10F NET3008 Midterm-SolnsMarks

8/8/2019 10F NET3008 Midterm-SolnsMarks

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2010-10-22 Name: ___________________________

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10F NET3008 Midterm-Solutions

1. Router C is connected to the Frame Relay cloud via multipoint interface S0/0 at an access rate of 512kbps with each location configured using the CIR and BW values shown. Dynamic routing viaEIGRP 10 is being used between the hub at C to the spokes at E through H. The administrator isfinding that EIGRP traffic is occasionally slowing payload traffic to a crawl at router C.

a. What exactly is the problem?- by default, up to half the BW on each spoke can be used for EIGRP traffic- with values configured as shown, the total inbound EIGRP traffic from the spokes toC can be as much as (128 x 3) + 64 = 448 kbps; also, if all VCs were configured at Cseparately with their actual BWs, outbound EIGRP traffic would also be at this rate,rather than being constrained to 256k (half of the 512k we have available)- since 448k approaches the 512 kbps at C, this can choke the traffic there

b. Assuming all devices are connectedto the FR cloud via s0/0, give theIOS commands that would bedirectly relevant to solvingthe problem.

IOS Command Reminders:

bandwidth kbps ip bandwidth-percent eigrp AS-num percent int int-name-type . sub-if-num {point-to-point | multipoint}

- At C, we group like bandwidth VCs under the same sub-if - we configure BW values such that the sum accurately reflects that available at the maininterface (512K on S0/0 = BW of s0/0.1 + BW of s0/0.2)- we also configure EIGRP bandwidth usage on both sub-interfaces to ensure that on anysingle VC we use exactly whats allowable (i.e. 50% of the actual BW available)** recall that the default behaviour at a multipoint i/f (or sub-if), is for EIGRP to simply dividethe total BW by the number of configured VCs and then use up to 50% of that for routing oneach VC** the parent interface is bound by 50% of the sum of the configured BW values on all sub-ifs

C(config)# int s0/0.1 point-to-pointdescr single VC to router H

band 128int s0/0.2 multipoint

descr bundles 3 VCs to routers E, F & G band 384

ip bandwidth-percent eigrp 10 100

E(config)# int s0/0descr single VC to router C

band 128ip bandwidth-percent eigrp 10 100

Also configure at routers F & G, as shown above for router E. No change to router H.

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Mark allocation:1 - sub-if to router H

1 - multipoint sub-if to E, F & G3 x each bandwidth command2 x each ip bandwidth-p cmd1 - indicating same config appliesto E, F & G

Mark allocation:1 mark for each point above


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2. Consider the topology below along with the partial configurations shown for R4. Notice that R4 isparticipating in two separate routing domains, one using OSPF and the other, EIGRP. All addresses inthe OSPF domain are numbered in the 172.16.0.0/16 range, while those in the EIGRP domain arenumbered in the 172.30.0.0/16 range. With all links up, the given configuration provides fullreachability between routing domains as well as the Internet.

a. Exactly how does the manual summary route configured on s0 affect R4s own routing table?

- it will auto-generate a summary discard route for that entire block, with AD=5172.16.0.0/16, egress to null0

b. Briefly explain why the default-info originate command in R4 should be changed to

include the always option. (i.e. Without it, what problem could occur?)

- without always, if connectivity to ISP was lost, the quad-0 route would be removedand R4 would stop propagating default into the OSPF routing domain- this would result in loss of reachability from the OSPF domain to the EIGRP domain

c. Provide a different solution (other than adding the always option, as suggested above).

- introduce a floating static quad-0 so it satisfies OSPF even if the real quad-0 isremoved due to problems at the Internet link; due to its high AD, this route will floatand only be placed in the routing table if the actual Internet route to ISP is lost; it willalso explicitly discard Internet-bound traffic while the ISP link is down

R4(config)# ip route 0.0.0.0 0.0.0.0 null0 250

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Mark allocation:2 marks foreach part


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c. Complete the table below to trace the packet at each step as HostA pings 172.50.205.2.

Packet forwarded to: Explanation: router Admin1 Admin1 configured as gateway for HostA router Border1 since destn network doesnt exist, pkt will follow the default route

being propagated through the EIGRP domain by Border1 router ISP still no route match so pkt follows Border1s quad-0, out s0/0 router Border1 IP destn matches ISPs static 172.50.0.0/16 back to Border1

last 2 steps above, will loop repeatedly until packet TTL expires & its discarded

4. Consider the following hub & spoke topology between the Cisco router at HQ in Ottawa and anumber of small branches in the outlying regions, each of which has a single router. Static routing iscurrently in use, but due to rapid growth in the number of remote branches being added, thecustomer is seeking a solution with less administrative burden.

Below are the static routes that have been configured on the different routers:

Ottawa(config)# ip route 192.168.1.0 255.255.255.0 192.168.0.1Ottawa(config)# ip route 192.168.2.0 255.255.255.0 192.168.0.2Ottawa(config)# ip route 192.168.3.0 255.255.255.0 192.168.0.3

Perth(config)# ip route 0.0.0.0 0.0.0.0 192.168.0.254

Pembroke(config)# ip route 0.0.0.0 0.0.0.0 192.168.0.254

Kingston(config)# ip route 0.0.0.0 0.0.0.0 192.168.0.254

Specify the configuration commands that must be added, changed and deleted in order for you toimplement On Demand Routing for this topology. Also state any assumptions and/or conditionsrequired for its successful operation.

- delete all 6 configuration lines shown above by using the no form of the same command

- add configuration line: Ottawa(config)# router odr

Conditions/Assumptions:- all spoke routers must be Cisco- all spoke routers must have no routing protocol configured- all routers must have CDP enabled

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6 Marks

2 Marks

2 Marks

1 Mark

3 Marks


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5. Consider the command dialog below

R3#sh ip ospf data router 3.3.3.3

OSPF Router with ID (3.3.3.3) (Process ID 1)

Router Link States (Area 5)

LS age: 44Options: (No TOS-capability, DC)LS Type: Router LinksLink State ID: 3.3.3.3Advertising Router: 3.3.3.3LS Seq Number: 80000003Checksum: 0xA053Length: 72Number of Links: 3

Link connected to: a Transit Network(Link ID) Designated Router address: 172.16.17.18(Link Data) Router Interface address: 172.16.17.17

Number of TOS metrics: 0TOS 0 Metrics: 1

Link connected to: another Router (point-to-point)(Link ID) Neighboring Router ID: 9.9.9.9(Link Data) Router Interface address: 172.16.14.91

Number of TOS metrics: 0TOS 0 Metrics: 781

Link connected to: a Stub Network(Link ID) Network/subnet number: 172.16.14.64(Link Data) Network Mask: 255.255.255.192

Number of TOS metrics: 0

TOS 0 Metrics: 781

a. What number of LSA type is represented by this output? LSA Type 1 b. Based on the given output, draw as much of the network as possible. Include the following

information on your diagram, wherever it is known: network prefix, network mask, interfaceIP, router ID, OSPF area, DR/BDR role.

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1 Mark

13 marks, 1 each on diagram as follows:1. Router R3 - 3.3.3.32. connected Ethernet ...3. ... to another router (unknown ID)4. ... which is a DR in that segment-----------

5. R3's IP = 172.16.17.176. other router's IP = 172.16.17.187. in Area 58. 3.3.3.3 also connected over serial link -----------9. to another router10. ... of ID 9.9.9.911. R3's IP = 172.16.14.9112. network ID = 172.16.14.64 - on serial link 13. mask /26 - on serial link

Mark allocation

Note: minus 0.5 mark for eachaspect on diagramnot supported bythe data shown


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6. Your friend Jim who works for ITA has sent you the partial topology diagram below in order to tapinto your OSPF expertise.

He says he has set the router ID of R10 to 255.255.255.255 to ensure it becomes the DR in network 10.2.100.0/24, and also has R11's router ID set to 255.255.255.254 to make it the BDR. However,

he has found that R10 has also become the DR in network 10.2.132.0/22, even though he needs R10to be the BDR there, with R14 being the DR. He would like your advice on his approach and howhe can achieve his goal. What do you tell him?

- [explain why/when RID works ] router ID is only used during DR/BDR elections if all interfacesin a multi-access segment are at equal priority (e.g all are at their default of 1due to nointerface priority having been configured)

- in this case that segments DR will be chosen as the device having the highest router ID(and for BDR, the second highest)- [never count on RID use i/f priority instead ] Jims approach is flawed one should never setrouter ID to influence DR elections because that affects the entire device, regardless of the

segments to which it is connected- [explain why ] since DR roles are by network segment, we can precisely (and flexibly) select asegments DR or BDR by boosting the priority of its connected interface accordingly- [using i/f priority buys you one more thing ] we can even ensure that a device never becomesDR/BDR in a segment by configuring its connected interface priority to zero- one way to achieve Jims goal, is to set interface priorities as follows:

- f0/4 = 10- f0/3 = 5- f0/1 = 10- f0/0 = 5 (in case other routers are later added to this segment)

7. Consider the following categories describing aspects of an area within an OSPF routing domain.

A. Number of ABRs: a ) 1 b ) 2 c) 3 or more d ) not relevant

B. Number of IA routes (if a regular area): e) 5 f ) 50 g) 5,000 h ) not relevant

C. Number of External routes (if a regular area): j ) 5 k ) 50 l) 5,000 m ) not relevant

Choose one item from each of categories A, B & C above, that together, would characterize an areathat's well suited to be configured as a TSA but NOT as a Stub. a, g, j

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4 Marks for solution using i/f priority(only 1 mark if a solution is provided using RID values)

4 Marks


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8.

In a recent lab, we dealt with the topology above, where EIGRP auto-summary was enabled on R1and R4 (but not on R2 and R3). Otherwise, assume that all other issues have been corrected andEIGRP has fully converged with all links up/up.

a. Describe the packet (type, source, dest) that would be generated if we issue the commandR1# ping 10.1.4.1 source loop0. ICMP echo request from 10.1.1.1 to 10.1.4.1

b. Explain in detail why this ping fails, with direct reference to the various 10 routes in thistopology, how they are advertised and adopted (or not) throughout. (i.e. explain step-by-step

what happens, and why it happens)

- auto-summ on R4 has generated a 10/8 route which is advertised out s0/0/0 to R3- this prevents R4 from advertising 10.1.4.0/24 to R3- depending upon link speeds (which affect the route metrics), this 10/8 route could bepropagated by R3 to R2, eventually reaching R1- auto-summ on R1 also auto-generates a 10/8 summary route (advertised to R2)- however, R1 will NOT adopt any 10/8 route received from R2 because it has that sameroute (same prefix/mask) with a lower admin distance (EIGRP summary is 5 versus 90 forany route received from R2)

- therefore, regardless of the link speeds, the only 10 routes in R1 are its connected

10.1.1.0/24 and the 10/8 summary- since the ping destination 10.1.4.1 doesnt match the connected route, the best match isthe 10/8 summary discard- the ping is routed using this route, thereby sending it to null0 (i.e. discarding it) at R1

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c. Assume now we disable auto-summary on R4, but leave it enabled on R1. With the situationchanged, explain again (as in part b.) what happens, and why it happens, when we retry thecommand R1# ping 10.1.4.1 source loop0.

9. - due to auto-summ being off at R4, it is now eligible to advertise its 10.1.4.0/24 network toR3 (as its no longer advertising 10/8 to R3)

- this 10.1.4.0/24 route is propagated by R3 to R2, eventually reaching R1- R1 still has its own 10/8 summary, but 10.1.4.0/24 is NOT the same route, so its adoptedand entered into R1s routing table- now the ping destination of 10.1.4.1 matches this new route, so its followed routing thepacket out s0/0/0 to R2- the echo request packet is then forwarded from R2 to R3, eventually reaching R4- R1, meanwhile, is still advertising the 10/8 summary to R2 (due to auto-summ) whopropagates it to R3, eventually reaching R4 (now that theres no competing 10/8 summarycoming from R4, this is a certainty)- R4 (not having its own 10/8 summary), will adopt this 10/8 route (originated by R1)- so the only 10 routes in R4 are its own connected 10.1.4.0/24 and R1s 10/8 summary

- on the echo reply packet destined for 10.1.1.1, R4s best match is the summary route- the reply packet is sent to R3, forwarded to R2 and eventually reaches R1, so the pingsucceeds!!

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