JAN2012 - Chapter 1 CCNA Exploration 4.0 EWAN Answer and Question

1. Which statement is true about the differences between a WAN and a LAN?
WANs generally support higher bandwidth than LANs support.
A WAN link typically traverses shorter geographic distances than a LAN link traverses.
A WAN often relies on the services of carriers, such as telephone or cable companies, but a LAN does not.
All WAN implementations generally use the same Layer 2 protocol but there are many accepted LAN Layer 2 protocols in use.

2. A U.S. company requires a WAN connection used only to transfer sales data from individual stores to the home office. All transfers will occur after business hours. The required bandwidth for this connection is estimated to be less than 38 kbps. Which type of connection requires the least investment for this company?
ATM
ISDN
analog dialup
T1 Leased Line

3. What are two advantages of an analog PSTN WAN connection? (Choose two.)
low cost
availability
traffic encryption
available bandwidth
support for voice and video

4. Which WAN technology uses a fixed payload of 48 bytes and is transported across both switched and permanent virtual circuits?
ATM
ISDN
Frame Relay
metro Ethernet

5. Which three WAN devices can be found in the cloud? (Choose three.)
ATM switches
core routers
CSU/DSU
Ethernet switches
Frame Relay switches
repeaters

6. Which term describes a device that will put data on the local loop?
DLCI
DTE
DCE
BRI
PRI

7. What is an advantage of packet-switched technology over circuit-switched technology?
Packet-switched networks are less susceptible to jitter than circuit-switched networks are.
Packet-switched networks can efficiently use multiple routes inside a service provider network.
Packet-switched networks do not require an expensive permanent connection to each endpoint.
Packet-switched networks usually experience lower latency than circuit-switched networks experience.

8. Which statement is true about data connectivity between a customer and a service provider?
Normally the CSU/DSU is the designated demarcation point for the service provider but not the customer.
The segment between the demarcation point and the central office is known as the “last mile.”
The local loop is the segment between the CSU/DSU and the serial port on a router.
Putting data on the local loop is the responsibility of the DTE.

9. A company needs a WAN connection that is capable of transferring voice, video, and data at a minimum data rate of 155 Mbps. Which WAN connection is the best choice?
X.25
DSL
ATM
ISDN BRI
ISDN PRI

10. Which statement is true of the functionality of the layers in the hierarchical network model?
The purpose of the access layer is to provide very high bandwidth communications between network devices.
Most security screening to prevent unauthorized entry to the network happens at the core layer.
Untrusted external connections are segmented from the rest of the network at all three levels.
The distribution layer aggregates WAN connections at the edge of the campus.

11. Why is the call setup time of a circuit-switched WAN implementation considered a drawback?
Routing protocols are incompatible with this function.
It restricts the communication sent to voice traffic only.
A telephone must be used to initially start transferring data.
Data cannot be transferred until a circuit has been established.

12. For digital lines, which device is used to establish the communications link between the customer equipment and the local loop?
CSU/DSU
Frame Relay switch
ISDN switch
modem
PBX switch

13. Which packet-switched WAN technology offers high-bandwidth connectivity capable of managing data, voice, and video all on the same infrastructure?
Time Division Multiplexing (TDM)
metro Ethernet
Integrated Services Digital Network (ISDN)
Public Switched Telephone Network (PSTN)

14. Which networking device is typically used to concentrate the dial-in and dial-out traffic of multiple users to and from a network?
core router
access server
Frame Relay switch
ATM switch

15. Which two devices are commonly used as data communications equipment? (Choose two.)
modem
router
CSU/DSU
ISDN switch
Ethernet switch

16. Which two features are identified with Frame Relay connections? (Choose two.)
53-byte cells
DLCI
DSLAM
PVC
SPID

17. Which statement about WAN protocols is correct?
ATM differs from other WAN protocols in that it uses variably sized packets.
Most WAN protocols use HDLC or a variant of HDLC as a framing mechanism.
The frame header consists of the frame check sequence and cyclic redundancy check.
ISDN differs from Frame Relay, HDLC, and ATM in that it is packet-switched rather than circuit-switched technology.

18. Which switching type will allow the communication devices in the provider network to be shared and only allocated to an individual subscriber during data transfer?
circuit-switched
dedicated switched lines
frame-switched
packet-switched

19. What can cause a reduction in available bandwidth on a cable broadband connection?
smaller cells
number of subscribers
committed information rate
distance from the central office of the provider

20. What three terms are associated with ISDN PRI? (Choose three.)
cell
DLCI
circuit switching
packet switching
data bearer channels
time-division multiplexing

21. At which two layers of the OSI model does a WAN operate? (Choose two.)
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Presentation Layer
Application Layer

22. What type of connectivity is established when VPNs are used from the remote site to the private network?
PVCs
DLCIs
tunnels
dedicated Layer 2 links

Update : Links

UCEN 3133 Advanced Computer Networks - Tutorial 5 - Year 2012

Tutorial 5 – Routing and Traffic Engineering

1. Why is conventional IP routing mechanisms such as RIP incapable of supporting constraint based routing (CBR) ? Based on your answer:

     (i)            Explain why CBR routing is supported by a variant of link state routing protocols such as OSPF

   (ii)            Give two reasons why a MPLS network is particularly suited to support CBR

Constraint-based routing requires route calculation at the source as different sources may have different constraints for a path to the same destination, and the constraints associated with a particular source router are known only to that router, but not to any other router in a network. In IP routing such as RIP, every router in a network is involved in computation of a route is in a distributed fashion.

i) Link-State Routing Protocols floods the network with information about all links in the network (in addition to constraint-related information associated with that link), hence allowing the source router to perform the entire route calculation

ii) The LSRs within the core MPLS network only switch on the basis of these labels and no longer make any forwarding decisions of their own based on the IP header (which is what happens in conventional routing).

Mapping between an FEC and an LSP is completely confined to the edge LSR at the head end of the LSP. The decision as to which packets will take a particular explicit route is completely achieved by this edge LSR, noone else is involved.

2. What is the common additional feature used in extending RSVP and LDP to support CBR, and briefly explain how this feature is used as part of CR-LDP ?

The Explicit Route Object (ERO) contains the explicit route that the message has to take. Forwarding of a message containing a ERO by a router is determined not by the IP destination address, but the content of the ERO. The ERO consists of an ordered sequence of “hops,” where the sequence specifies an explicit route and each hop is represented by an “abstract node.”, which is a group of one or more routers

1. A LSR determines the explicit route to be established and constructs a ERO that contains this route.
2. This LSR then constructs a LDP LABEL REQUEST message and includes ERO in this message, and finds the first abstract node in ERO and forwards this message onwards to it.
3. The receiving LSR removes the abstract node associated with it and forwards it onwards to the next  LSR specified in the ERO
4. This process is repeated until it finally arrives at the destination LSR.
5. This LSR then constructs a LABEL MAPPING message and sends it back on the same route – each LSR along the way will use the label to populate its label forwarding message.
6. When it reaches the original LSR, the labels would have established a complete LSP from the original LSR to the destination.

3. List 5 common traffic parameters that are part of the CR-LDP specification and briefly explain their purpose.

• Peak data rate (PDR), Peak burst size (PBS)
• Committed data rate (CDR), Committed burst size (CBS)
• Excess burst size (EBS)

Peak data rate and burst size together define a token bucket, which characterizes the maximum rate of traffic that is expected to be sent down this LSP. Committed data rate and burst size define a token bucket characterizing the average rate at which traffic is expected to be sent on this LSP. Excess burst size defines another token bucket that can be used to characterize the amount by which bursts may exceed the committed burst size.

4. Resource reservation for QOS purposes using RSVP may cause problems in a MPLS network that uses link state routing protocols such as CSPF. Explain briefly the cause of this problem, and how it is addressed.

Establishing a route for a particular traffic flow may require resource reservation along the route using RSVP. Once resources (such as bandwidth) of a link are reserved, the attributes of these links will change (since bandwidth is usually one of the attributes). In link state routing protocols such as CSPF, when the attribute of a given link changes, the node to which the link is connected floods this information throughout the network. Since resource reservation happens quite frequently, flooding also happens frequently which results in high network overhead.

This can be addressed by establishing an upper bound on the frequency of flooding of information when a link attribute changes.

5. List the drawbacks of using RSVP and CR-LDP respectively in a MPLS network.

RSVP makes reservations for individual microflows between single applications, which does not scale well as  the number of microflows in a large IP network is very large. RSVP’s soft state also requires constant refreshing which consumes bandwidth and processing resources.

CR-LDP, by contrast, runs on top of TCP. TCP’s congestion avoidance may limit the transfer of information between LSRs. There is overhead involved in establishing an adjacency between two LSRs because they must go through TCP’s handshake sequence before initiating an LDP session.

6. What is meant by a routing transient and name 2 factors that influence the duration of a routing transient within a network based on conventional IP routing

Routing transients refer to episodes in a network where routing information across a network is changing, mainly due to failures of links or routers or both. At such times, the routing information stored at different routers may be temporarily inconsistent.

The duration of routing transients depends on two factors: - The time it takes for a router adjacent to the failed link (or node) to detect the failure, and the time it takes to distribute this information among all the routers and for them to recompute their forwarding tables based on this information (converge on the failure).

7. What is the motivation behind the use of a protection LSP in a MPLS ? Briefly explain how it is set up and used in that context.

Protection LSPs are used to handle link failures in an MPLS, so that when a link fails, the LSR attached to that link can channel all traffic destined for that link on to the alternative protection LSP.

When a link between two LSRs fail, the information about this failure will be distributed (via OSPF or IS-IS) to all the LSR. Once the original LSR gets this information, it can use constraint-based routing to compute a new route (the protection LSR). Or it may already have a precomputed alternative route

To route traffic onto the new route, the LSR pushes a new label corresponding to the protection LSP into the label stack of all incoming packets, allowing them to be switched over the protection link.

8. The Constrained Shortest Path First (CSPF) algorithm is typically employed in a MPLS network for TE purposes during the construction of routing tables. In the process of constructing a routing table based on CSPF, what are the tiebreaking factors used to arbitrate between two paths of equal hop cost ?                    

For tie break

1. Take the path with the largest minimum available bandwidth.
2. If there is still a tie, take the path with the lowest hop count (the number of routers in the path).
3. If there is still a tie, take one path at random.

9. Consider a MPLS network of routers (A-E) as shown below. The parenthesis pair that labels each link between the routers is used to denote the hop cost and bandwidth available, respectively. For example, the link between A and B has a hop cost of 2 and a bandwidth of 90 Mbps. CSPF is now used to calculate the best path to router D from router A, given a constraining bandwidth of 70 Mbps. Show all steps involved in constructing router A’s table to determine this best path. In your working, show all tentative routes possible and mark them as cancelled if they do not qualify to be used.

Configured constraint 70 Mbps

PATH list	TENT List
{A, 0, self, N/A}

PATH list	TENT List
{A, 0, self, N/A}	{B, 2, B, 90} {C, 8, C, 80} {D, 10, D, 100}

PATH list	TENT List
{A, 0, self, N/A} {B, 2, B, 90}	{E, 3, B, 90} {C, 7, B, 90}  [1] {C, 8, C, 80} -> cancel {D, 10, D, 100} {D, 10, B, 80} -> cancel

PATH list	TENT List
{A, 0, self, N/A} {B, 2, B, 90} {E, 3, B, 90}	{C, 7, B, 90} {D, 10, D, 100} {D, 10, B, 90} -> cancel   [2]

PATH list	TENT List
{A, 0, self, N/A} {B, 2, B, 90} {E, 3, B, 90} {C, 7, B, 90}	{D, 10, D, 100} {D, 8, B, 50} -> cancel    [3]

PATH list	TENT List
{A, 0, self, N/A} {B, 2, B, 90} {E, 3, B, 90} {C, 7, B, 90} {D, 10, D, 100}

[1] {C, 7, B, 90} – The required format is {destination, cost, next hop, minimum bandwidth}. If we take the newly added node to the PATH list {B, 2, B, 90}, this means that the destination is B, cost = 2, next hop to get to B is also B, and minimum bandwidth on all the routes to B is 90. Now when we consider connection to C from B, the entry for the TENT list becomes {C, 7, B, 90}. 90 because if we consider from A->B->C, the minimum bandwidth on all the links encountered is 90. The logic of taking the minimum bandwidth is that the delay along any given path is most affected by the link on that path with the smallest bandwidth. So if a packet was going from A -> C, the slowest  travel time would be from A to B. Therefore, if we have two paths to C with exactly the same cost, then as a tie breaker we would pick the path with the highest bandwidth (remember the rule).

[2] {E, 3, B, 90} -> This means the shortest path to E from A has a total cost of 3, with next hop B, and minimum bandwidth among all the links so far is 90. E has a connection with metric {7,100} to D. So the total cost from A to D now becomes 10. The next hop is still B. The minimum bandwidth is still 90 (because the E-D bandwidth is 100). So, the correct entry becomes {D, 10, B, 90}, which is cancelled because its bandwidth is lower than the existing entry of {D, 10, D, 100} -> remember, in tie breaker we choose the one with the highest bandwidth

[3] {C, 7, B, 90} -> This means the shortest path to C from A has a total cost of 7, with next hop B, and minimum bandwidth among all the links so far is 90. C  has a connection with metric {1,50} to D. So the total cost from A to D now becomes 8. The next hop is still B. The minimum bandwidth becomes 50 (because this is lower than the previous lowest of 90). So, the correct entry becomes {D, 8, B, 50}, which is cancelled because the bandwidth of 50 is less than the initial configured constraint of 70 as given in the question.

10. A multimedia network that provides QOS guarantees uses a leaky bucket policer in one of its routers to ensure that the incoming packet traffic does not exceed the TSpec specification agreed upon during an initial session of Integrated Services (IntServ). The following are the features of this policer:

·         The token buffer can hold at most three (3) tokens, and is initially filled with two (2) tokens at time slot t = 0.

·         New tokens arrive into the bucket at a rate of two (2) tokens per time slot. Packets arrive at the beginning of a time slot and enter the packet queue, where they are processed and transferred to the output link in a First In First Out (FIFO) manner.

·         The size of the packet queue is four  (i.e. it can queue a maximum of 4 packets at any given time slot); any extra arriving packets are dropped.

·         Packets that obtain available tokens in a given time slot go together on the same time slot in the output link.             

Time slot	Incoming Packets
0	A B C D
1	E F
2	G
3	-
4	-
5	H I J K
6	L M N O
7	P Q
8	-
9	R S T

The table shows incoming packets from the network into the router with the policer, from time slot t = 0 to time slot t = 9. Based on this information, construct a new table with columns showing the packets in queue, tokens in bucket and packets on output link from time slot t = 0 to t = 9.                                            

Time slot	Packets in queue	Tokens in bucket	Packets at output
0	A B C D	2	A B
1	C D E F	2	C D
2	E F G	2	E F
3	G	2	G
4	-	3	-
5	H I J K	3	H I J
6	K L M N	2	K L
7	M N P Q	2	M N
8	P Q	2	P Q
9	R S T	2	R S

UCEN 3133 Advanced Computer Networks - Tutorial 2 - Year 2012

Tutorial 2 – MPLS Overview

1. Sketch the diagram of an Edge LSR (Label Switching Router). Identify 3 components that are important with respect to label switching at the periphery of an autonomous MPLS network, and briefly explain how they function.

     (i)            The Forwarding Information Base (FIB) is a standard IP forwarding table extended with labelling information. It allows labelled packets from within the MPLS core network to routed forwards as pure IP packets

   (ii)            Label Information Base (LIB) - holds all labels assigned by this LSR and the  mappings of these labels to labels received from any neighboring LSRs, during the process of label binding exchange.

 (iii)            Label Forwarding Information Base (LFIB) is used during the actual forwarding of labelled packets and holds only labels that are in use currently

2. Explain how labels can be useful as a preliminary measure  towards the enforcement of some form of QOS guarantee in a MPLS, making reference in your answer to Forward Equivalence Class (FEC)

Forwarding Equivalence Class (FECs) is a way of grouping incoming packets in some way so that all packets identified with a particular FEC are treated in the same manner, for example being switched over the same path (LSP). This allows some form of QOS to be enforced on the LSRs of the label switched path (LSP) that the packets flow through. Labels are associated with each particular FEC, and the FEC is established once at the entry of the IP packet into an ingress LSR.

3. Give examples of 3 items that can be used in classifying a Forward Equivalence Class (FEC)

     (i)            A specific source or destination IP network address

   (ii)            A specific class of traffic (e.g. interactive voice, streaming media) data within the packet

 (iii)            The particular interface on a router which the packet arrives on

4. Explain what is meant by label disposition or imposition with respect to an edge LSR.

Label imposition is the act of appending a label, or a stack of labels, to a packet in the ingress point of the MPLS domain. Label disposition is the act of removing the last label from a packet at the egress point before it is forwarded to a neighbor that is outside the MPLS domain.

5. Consider that an egress LSR in autonomous MPLS (AS 1) is communicating with its peer in another autonomous MPLS network (AS 2) using a suitable protocol (BGP for example). The process of negotiation between these peers results in a decision that exiting IP packets with a specific subnet prefix from the egress LSR in AS1 is to be routed along a specific Label Switched Path (LSP) in AS 2. Sketch an outline of how this might be accomplished using labels. What is the advantage of this approach ?

The ingress edge LSR at AS1 negotiates with the egress LSR of AS1, to append a label to all incoming packets into AS1. This label specifies that packets with a specific subnet prefix are meant to be switched immediately onwards to AS2 once they exit  from AS1. The AS1 ingress LSR builds a label stack by pushing another label corresponding to an LSP that terminates at the IP address of the AS1 egress LSR. This packet with two labels is forwarded along a LSP in AS1. All the other LSRs in the LSP in AS1 only perform switching on the top most label in the stack. Finally, at the egress LSR of AS1 this label is popped leaving the bottom label – which is used to forward the packet on to the ingress router of AS2