs on the network try to send packets at the same time a collision is
said to occur.
When that happens all of the PCs have to go though a routine to resolve the conflict.
The process is proscribed in the Ethernet Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) protocol.
An Ethernet switch automatically divides the network into multiple segments, acts as a high-speed,
selective bridge between the segments, and supports simultaneous connections of multiple pairs of
computers which don't compete with other pairs of computers for network bandwidth.
accomplishes this by maintaining a table of each destination address and its port.
It
When the switch
receives a packet, it reads the destination address from the header information in the packet, establishes
a temporary connection between the source and destination ports, sends the packet on its way, and then
terminates the connection.
3. In Transparent Bridging, end stations are totally unaware that a bridge interconnects the two LAN
segments, and it is invisible to the operation of the LAN. Transparent bridges perform no protocol
translation (such as from Ethernet to FDDI, or to Token Ring), and they do not calculate paths through
the network to any end node. Bridges make their forwarding decisions based on the data link layer's
MAC address. Transparent bridging also employs learning to associate MAC addresses with ports.
4. The role of the Spanning Tree Protocol is to prevent loops in the topology. How it works is as
follows: One bridge, called the root bridge is elected to be the root of the tree. Each bridge determines
which of its ports has the best path to the root bridge. On each LAN, the bridges elect one bridge,
called the designated bridge, which, among all bridges on the same LAN, has the best path to the root
bridge. Then, all bridges disable all ports that are not root ports or designated ports. What results is a
spanning tree of bridges. Since a tree topology does not have a loop, forwarding along these routes
guarantees that loops are entirely avoided.
5. a. Root bridge - A bridge that is elected as the root of the tree, where all data is forwarded to.
b. Root port - the port on a bridge where data will move closest to the root bridge.
c. Designated bridge - On each LAN, the bridges elect a designated bridge, which has the best path
to the root bridge.
d. Designated port - The port that connects a bridge to the LAN where it is a designated bridge.
e. Blocked port - A disabled port on a bridge, because it is not a root port or a designated port.
6. A port on a bridge is disabled if it is not a root port or a designated port.