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Q. Describe why postal addresses and telephone numbers are routable.
A postal address has three components that can be used to deliver mail: state, city, and street. A phone number has an area code and exchange. At the core layer, mail can be delivered to the next post office based on only the state or city and state information. A phone number is delivered at the core layer based on the area code.
Q. What is the purpose of a default route?
A default route is used if there is not a specific entry in the routing table for the destination.
Q. Describe the difference between routing and switching.
Routing moves a letter or telephone call to the access layer (as in a street or telephone exchange). Switching makes the final delivery. A switching decision is made on the part of the address that is not used in routing (as in the street number or last four digits of a phone number).
Q. What does the term information hiding mean in relation to route summarization?
At the core layer in the postal system, the only information that is needed to make a routing decision is the state or city/state information. The specific street names and street numbers are hidden, the core layer does not need this information. At the core layer in the telephone system, the area code is used to make a routing decision. The specific exchange or last four digits of the phone number are not needed, or hidden, from the core layer.
Q. How does the use of a hierarchical routing structure (access, distribution, and core) enable a scalable delivery system?
If a delivery system is not divided into access, distribution, and core layers, every point in the system needs to maintain every possible destination address to make a delivery decision. The use of a layered system means each layer needs only the information necessary to deliver to the next layer, either above or below.
Q. Why are multiple protocols used, such as a package, addressing, delivery, and transportation, instead of using one protocol defining everything?
Using multiple protocols is modular and allows changes to one protocol without affecting the others. For example, if the addressing protocol is dependent on the delivery protocol, changes to one would imply changes need to be made to the other.
Q. Can you think of another familiar system that routes using a hierarchical delivery system?
The airport system. At the core routing level, there are major hub airports such as Denver, Chicago, New York, and Atlanta. The core airports are responsible for routing people and cargo to major geographical areas. Core airports connect with regional airports that serve a specific area; regional airports are at the distribution layer. Finally, to reach your final destination, you can take a bus, a cab, a train, or rent a car. This can be considered the access layer.
Q. Explain how a letter from New York City to San Diego is routed using the address information.
The source address is not used unless the letter needs to be returned to the sender. Using the destination address, the access level post office in New York examines the state, city, and street information to determine if it is directly connected to the destination. If not, the letter is sent to the distribution layer post office using a default route. The distribution layer post office also examines the state, city, and street information to determine if it is directly connected to an access layer post office servicing the particular street. If it isn’t, the letter is routed to the core level using a default route. The core level post office examines the state name, and if the state name does not equal New York, the letter is delivered to the core post office for the state of California. The California core post office delivers the letter to the distribution post office that handles the city of San Diego. The San Diego distribution post office delivers the letter to the access post office that handles the destination street. Finally, the access level switch delivers the letter to the proper destination.
Q. What are the access, distribution, and core components of a postal address?
The street name and number are the access layer components. The city name is the distribution layer component. The state name is the core layer component.
Q. What are the access, distribution, and core components of a North American phone number?
The last four digits are the used at the access layer to identify a particular telephone. The next three numbers are used at the distribution layer to identify an exchange that services several phone numbers. The area code is used at the core level for routing between different regions.
This is a speed drill. Using only your head, convert the following binary numbers to decimal. 11100000
224, 252, 128, 240, and 63 (64 – 1)
Q. Convert FACE1234 16 to dotted decimal.
FA 16 = 250 10, CE16 = 20610, 1216 = 18 10, and 34 16 = 52 10
FACE1234 16 = 250.206.18.52 dotted decimal
Q. Convert 10100010111101011001110110001011 2 to hexadecimal.
Q. Convert 101000101111010110011101100010112 to dotted decimal.
Convert to dotted hexadecimal first, and then convert each hexadecimal number pair to decimal. A2.F5.9D.8B then 184.108.40.206
Q. Convert 101000101111010110011101100010112 to octal.
Q. True or False. Converting between number bases is fun.
Answer is True
Q. Convert 12345670 8 to hexadecimal.
Convert each octal digit into three binary digits, and then convert the binary result to hexadecimal. 001 010 011 100 101 110 111 000
0010 1001 1100 1011 1011 1000
Q. Convert 734215 8 to binary.
111 011 100 010 001 101
Q. Convert 734215 16 to binary.
0111 0011 0100 0010 0001 0101 2
Q. What is the broadcast address for network 220.127.116.11/23?
Q. Subnet 18.104.22.168/24 into the maximum number of networks that can support 28 hosts each.
22.214.171.124/27 and the subnets are: 126.96.36.199/27
Q. What is the broadcast address for network 188.8.131.52/27?
Q. What is the prefix and subnet mask that summarizes the following networks:
Q. Using the following routing table, determine the best route to reach the host at address 184.108.40.206. Network Output Interface
220.127.116.11/8 Serial 0
18.104.22.168/11 Ethernet 1
22.214.171.124/22 Ethernet 2
126.96.36.199/11 because it matches more network bits than 188.8.131.52/8. Network 184.108.40.206/22 and 220.127.116.11/22 do not match on the network address.
Q. What is the range of host addresses for network 172.16.53.96/27?
172.16.53.97 through 172.16.53.126
Q. How many two-host subnets can be made from a /24 network?
64 because 2 bits are needed for the hosts on the network, leaving 6 bits for the subnet.
Q. What is the full IPv6 address represented by FF02::130F:5?
Q. Explain the difference between a classful and a classless routing protocol.
Classless routing protocols advertise subnet mask information along with the network prefixes. Classful routing protocols do not. Therefore, for a classful protocol, all subnets for the major network number being used must be the same length. Also, classful protocol cannot support discontiguous networks prefixes.
Q. What are the states that an EIGRP route can be in and what do these states mean?
The passive state means that a router has a successor for a route. The active state means that a router does not have a successor or feasible successor for a route and is actively sending queries to neighbors to get information about the route.
Q. Explain the relationship between reported distance and feasible distance and how they determine successors and feasible successors.
The reported distance to a route that is sent to another router is the feasible distance on the reporting router. Feasible distance is the reported distance plus the metric between the receiving and reporting routers. The route with the lowest feasible distance is the successor. Any routes with a reported distance that is less than the feasible distance are feasible successors.
Q. Determine the EIGRP command to summarize the following networks: 10.1.0.0/19
You need to examine the third byte because that is the byte where the four prefixes differ: 0 = 0 0 0 0 0 0 0 0
32 = 0 0 1 0 0 0 0 0
64 = 0 1 0 0 0 0 0 0
96 = 0 1 1 0 0 0 0 0
The last 7 bits are irrelevant, so the mask is 1 0 0 0 0 0 0 0 and the EIGRP command is ip summary-address eigrp 1 10.1.0.0 255.255.128.0.
Q. What are the purposes of areas in an OSPF network?
Areas allow the design of a hierarchical network. Routes can be summarized or blocked in an area to reduce the amount of routing information on internal OSPF routers.
Q. Why are intra-area summary routes not allowed?
OSPF databases on routers in the same area must be identical. If route summarization was allowed within an area, some routers would have specific routes and some routers would have summary routes for routes in the area. If this were allowed, the databases for the area would never agree.
Q. What types of routes are allowed into a stub area?
OSPF intra-area and interarea routes, and a default route. External routes are not advertised into a stub area.
Q. What types of routes are allowed into a totally stubby area?
OSPF intra-area routes and a default route. OSPF interarea and external routes are not advertised into a totally stubby area.
Q. What types of routes are allowed into a NSSA?
OSPF intra-area and interarea routes, and possibly a default route. External routes from ABRs are blocked, and external routes from ASBRs are converted to N1 or N2 routes.
Q. What types of routes are allowed into a totally NSSA?
OSPF intra-area routes and a default route. External routes from ABRs are blocked, and external routes from ASBRs are converted to N1 or N2 routes.
Q. What is the difference between an E1 and E2 OSPF route?
An E1 route contains the OSPF cost to reach the ASBR plus the cost from the ASBR to the external route. An E2 route contains only the cost from the ASBR to the external route.
Q. What are the three types of OSPF routers?
ABR, internal router, and ASBR.
Q. What are the six OSPF route types?
Intra-area, interarea, E1, E2, N1, and N2.
Q. Where can routes be summarized in an OSPF network?
OSPF routes are summarized on an ABR. External routes are summarized on an ASBR.
Q. How is the OSPF router ID determined?
If physical interfaces are only used, the OSPF router ID is the highest IP address assigned to an active physical interface. If loopback interfaces are used, the OSPF router ID is the highest IP address assigned to a loopback interface. If the router-id command is used with the OSPF configuration, the address used with this command will be the router ID.
Q. How does OSPF determine the DR on a multi-access network?
The router with the highest interface priority will be the router ID. If all the interface priorities on the multi-access network are the same, the router with the highest router ID will be the DR.
Q. How does OSPF determine an interarea shortest path?
First, calculate the shortest path to an ABR.
Second, calculate the shortest path across area 0 to an ABR that is attached to the destination area. Third, calculate the shortest path across the destination area from the ABR to the destination network.
Q. What is the purpose of an OSPF virtual link?
To connect a nonzero area to the backbone if the nonzero area becomes disconnected from the backbone. A virtual link can also be used if the backbone, or area 0, becomes discontiguous.
Q. How is the OSPF cost of an interface calculated?
By default, the cost of an OSPF interface is 100,000,000/(Interface Bandwidth). The constant 100,000,000 can be changed using the auto-cost reference-bandwidth command.
Q. The following OSPF routes originate in OSPF area 1: 18.104.22.168/28
Q. What is the command to summarize these routes on the ABR between area 1 and the backbone?
Area 1 range 22.214.171.124 255.255.255.192
Q. How many OSPF databases are on an OSPF router?
The number of OSPF databases on a router is equal to the number of OSPF areas configured on the router.
Q. What is the administrative distance of OSPF routes?
Q. If a router learns about the same network prefix through RIP, IGRP, EIGRP, and OSPF, which route will be preferred?
EIGRP has an administrative distance of 90. IGRP has an administrative distance of 100. OSPF has an administrative distance of 110. RIP has an administrative distance of 120. Therefore, the EIGRP route is preferred.
Q. Describe the structure and format of an NSAP address.
An NSAP address has a length of 8 to 20 bytes and consists of three components:
One to 13 byte area ID
Six byte system ID
One byte NSAP selector that is always equal to zero for a router
Q. Assume a router has a loopback address of 126.96.36.199. Convert the loopback address to an IS-IS system ID.
The loopback address written in dotted decimal and using three digits for each byte has a value of 135.077.009.254. The system ID is 13.50.77.00.92.54.
Q. Describe the difference between an OSPF and IS-IS backbone.
OSPF has a backbone area or area 0. All nonzero areas must be connected to the backbone through a router or a virtual link. IS-IS has a backbone area made up of a contiguous chain of Level 2 capable routers.
Q. In IS-IS, what does Level 1 routing mean?
Level 1 routing is routing between destinations in the same IS-IS area
Q. What is the OSPF counterpart to Level 1 routing?
Q. In IS-IS, what is the function of a Level 1-2 router?
A Level 1-2 router has two IS-IS databases. The Level 1 database is used for routing to destinations within the router’s configured area. The Level 2 database is used to route between destinations in different areas.
Q. What is the OSPF counterpart to a Level 1-2 IS-IS router?
An Area Border Router (ABR).
Q. What is the difference between the types of routes allowed by default into IS-IS and OSPF areas?
By default, all routes are advertised into all OSPF areas. This includes interarea OSPF routes and external routes that have been injected into OSPF. By default, IS-IS does not advertise interarea or external routes into an area, but injects a default route.
Q. What is route leaking?
Redistribution of Level 2 routes into an area as Level 1 routes.
Q. Compare IS-IS metrics with OSPF metrics.
An OSPF interface metric is determined from the interface bandwidth. By default, all IS-IS interface metrics are equal to 10.
Q. What is the difference between an IS-IS narrow and wide metric?
A narrow metric uses 6 bits for the interface metric and 10 bits for the path metric. A wide metric uses 24 bits for the interface metric and 32 bits for the path metric.
Q. Describe four differences between the operation of IBGP and EBGP.
IBGP is the protocol used between routers in the same autonomous system. EBGP is the protocol used between routers in different autonomous systems.
IBGP routes must be synchronized before they can be transferred to the IP routing table (unless synchronization is disabled).
EBGP sets the next hop attribute to the IP address of the interface used to communicate with the EBGP peer. The next hop attribute is not modified when an IBGP router advertises a prefix to an IBGP peer if the prefix was learned from an EBGP neighbor.
EBGP advertises all prefixes learned from an EBGP neighbor to all other EBGP neighbors. IBGP routers do not advertise prefixes learned from one IBGP neighbor to another IBGP neighbor.
Q. What is the purpose of the AS_PATH attribute?
If a router has more than one route to the same IP prefix, the best path is the one with the shortest AS_PATH (assuming other BGP attributes are equal).
Q. What is the purpose of the WEIGHT attribute?
If a router has more than one route to the same IP prefix, the best path is the one with the highest WEIGHT value.
Q. What is the scope of the WEIGHT attribute?
WEIGHT has only local significance and is not advertised to BGP peers.
Q. What is the purpose of the LOCAL_PREF attribute?
If a router has more than one route to the same IP prefix, the best path is the one with the highest LOCAL_PREF (assuming the WEIGHT attribute for the routes is equal).
Q. What is the scope of the LOCAL_PREF attribute?
The LOCAL_PREF attribute is advertised throughout the autonomous system.
Q. What is the purpose of the metric or MULTI_EXIT_DISC attribute?
MED is used to prefer a path into an autonomous system. A lower MED value is preferred.
Q. What is the order of preference for the BGP attributes AS_PATH, LOCAL_PREF, MED, and WEIGHT?
WEIGHT, LOACL_PREF, AS_PATH, MED
Q. Name all the ways for installing a prefix in the BGP routing table.
Using the network command to transfer a router from the IP routing table to the BGP routing table
Redistributing routes from the IP routing table to the BGP routing table
Learned from a BGP neighbor
Q. What is the first thing that BGP checks to determine if a prefix is accessible?
BGP checks the NEXT_HOP attribute to determine if the NEXT_HOP is accessible or in the IP routing table.
Q. What is synchronization?
Synchronization is a property of IBGP. An IBGP router will not accept a prefix received from an IBGP neighbor if the prefix is not already in the IP routing table.
Q. Name two methods for reducing the number of IBGP connections.
Route reflector and confederation.
Q. Why does IBGP require a full mesh?
BGP uses the AS_PATH attribute for loop detection. If a router sees its own AS number in a BGP advertisement, the advertisement is dropped. IBGP routers have the same AS number so the AS number cannot be used for loop detection. IBGP neighbors will not advertise prefixes learned from one IBGP neighbor to another IBGP neighbor; therefore, a full mesh is required.
Q. What is the major difference between BGP and IGP route summarization?
When a summary address is created with an IGP (EIGRP, OSPF, and IS-IS), the specific routes of the summary are not advertised. BGP advertises the summary, and all the specific routes of the summary unless they are specifically suppressed.
Q. Why is BGP a better choice for Internet routing than the IGPs?
RIP version 1 and IGRP are classful protocols and do not advertise subnet mask information. RIP version 2 has a limited network diameter of 15 hops. EIGRP, OSPF, and IS-IS use computational intensive algorithms for determining a shortest path. BGP relies on simple techniques for best path selection and loop detection, and can handle the number of network prefixes required for Internet routing.
Q. What are the four general types of BGP attributes?
Well-known mandatory, well-known discretionary, optional transitive, and optional nontransitive.
Q. Explain the difference between the forwarding of a unicast IP packet and the forwarding of a multicast IP packet.
Unicast IP packets are forwarded based on the destination IP address. Multicast packets are forward based on the source IP address. If a multicast packet is received on the interface used to send a unicast packet back to the source, the multicast packet is forwarded to multicast neighbors. If the multicast packet is received on an interface that would not be used to send a unicast IP packet back to the source, the packet is discarded.
Q. Explain the difference between dense mode and sparse mode multicast.
Dense mode multicast assumes all multicast neighbors want to receive all multicast traffic unless the neighbors have specifically pruned the traffic. Sparse mode multicast assumes multicast neighbors do not want to receive multicast traffic unless they have asked for it. Dense mode uses source-based delivery trees while sparse mode uses shared delivery trees where traffic is first sent to an RP.
Q. What is the multicast Ethernet address for IP address 188.8.131.52?
The base Ethernet multicast address is 01 00 5E 00 00 00. The first byte of the IP multicast address is not used. If the second byte is greater than 127, subtract 128, giving a value of 0. The third and fourth bytes of the IP address are used as is after converting to hex. Their values, in hexadecimal, are 40 and 0C. So the Ethernet multicast address for the IP multicast address 184.108.40.206 is 01 00 5E 00 40 0C.
Q. Determine at least four IP multicast groups that have the multicast Ethernet address of 01 00 5E 00 40 0C?
The low order 32 bits of the IP address determine the multicast Ethernet address. The first four bits are always 1 1 1 0 and the next five bits can be anything. Therefore, the IP multicast addresses that map to the multicast Ethernet address of 01 00 5E 00 40 0C are
1110 0000 0000 0000 0100 0000 1100 = 220.127.116.11
1110 0000 1000 0000 0100 0000 1100 = 18.104.22.168
1110 0001 0000 0000 0100 0000 1100 = 22.214.171.124
Q. Why are the Cisco multicast routing protocols referred to as protocol independent?
Multicast forwarding decisions are based on the entries in the unicast IP routing table. Multicast is not dependent on how the unicast IP routing table was built; you can use any dynamic interior routing protocol, static routes, or a combination of the two.
Q. What protocols do switches use to prevent the broadcasting of multicast traffic?
CGMP and IGMP Snooping
Q. Describe the operation of Anycast RP.
Two or more RPs are configured with the same IP address. The IP addresses of the RPs are advertised using a unicast IP routing protocol. Each multicast router chooses the closest RP. If an RP fails, the routers switch to the next nearest RP after the unicast IP routing protocol converges. The MSDP is used between RPs to exchange active multicast source information.
Q. What is the range of IP multicast addresses?
Q. What is the purpose of the interface command ip multicast spares-dense-mode?
Used with PIMSM Auto-RP and version 2. If the RPs fail, the router reverts to dense mode.
Q. Explain the function of a rendezvous point?
A RP is the focal point for multicast traffic. Traffic is forwarded to the RP from multicast sources. The RP then forwards traffic to multicast receivers.
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