Routing and Switching posted by
Sachin Jain
What do you mean by the term
‘routing’? What a router must know to route a packet?
Comment on Static Routing, Default
Routing and Dynamic Routing.
How many classes of routing protocols
are there. Describe each?
Why do Routing Loops occur and how to
overcome them?
Define OSPF and what features are
provided by OSPF
Define Routing Information Protocol
(RIP) and what is the difference between RIPv1 and RIPv2?
What is IGRP? Differentiate between
IGRP and RIP.
What is EIGRP and what conditions are
needed to neighbourship establishment in context to EIGRP?
OSPF is supposed to be design in a
hierarchical fashion; what are the reasons for creating OSPF in a hierarchical
fashion?
What do you mean by ‘Switching’?
What are the services provided by Switching?
What is Spanning Tree Protocol (STP)
and define some STP terms?
Three switch functions are address
learning, forward/filter decision, loop avoidance. Define these functions.
How STP works and what is the
purpose for STP?
Define Spanning-Tree port states.
What are Manageable and unmanageable
switches? What is the advantage of manageable over unmanageable?
If your routing table has a static,
a RIP, and an IGRP route to the same network, by default which route will be
used to route packets?
If a switch receives a frame and the
source MAC address is not in the MAC address table but the destination address
is, what will switch do with the frame?
If you want to improve switched
network performance by increasing the bandwidth available to hosts and limit
the size of broadcast domains. Which of the following options will achieve this
goal—(i) Bridges, (ii) Switches or (iii) Switches configured with VLANs?
If two connected routers are
configured with RIP routing. What will be the result when a router receives a
routing update that contains a higher-cost path to network already in its
routing table?
What do you mean by the term ‘routing’? What a router must know
to route a packet?
‘Routing’ is used to deliver a packet from one device to another device through
communication network. Routing is performed by the router and each router
maintains a routing table. A routing table contains the information of the best
possible paths from source router to the destination router.
A router must know the following to route a packet:
a. Address of the destination
b. Neighbor routers
c. Routes to all remote networks
d. The best route to each remote network
e. Way to maintain and verify routing information
Comment on Static Routing, Default Routing and Dynamic Routing.
To route a packet over the communication network, a network administrator has to
configure a router. These configurations are of three types:
Static Routing: A network administrator manually configures
the routes for a router. A static route has higher priority than a dynamic
route.
Default Routing: Default routing is used only for the network
that has only a single connection to router. Static routes are also manually
configured.
Dynamic Routing: Dynamic routing used various routing
protocols to route packets. A route is automatically updated as a topology
change occurs. Dynamic routing is easier than static and default routing.
How many classes of routing protocols are there. Describe each?
There are three classes of routing protocols: Distance vector, Link State and
Hybrid
Distance Vector: This routing protocol discover the best path
to a remote network by judging distance. This type of protocols counts the hop;
hops are the number of the routers from which a packet goes. The vector points
to the direction of remote network.
Example: RIP and IGRP
Link State: A router send updates containing the state of
their own link to the other routers. This is also called shortest path first
protocol. Three different tables are maintained by the router using this
protocol. One is routing table, one of directly attached neighbors and one for
the topology of entire internetwork. Link state enables a router to know about
internetwork.
Example: OSPF
Hybrid: Hybrid protocols contain the features of both Distance
vector and link state protocols.
Example: EIGRP
Why do Routing Loops occur and how to overcome them?
a) Routing loop is common problem of various types of networks. Distance vector
routing protocol keeps track of any change to the internetwork by broadcasting
periodic updates. This works okay but if a network outage happens then routing
loops occurs in the network. The main reason to occur routing loops is that
each router is not updated at the same time and the fake information of a
router’s link broadcasted.
Three rules for distance vector routing protocol are developed to overcome
routing loops:
i. Split Horizon: According to this rule, never broadcast a route out of the
interface through which it received. It helps to avoid loops between adjacent
routers.
ii. Route Poisoning: When a network goes down than the router associated to
that network initiates route poisoning. This shows an unreachable network.
iii. Holddown timer: Holddown timer says that if a route advertised as down, do
not listen to routing updates from that route for a specified period of time.
Define OSPF and what features are provided by OSPF?
OSPF is abbreviated as Open Shortest Path First. OSPF is an open standard and
supported by a large variety of network vendors, including Cisco. OSPF works on
the concept of Dijkstra algorithm in which a shortest path is maintained and
routing table is populated on that path.
Some features of OSPF are:
i. Minimizes routing update traffic
ii. Open standard routing protocol
iii. Unlimited hop count
iv. Routing update traffic is lesser than any other routing protocol.
Define Routing Information Protocol (RIP) and what is the
difference between RIPv1 and RIPv2?
RIP is a distance vector routing protocol which uses hop count to find the best
way to a remote network. RIP sends its complete routing table to neighbor
router in every 30 seconds. RIP uses maximum 15 hop count and is suitable only
for small networks.
RIPv1 uses classful routing that means all the devices in the network must use
the same subnet mask and do not send subnet mask information with routing
updates. While RIPv2 uses classless routing and subnet mask information is sent
with routing update.
What is IGRP? Differentiate between IGRP and RIP.
IGRP is abbreviated as Interior Gateway Routing Protocol. This is created by
Cisco and all the routers must be of Cisco to run this protocol. IGRP is
developed to overcome the problem with RIP. RIP can be used only for small
network because it uses maximum 15 hop-counts while IGRP can be used for bigger
networks because it uses 255 hop-counts.
i. IGRP uses autonomous system number and supplies this number to all routers
while in RIP there is no autonomous number.
ii. IGRP updates its Routing table every 90 seconds and RIP updates its routing
table in every 30 seconds.
iii. IGRP has administrative distance of 100 while RIP has 120.
iv. IGRP uses maximum 255 hop-counts while RIP uses 15 maximum hop-counts.
What is EIGRP and what conditions are needed to neighbourship
establishment in context to EIGRP?
EIGRP is abbreviated as Enhanced Interior Gateway Routing Protocol. EIGRP is a
proprietary Cisco protocol that runs only on Cisco routers. EIGRP is a popular
routing protocol now days. EIGRP is a classless, enhanced distance routing
protocol.
Before EIGRP routers wants to exchange routes, they have to establish a
neighborhood relationship. To establish this neighbourship three conditions
must meet:
i. Autonomous System Numbers must match
ii. Hello packet must received
iii. Metrics value must identical
OSPF is supposed to be design in a hierarchical fashion; what are
the reasons for creating OSPF in a hierarchical fashion?
OSPF (Open Shortest Path First) is supposed to be design in hierarchical fashion
so that a large internetwork could be break into smaller network. These smaller
networks are called areas.
Following are some reasons for creating OSPF in hierarchical design:
i. To reduce the routing overhead
ii. To break a bigger internetwork into smaller internetworks caller areas
iii. To accelerate convergence
What do you mean by ‘Switching’? What are the services provided
by Switching?
Generally switching refers to the layer 2 switching. Switching is a process
which uses the hardware address or MAC address of a device to switch a packet
from one device to another.
Services provided by switching are:
i. Switches use Application-Specific Integrated Circuits (ASICs) to make and
maintain their filter table.
ii. Time required to transfer a packet is low i.e. low latency
iii. Cost is low
What is Spanning Tree Protocol (STP) and define some STP terms?
The main function of STP is to prevent the network loops occurring in switching
network. STP monitors the network to find all links and shut down the redundant
links. For this STP uses Spanning tree algorithm.
Some STP terms are:
Root Bridge: To select a root bridge an election is done by
all switches in a switching network. All the decisions like- which port is to
be blocked and which port is forwarding are made from the viewpoint of Root
Bridge.
BPDU: BPDUs (Bridge Protocol Data Unit) are sending from one
switch to another to elect a root bridge.
Bridge ID: Bridge ID is an identifier of a Switch.
Three switch functions are address learning, forward/filter
decision, loop avoidance. Define these functions.
Address Learning: In Layer 2 switching, each interface of a
switch learns the source hardware address (MAC address) and save this into its
MAC database table. This table is also known as forward filter table.
Forward/filter decisions: Forward/filter decision is taken by
the switch to forward a frame to a specific destination port. The frame which
comes on an interface, switch sees its MAC address table and forward this frame
to a specific destination port.
Loop Avoidance: Network loops can arise if multiple
connections are formed between switches for redundancy purposes. TO prevent
network loops STP (Spanning Tree Protocol) is used and redundancy is also
maintained.
How STP works and what is the purpose for STP?
Firstly STP elects a Root Bridge and forward to all ports and this root bridge
acts as a point of reference for all other devices in STP domain. When all
switches agreed on root bridge, every switch must find its one and only
allotted root port. Each and every link between switches must have one and only
one designated port in such a way that it must provide the highest bandwidth to
link.
The main purpose for STP is to prevent the network loops.
Define Spanning-Tree port states.
The port of switches running STP can has five different port state:
- Blocking: A blocking port only listens to BPDUs while cannot forward frames.
- Listening: Listening port only listen to BPDUs and preparing to forward
frames.
- Learning: The switch listen to BPDUs and learns all the paths in the LAN
network.
- Forwarding: In this port state, switch port can forward and receive the
frames.
- Disabled: In disabled state a switch port is administratively down and that
port do not participate frame forwarding.
What are Manageable and unmanageable switches? What is the
advantage of manageable over unmanageable?
Manageable Switch: Managed switches allows the layer 3
functionality and can be used as a router. It has own IP address and can be
easily configured. It also has ability to traffic control, port blocking and
VLANs configuration.
Unmanageable Switch: Unmanageable switches are layer 2
switches and learn only hardware address (MAC address). It learns the MAC
address of all the connected devices and when a frame came from some source it
broadcast it to all ports and throws frame to destination (by using its MAC
table).
Advantages of manageable over unmanageable are:
i. IP address can be assigned to manageable switch and can it can be access
through ‘telnet’ command.
ii. More secure than unmanageable because any port can be block at any time.
iii. Managed switch has router like capabilities.
iv. Managed switch can manage the bandwidth of link.
If your routing table has a static, a RIP, and an IGRP route to
the same network, by default which route will be used to route packets?
Administrative distance rates the trustworthiness of the any routing protocol.
AD value is an integer from 0 to 255 and trustworthiness increases with the
increasing order of the AD value.
Static route will be used to route packets. Static routes have an
administrative distance of 1 by default. IGRP has an administrative distance of
100, and RIP has an administrative distance of 120, by default.
If a switch receives a frame and the source MAC address is not
in the MAC address table but the destination address is, what will switch do
with the frame?
Since the source MAC address is not in the MAC address table, firstly the switch
will add the source MAC address and the port it is connected to into its MAC
address table and then forward the frame to the outgoing port and frame reached
to the right destination address.
If you want to improve switched network performance by
increasing the bandwidth available to hosts and limit the size of broadcast
domains. Which of the following options will achieve this goal—(i) Bridges,
(ii) Switches or (iii) Switches configured with VLANs?
Switches configured with VLANs will improves the network performance. By
creating and implementing VLANs in our switched network, we can break up
broadcast domain and limit the size of broadcast domain. VLANs greatly enhance
network security which also improves the switched network performance.
If two connected routers are configured with RIP routing. What
will be the result when a router receives a routing update that contains a
higher-cost path to network already in its routing table?
When a routing update is received by a router, the router first checks the
administrative distance (AD) value and always choose the route with the lowest
administrative distance value. However, if two routes are received and they
both have same administrative distance value, then the router will choose the
one route with the lowest metrics, or in RIP’s case, hop count.
RIP allows a maximum hop-count of 15, so anything that requires 16 hops is
consider as unreachable. Hence if a router receives a routing update that
contains a higher-cost path but have lower hops than the packet will be
transferred through that path.
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