“Do I Know This Already?” Quiz
3. a, b, c
4. a, d
5. a, c
6. c, d
10. a, c
14. a, d
1. What is the routing algorithm used in OSPF?
The Dijkstra algorithm is the routing algorithm used in OSPF.
2. State one method by which a link-state routing protocol attempts to reduce the network overhead.
Link-state routing protocols are used in larger networks because the method that they use to update the routing tables requires fewer network resources.
Link-state routing protocols attempt to reduce network overhead by:
— Using multicast addressing
— Sending triggered updates
— Sending network summaries infrequently if at all
— Using small packets from every router to describe their local connectivity, instead of the entire routing table
3. What is the purpose of the Dijkstra algorithm?
This is a routing algorithm that iterates on the length of path to determine a shortest path ﬁrst tree (SPF tree). It is commonly used in link-state routing protocols to determine which route to use.
4. Name two link-state IP routing protocols.
OSPF and IS-IS are two link-state IP routing protocols.
5. Name the TCP port used by BGP.
TCP port 179 is used by BGP.
6. State the metric used by OSPF.
The metric cost can be manually conﬁgured to represent anything you want, though Cisco has programmed its systems to use the inverse of bandwidth as the default.
7. How often does Integrated IS-IS send out new LSAs?
Integrated IS-IS sends out LSAs every 15 minutes or whenever a change in the network is seen.
8. State one way that OSPF is an improvement over RIPv1.
OSPF is an improvement over RIPv1 for large networks because of the following:
— It uses bandwidth more efﬁciently, sending incremental updates.
— The updates are not broadcast as in RIPv1 but are directed to multicast addresses 184.108.40.206 and 220.127.116.11.
— It propagates changes in the network more quickly, with incremental updates and neighbor relationships.
— It is not limited in size by a maximum hop count of 15.
— It allows for variation in network size throughout the organization, using VLSM.
— It is capable of using the MD5 speciﬁcation.
— The metric can be deﬁned manually, allowing for greater sophistication in the path determination.
— It is more responsive to network changes, is ﬂexible in network addressing and design, and scales to a larger size.
9. State one key attribute of OSPF.
The key attributes of OSPF include the following:
— Maintaining a connection-oriented relationship with other routers on the same physical segment. These are known as adjacent neighbors.
— Sending the minimum amount of information in an incremental update when there has been a change in the network. This allows for fast network convergence.
— Adding another level of hierarchy to the IP address by designing networks into areas.
— Using VLSM and summarization.
— Assigning speciﬁc functionality to different routers to streamline the process of communication change in the network.
— Operating within an organization as an interior routing protocol.
10. State one key attribute of IS-IS.
The following are attributes of IS-IS:
— Routes CLNP trafﬁc.
— Routes IP trafﬁc.
— Allows VLSM and summarization.
— Uses network design of areas to limit CPU-intensive computation.
— Assigns functionality to routers to streamline the communication of network change.
Level 1 routers deal with interarea updates, whereas Level 2 routers communicate between areas.
— Sends incremental updates to conserve both bandwidth and CPU.
— Operates within an autonomous system as an internal routing protocol.
11. State one key attribute of BGP.
The following are key attributes of BGP:
— Full routing updates are sent at the beginning of the session.
— Trigger or incremental updates are sent only after the initial setup.
— Connections between BGP routers are maintained by periodic hellos. The Hello protocol is connection-oriented, using TCP, port 179.
— It uses the hierarchical structure of autonomous systems.
— It has a complex metric, called attributes, by which trafﬁc paths can be manipulated.
12. What is the default hello update timer for IS-IS on broadcast media?
The update hello timer for IS-IS on broadcast media is every 10 seconds.
13. On a broadcast link, how long does OSPF wait by default before it determines that a neighbor is dead?
On a broadcast link, OSPF would wait 40 seconds before declaring the neighbor as unavailable or dead. This would result in the creation and ﬂooding of an LSA.
14. What is iBGP?
The acronym iBGP stands for internal BGP; it is used to traverse an autonomous system. An internal routing protocol is required to carry updates from one part of the autonomous system to another.
15. When does OSPF send updates?
Incrementally. The update only contains the network change.
However, 30 minutes after the last update is received, a compressed version of the table is propagated.
16. When does BGP send updates?
BGP sends incremental updates that contain only the network change.
17. What is a topological database?
Used by EIGRP and OSPF, this table records all the routes in the network before determining which will be entered into the routing table.
18. What is an adjacent neighbor?
A neighbor is a router that is directly connected to another router. They must also have the same mask and hello parameters. Once a neighbor relationship is formed, routing updates can be exchanged. An adjacent router is a router that has exchanged routing information with its neighbor, becoming an adjacent neighbor.
19. What is a triggered update?
A triggered update is when a routing update is sent asynchronously in response to a change in the network topology. If there is a change in the metric, the update is sent immediately without waiting for the update timer to expire.
20. What is required for iBGP to operate?
The iBGP is used to send routing information internally across an autonomous system, using it as a transit area to another autonomous system. The iBGP needs a fully meshed BGP network, but these routers do not need to be directly connected. The way the BGP updates can be sent to the other BGP routers, or the BGP data trafﬁc can ﬁnd the remote destination, is by listening to the interior IP routing protocol. Although the remote peer does not have to be directly connected, there is an entry in the routing table for the remote peer so that the routers can communicate with each other.
“Do I Know This Already?” Quiz
1. What information is held in the topology table?
The topology table holds a map of every link in the area. Every topology table in the area is the same. This is sometimes referred to as the link-state database.
2. What command is used to determine manually which router on a LAN will become the DR?
The ip ospf priority number command is used to determine the DR manually. The higher the priority, the greater the likelihood is of success.
3. How many subnets are required in an OSPF configuration over a point-to-point network that has multiple connections?
It is necessary to have one subnet per connection. Thus, if there are four point-to-point links, four subnets are required.
4. State the different types of packets used to build a routing table for the first time.
Five packets are used to build the routing table for the ﬁrst time:
— The hello packet —This is used to ﬁnd neighbors and to determine the designated and BDR. The continued propagation of the hello packet maintains the transmitting router in the topology database of those that hear the message.
— The database descriptor —This is used to send summary information to neighbors to synchronize topology databases.
— The LSR —This is a request for more detailed information, which is sent when the router receives a database descriptor that contains new information.
— The LSU —This is the LSA packet issued in response to the request for database information in the LSR packet.
— The link-state acknowledgement —This acknowledges the LSU.
5. In creating an adjacency, what is the exstart state?
The exstart state is a stage in the forming of an adjacency between neighbors. This stage is the stage when the DR and the BDR have been elected. The master/slave relationship has been established, as has the initial sequence number of the DDP packets.
6. What is the database descriptor and when is it used?
Referred to as DBDs or database descriptor packets (DDPs), these are packets exchanged between neighbors during the exchange state. The DDPs contain summary information taken from the LSAs, which describe the links of every router in the neighbor’s topology table.
7. Explain the difference between an LSR and an LSA.
A link-state advertisement (LSA) is a packet describing a router’s links and the state of those links. There are different types of LSAs to describe the different types of links.
An LSR is a link-state request, which is used when the router receives a DDP complete with summary information taken from the LSA. It compares the LSA against the topological database. If either the LSA entry is not present or the entry is older than the DDP, it will request further information via an LSR.
8. What packet is used to maintain the neighbor table?
The hello packet is used to maintain the neighbor table. Whenever a hello is heard, the source address in the hello packet is used to reset the hello interval timer. This shows that the neighbor is still active.
9. What is the metric used by OSPF standards? Is this the same metric that Cisco uses?
OSPF deﬁnes cost as the OSPF metric, but does not deﬁne what cost represents. Thus, any determinant could be used and deﬁned manually as cost. Cisco has set a default metric to be the inverse of bandwidth, making the fastest link the most preferred link. This default can be overridden by manual conﬁguration.
10. Explain the meaning of the letters BDR.
BDR stands for backup designated router. This router acts as the backup to the DR in case the DR fails. The BDR performs none of the DR functions while the DR is operating correctly.
11. What is used to elect the DR when the election is dynamic?
When selected dynamically, the DR is elected arbitrarily. The election is made on the basis of the highest router ID or IP address present on the network segment. It is wise to be aware that the highest IP address is the numerically highest number, not the class ranking of the addresses. Therefore, a remote, small router with a Class C address might end up as a DR.
12. When a new router joins the OSPF network, will it learn about the rest of the OSPF network through the flooding method or the exchange method?
When a new router connects to a network, it will ﬁnd a neighbor using the Hello protocol and will exchange routing information.
13. If an LSA is received that is present in the OSPF database, and the receiving LSA is older than the one currently held by the router, what action is taken?
The receiving router will send a copy of the LSA it holds in its database to the source of the old LSA and then discard the old LSA it received.
14. A router has made a neighbor relationship with another router and exchanged DDP. Having compared the routing information from its neighbor, the router realizes that its topology database is incomplete. Name the different stages or states that a router goes through to update its topology database.
When the router has received the DDPs from the neighboring router, it compares the received network information with that in its topology table. In the case of a new router, such as the 2500, all the DDPs are new. Remember that the DDPs are simply a summary of the routes about which the neighbor knows. If there is a discrepancy between the information in the received DDPs and the router’s topology database, then the router requests more detailed information from its neighbor on those routes of which it was unaware. The different stages or states that the router goes through gathering routing information to update the topology database from a neighbor are shown in the following list:
— The loading state —If the receiving router, the 2500, requires more information, it will request that particular link in more detail using the LSR packet.
The LSR will prompt the master router to send the LSU packet. This is the same as an LSA used to ﬂood the network with routing information. While the 2500 is awaiting the LSUs from its neighbor, it is in the loading state.
— The full state —When these LSRs are received and the databases are updated and synchronized, the neighbors are fully adjacent.
15. How many equal-cost paths will Cisco enter into the routing table?
The RFC 2328 that deﬁnes OSPF does not state the number of equal-cost paths that can be entered into the routing table. Cisco has deﬁned this to be four paths by default, which can be conﬁgured to contain up to six equal-cost paths.
16. An LSA is received by a router, and when checked against the topology database, it finds the LSA is new or a change in the status of an existing route that has been received. What action will the receiving router take?
The LSA is ﬂooded out of all the interfaces, excepting the interface through which it was received. The LSA is copied into the topology database, replacing the original LSA if it existed. The received LSA is acknowledged. The SPF algorithm is run to update the routing table.
17. Which NBMA configuration options are Cisco-specific?
The conﬁguration options proprietary to Cisco for NBMA are:
— Point-to-multipoint nonbroadcast
18. What is the difference between a point-to-point interface and a point-to-multipoint interface?
In a point-to-point network, the concept of broadcast is not relevant because the communication is direct to another router. There is very little network overhead. An IP subnet is required for each point-to point link.
In point-to-multipoint connections, OSPF simulates a broadcast, the network trafﬁc is replicated and sent down each physical link and uses multicast addressing.
19. What is the default network type for serial interfaces with HDLC encapsulation, and how often is the hello packet sent?
The default network type for serial interfaces with HDLC encapsulation is point-to-point and the hello packet is sent out every 10 seconds.
20. On a multiaccess link, what role does the BDR play?
The BDR listens to all the OSPF network trafﬁc, which is addressed to both the designated and BDRs. All the routers on the medium have an adjacency with both DRs. The difference is that the BDR listens but does not respond. If the DR fails, the BDR becomes the DR.
“Do I Know This Already?” Quiz
2. c, d
3. b, d
11. b, c
12. a, d
13. b, c, d
1. What command is used to determine manually which router on a LAN will become the DR?
The priority command is used to determine manually the DR. The higher the priority, the greater the likelihood is of success. Remember that the default=1 and p=0 means that the router cannot win.
2. What parameter is used to calculate the default metric of a route in OSPF on a Cisco router?
The bandwidth parameter conﬁgured on an interface is used to determine the default cost or the value of the path with the lowest cost.
3. It is possible to have more than one OSPF process on a router. How would you do this?
The router command creates the OSPF process with an ID number to identify it. To create another process on the same router, issue the same command again with a different ID number.
It is possible to have more than one process, although it is rarely conﬁgured. The process ID in the command router ospf process-id not only starts the process, but also identiﬁes the process; repeating the command with another ID number will create another process. One possible scenario for this conﬁguration is a service provider that wants to separate its OSPF domain from its customer.
4. Explain the command ip ospf network non-broadcast .
The ip ospf network non-broadcast command is the RFC-compliant mode for NBMA. It is the default mode for interfaces and point-to-multipoint subinterfaces. It is used in a full or partial meshed network, and OSPF operates as if on a nonbroadcast network. It is necessary to deﬁne manually the DR to be a hub router that is connected to all the other routers. Neighbors must be deﬁned manually.
5. In which of the NBMA configuration choices is it necessary to state the neighbors manually? Why is this necessary?
It is necessary to manually conﬁgure the neighbors in the industry-standard NBMA mode and in the Cisco point-to-multipoint nonbroadcast mode.
You need to deﬁne the neighbors to the router because the router believes that it is a nonbroadcast medium, so it cannot send out the multicast trafﬁc to ascertain the neighbors.
6. In a Frame Relay environment, which is fully meshed, which OSPF configurations might be chosen? Give reasons for your choice.
The industry-standard NBMA conﬁguration can be chosen in a fully meshed environment. It requires an additional manual conﬁguration of the neighbors, but the network will elect the DR and the BDR. There might be some design concerns about running this mode in an unstable network, which could burden the CPU and the WAN links.
It is possible to use point-to-point subinterfaces without worrying about the OSPF network type because they will become neighbors.
The other alternative is the Cisco broadcast mode, which does not require the manual conﬁguration of neighbors.
7. The Cisco solution point-to-point mode does not require the configuration of DR and BDR. Explain briefly why.
The Cisco solution point-to-point does not require the election of either a DR or a BDR because there are only two nodes on the network. They form an adjacency immediately.
8. The address 18.104.22.168/21 has been allocated to an interface on the router. This interface alone is to be included in the OSPF process. State the command that would start the process on this interface.
There are several ways to conﬁgure the process to include the interface. The command network network-number wildcard-mask area area-number would be a subcommand to the global command router ospf process-id . The network command is used in both possible solutions; the difference is in the wildcard mask.
— network 22.214.171.124 0.0.0.0 area 2 —This will match every bit in the interface address.
— network 126.96.36.199 0.0.7.255 area 2 —This will also match the interface because it will resolve to the subnet assigned to the wire connected to the interface. This bit
allocation was chosen merely to demonstrate the technique. The allocation assumed is the subnet mask of 255.255.248.0. Note that the wildcard mask is the inverse of the subnet mask, ensuring that the individual subnet is selected for the interface.
9. The metric used by OSPF is cost. How would you change the metric on an interface?
Underneath the appropriate interface, issue the command ip ospf cost . The value for cost is an unsigned integer value expressed as the link-state metric. It can be a value in the range 1 to 65,535.
10. If the command ip ospf network non-broadcast is used, what additional statement is necessary?
If the command ip ospf network non-broadcast is used, the additional statement that is required is the neighbor statement. Because the network is a nonbroadcast network that cannot see its neighbors, the neighbors are to be manually conﬁgured.
11. What command shows which router on a LAN is the BDR?
The show ip ospf neighbor command will show the DR and the backup router. Another command that will show the DRs is the show ip ospf interface command.
12. Explain briefly what show ip ospf database will reveal.
The command show ip ospf database shows the contents of the topology database and gives a status on the LSAs that have been sent and received, including how long it has been since the last LSA was received.
13. What command is used to show the state of adjacencies?
The command show ip ospf interface shows the adjacencies that exist with neighbors.
14. Which command is used to show OSPF packets being sent and received in real time?
The command debug ip packet shows OSPF packets being sent and received in real time.
15. How would you show the OSPF process ID of the router?
The commands show ip ospf , show ip ospf database , and show ip ospf interface all show the OSPF process ID on the router.
16. What makes debug a dangerous command for your router?
The debug command has the highest process priority and is therefore capable of consuming all the resources on the router, thus becoming the problem as opposed to helping to solve the problem.
17. What is the sequence number and where is it held?
The sequence number is used to ensure the LSA that has been received contains the most recent information about the network. This prevents any packets arriving out of sequence from resulting in a change in the network that is incorrect.
18. In the show ip ospf command, there is a field called the SPF schedule delay. What is the purpose of this field, and what is the default time?
The SPF schedule delay is the time between OSPF receiving a topology change and starting an SPF calculation. The delay can be an integer from 0 to 65,535. The default time is 5 seconds. If the value is set to 0, this means that the SPF calculation is started as soon as a valid LSA is received.
There is a balance between responding to a topology change quickly and the use of CPU processing.
19. What is the advantage of the command show ip ospf interface in troubleshooting?
The show ip ospf interface command shows how the interface has been conﬁgured for OSPF. This allows for the immediate identiﬁcation of typing errors that result in a mismatch between neighbors.
“Do I Know This Already?” Quiz
3. b, d
8. a, b, c
9. a, c, d
12. a, c
1. In a totally stubby area, which routes are not propagated into the area?
There will be no summary or external routes propagated by the ABR into the area. Thus, there will be no Type 3, Type 4, or Type 5 LSAs sent into the other area.
2. Can a virtual link contain a stub area?
No, a virtual link cannot contain a stub area. A stub area cannot accept external LSAs, and by deﬁnition, the virtual link is traversing a different area.
3. An ABR must be resident in which area?
An ABR must be resident in Area 0, as well as in the area that is connecting to the backbone area. It has two topological databases, one for each area in which it is resident, so that it knows how to forward trafﬁc.
4. What LSAs will the ABR forward?
ABR forwards summary LSAs. It forwards both Type 3 LSAs and Type 4 LSAs. Type 3 LSAs are forwarded to the other ABRs, and Type 4 LSAs are forwarded to the ASBRs. ABR also forwards Type 3 LSAs from other areas into its own area. If the ABR has multiple links in the same area, it also forwards Type 1 and Type 2 LSAs in its capacity as an internal router.
5. State two advantages in creating areas in OSPF.
The advantages in creating areas in OSPF include the following:
— It is easier to manage and administrate smaller areas, where hopefully many of the design considerations and even conﬁguration are standardized.
— It uses a smaller topology table, which reduces the CPU, memory, and network bandwidth consumption.
— Fewer SPF calculations are involved because the topology table is smaller and there is less likelihood of change in the network.
— It uses a smaller routing table if summarization is in operation.
6. What is an external route, and on which type of router will this route be introduced?
An external route is a route that did not originate in the OSPF domain. It has been redistributed from another routing protocol or static routing. An external route is introduced into the OSPF domain by an ASBR.
7. Why is the use of summarization important in the design of OSPF?
Summarization is important in the design of OSPF because it supports a hierarchical design and allows for the summarization of IP subnets between areas, which reduces the size of the routing tables, which in turn reduces the CPU and memory requirements.
8. How many routers does Cisco suggest is the limit to have in a single area?
Cisco suggests that there should not be more than 50 routers in any single area.
9. What are the restrictions to be considered in the creation of a stub area or a totally stubby area?
Some restrictions govern creating a stub or totally stubby area. Because no external routes are allowed in these areas, the following restrictions are in place:
— No external routes are allowed.
— No virtual links are allowed.
— No redistribution is allowed.
— No ASBR routers are allowed.
— The area is not the backbone area.
— All the routers are conﬁgured to be stub routers.
10. A virtual link in OSPF is used to solve what problem?
The virtual link provides a disconnected area with a logical path to the backbone. The virtual link must be established between two ABRs that have a common area, with one ABR connected to the backbone. It can also be used to connect two area 0s together. This might be necessary when two companies merge, each with its own area 0, or if, due to the loss of a link, the area 0
11. State one disadvantage for making an NBMA cloud Area 0.
Creating the NBMA cloud as one OSPF area, preferably Area 0, causes summary LSAs to beﬂooded throughout the NBMA network. This results in a large number of routers recalculating whenever there is a change that requires the topology table to be updated, and the Frame Relay cloud can become saturated. If the Frame Relay cloud has a problem, then the entire network might suffer.
12. State one advantage in making the centralized routers and network resources dwell in Area 0 while the Frame Relay cloud and the stub remote LANs reside in satellite stub areas.
One advantage of this design is that any ﬂooding of external LSAs is prevented from entering the Frame Relay network, because it is a stub network. This reduces the network overhead.
13. How does creating a number of areas in OSPF reduce the number of SPF calculations?
The number of SPF calculations is reduced because the size of the topology table is reduced. This lessens the likelihood of a change in the network and, thus, SPF calculations.
14. How does a stub area differ from the backbone area?
A stub area differs from the backbone area in that it does not propagate external routes into its area. The backbone is obliged to forward these LSAs to ensure connectivity throughout the network.
15. How does a totally stubby area differ from a stub area?
A totally stubby area differs from a stub area in that it propagates neither external routes nor summary routes from other areas. This is a Cisco solution to minimize the amount of CPU and memory required of the routers within the area. Connectivity is achieved by the use of default routes, which are advertised to the internal routers.
16. State the different LSA types.
The different LSA update types are as follows:
— Router link —Sent by the router, stating the links directly connected. These are ﬂooded through the area. This update is identiﬁed by the type code Type 1.
— Network link—Sent by the DR, stating the links for the LAN for which it is the DR. These LSAs are ﬂooded throughout the area. This update is identiﬁed by the type code Type 2.
— Summary link —Sent by the ABR into the backbone. It states the IP subnets within the area that are to be advertised into other areas. This is where summarization would
be conﬁgured. This update is identiﬁed by the type code Type 3.
— Summary link (to an ASBR) —Sent from an ABR to a router that connects to the outside world (ASBR). It contains the metric cost from the ABR to the ASBR. This
update is identiﬁed by the type code Type 4.
— External link —Sent to the ASBRs to which the organization is directly connected. This update is identiﬁed by the type code Type 5.
— The NSSA External LSA —These LSAs are created by the ASBR residing in an NSSA. This LSA is similar to an autonomous system external LSA, except that this LSA is contained within the NSSA area and is not propagated into other areas.
17. Where does the backbone router reside, and what is its function?
OSPF has special restrictions when multiple areas are involved. If more than one area is conﬁgured, one of these areas must be Area 0. This is called the backbone. When designing networks, it is good practice to start with Area 0 and then expand into other areas later.
The backbone must be at the center of all other areas—that is, all areas must be physically connected to the backbone. The reasoning behind this is that OSPF expects all areas to inject routing information into the backbone; in turn, the backbone will disseminate that information into other areas.
18. There are two types of summarization. What are they?
The two types of summarization are as follows:
— Interarea route summarization —These routes are sent between areas. The ABR will summarize routes if the network within the area was designed using contiguous addresses, conforming to both a physical and a logical hierarchy.
— External route summarization —These are routes sent into OSPF from another routing protocol. This summarization also demands a hierarchical design using contiguous addresses. This is employed at the ASBR.
19. For how many LANS does Cisco suggest a router should serve as a DR or a BDR? Cisco suggests that a router should be a DR or a BDR for only one LAN.
20. Which router type creates LSA Types 3 and 4?
The ABR creates the LSA Types 3 and 4.
20. Which router type creates LSA Types 3 and 4?
The ABR creates the LSA Types 3 and 4.
“Do I Know This Already?” Quiz
3. a, d
4. a, c
10. a, c, d
11. b, c
12. a, b, d
1. Which command in OSPF shows the network LSA information?
The command show ip ospf [process-id area-id] database network displays the network linkstate information.
2. What command would you use to create a totally stubby area?
The command area area-id stub no-summary will create a totally stubby area. This is a subcommand to the router ospf process-id command. It is necessary only on the ABR, but all the other routers in the area must be conﬁgured as stub routers.
3. What is a virtual link, and what command would you use to create it?
A virtual link is a link that creates a tunnel through an area to the backbone (Area 0). This allows an area that cannot connect directly to the backbone to do so virtually. The command to create the link is area area-id virtual-link router-id. Note that the area-id that is supplied is that of the transit area, and the router-id is that of the router at the other end of the link. The command needs to be conﬁgured at both ends of the tunnel.
4. Where would you issue the command to summarize IP subnets? State the command that is used.
Summarization is done at area boundaries. The command to start summarization is the area range command, with the syntax area area-id range address mask. To summarize external routes, use the summary-address command on the ASBRs.
5. How would you summarize external routes before injecting them into the OSPF domain?
The command summary-address address mask is the command that you would use.
6. When is a virtual link used?
A virtual link is used when an area is not directly attached to the backbone area (Area 0). This may be due to poor design and a lack of understanding about the operation of OSPF, or it may be due to a link failure. The most common cause of an area separating from the backbone is
link failure, which can also cause the backbone to be segmented. The virtual link is used in these instances to join the two backbone areas together. Segmented backbone areas might also be the result of two companies merging.
7. Give the command for defining the cost of a default route propagated into an area.
The command to deﬁne the cost of a default route propagated into another area is area area-id default-cost cost.
8. Give an example of when it would be appropriate to define a default cost.
It is appropriate to deﬁne a default cost for the default route when a stub area has more than one ABR. This command allows the ABR or exit point for the area to be determined by the network administrator. If this link or the ABR fails, the other ABR will become the exit point for the area.
9. On which router is the area default cost defined?
The default cost for the default route is deﬁned on the ABR. The ABR will then automatically generate and advertise the route cost along with the default route.
10. Give the command to configure a stub area and state on which router it is configured.
The command syntax to conﬁgure a stub area is area area-id stub. This command is conﬁgured on the ABR connecting to the area and on all the routers within the area. Once the conﬁguration is completed, the Hellos are generated with the E bit set to 0. All routers in the area will only form adjacencies with other routers that have the E bit set.
11. What is the purpose of the area range command, and why is it configured on the ABR?
The area range command is conﬁgured on an ABR because it dictates the networks that will be advertised out of the area. It is used to consolidate and summarize the routes at an area boundary.
12. Give the commands to configure a router to place subnets 188.8.131.52 through to
184.108.40.206 in Area 1 and to put all other interfaces into Area 0. The commands are as follows:
network 220.127.116.11 0.0.7.255 area 1
network 0.0.0.0 255.255.255.255 area 0
13. Give the syntax to summarize the subnets 18.104.22.168 to 22.214.171.124 into another
The syntax is as follows:
summary-address 126.96.36.199 255.255.248.0
14. Explain briefly the difference between the area range command and the summary-address command.
The area range command is used to summarize networks between areas and is conﬁgured on the ABR. The summary-address command is used to summarize networks between autonomous systems and is conﬁgured on the ASBR.
15. Explain the following syntax and what it will achieve: area 1 stub no-summary .
The command area 1 stub no-summary creates a totally stubby area. The number after the word area indicates the area that is being deﬁned as a totally stubby area. This is necessary because the router might be an ABR with connections to many areas. Once this command is issued, it prevents summarized and external routes from being propagated by the ABR into the area. To reach the networks and hosts outside the area, routers must use the default route advertised by the ABR into the area.
16. Why would you configure the routing process to log adjacency changes as opposed to turning on debug for the same trigger?
The reason to conﬁgure the router process to log adjacency changes to syslog as opposed to running debug is an issue of resources. It takes fewer router and administrator resources to report on a change of state as it happens than to have the debugger running constantly. The debug process has the highest priority and thus everything waits for it.
17. Give some of the common reasons that neighbors fail to form an adjacency.
Many OSPF problems stem from adjacency problems that propagate throughout the network. Many problems are often traced back to neighbor discrepancies.
If a router conﬁgured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the following:
— Make sure that both routers are conﬁgured with the same IP mask, MTU, Interface Hello timer, OSPF Hello interval, and OSPF dead interval.
— Make sure that both neighbors are part of the same area and area type.
— Use the debug and show commands to trace the problem.
18. When configuring a virtual link, which routers are configured?
The conﬁguration is between the ABRs, where one of the ABRs resides in Area 0 and the other in the area that is disconnected from the backbone. Both of the ABRs are also members of the transit area. Having created the virtual link, both ABRs are now members of Area 0, the disconnected area, and the transit area.
19. What does the command area 1 default-cost 15 achieve?
The command area 1 default-cost 15 will assign a cost of 15 to the default route that is to be propagated into the stub area. This command is conﬁgured on the ABR attached to the stub area.
20. Explain what is placed in the parameters area-id and router-id for the command area area-id virtual-link router-id.
The parameter area-id is the area ID of the transit area. So if the ABR in Area 0 is creating a virtual link with the ABR in Area 3 through Area 2, the area ID stated in the command is Area 2. The router ID is the router ID of the router with whom the link is to be formed and a neighbor relationship and adjacency established.
“Do I Know This Already?” Quiz
4. a, b, c, d
14. a, b
15. b, c
16. a, b
19. a, b, d
20. a, b, d
1. Which system generates the pseudonode?
The DIS generates the pseudonode, using its own system ID and setting the following octet to be a nonzero number.
2. What is a CSNP? When is it used?
The complete sequence number packet describes every link in the link-state database. It is sent on point-to-point links when the link comes up to synchronize the link-state databases. The DIS on a multicast network will send out CSNPs every 10 seconds.
3. What is a PSNP? When is it used?
Partial Sequence Number Packets are sent on point-to-point links to explicitly acknowledge each LSP they receive. A router on a broadcast subnetwork will send a PSNP requesting the LSPs it needs to synchronize its link-state database.
4. A new router comes online on a multiaccess link, and the priority is the same as that of the DIS for the segment. What action is taken?
The routers exchange Hellos and immediately see that both routers have the same priority. If the new router had a higher priority, it would take over as the new DIS. However, if both routers have the same priority, the router with the highest MAC address will reign as the DIS.
5. What happens in the event of the DIS dying?
There is no backup designated router in IS-IS. Therefore, if the DIS meets an untimely death, a new DIS would be elected, based on priority or highest MAC address. If another router comes online with a higher priority, it will dislodge the existing DIS and rule in its place. This behavior is different from that of OSPF. Once a new DIS is elected, the link-state databases are purged and new LSPs are ﬂooded.
6. Integrated IS-IS can be used to send information about which routed protocols?
IS-IS is capable of carrying both IP and CLNS.
7. How often does the DIS send out a Hello packet?
The DIS sends out hellos every 3.3 seconds, three times the speed of other routers on the multiaccess link.
8. What is the name of the link-state algorithm used to create entries for the routing table?
The name of the link-state algorithm is the Dijkstra algorithm.
9. What is the relationship between the Hello timer and when the path is considered to have died?
The default timer is three times that of the Hello timer; thus the path will wait for 30 seconds before declaring the path dead and ﬂushing the LSPs from the link state database.
10. Integrated IS-IS areas are similar to which type of areas in OSPF?
Integrated IS-IS areas are similar to OSPF stub areas.
11. Describe one design restriction in configuring Level 2 routing.
There is only one hard and fast rule for the design of a Level 2 network: Level 2 routers must be contiguous; that is, the area cannot be fractured.
12. Given the following address:
Is this a NET or NSAP address? Give reasons for your choice.
The address is a NET address because the last octet is set to 0×00. This there is no network service deﬁned. This is the address of a router, not an end system.
13. What is a pseudonode and what is its purpose?
The pseudonode is the LAN identiﬁer for a broadcast subnetwork. The pseudonode is the System ID of the DIS plus the Circuit ID. The pseudonode has links to each of the ISs, and each IS has a single link to the pseudonode. The use of the pseudonode reduces the number of links required. Instead of n-1 links to each of the other ISs, there is one link per IS. The DIS generates link-state PDUs on behalf of the pseudonode. These LSPs are sent to all the connected ISs.
14. State two reasons why a router might not be able to find a neighbor.
For an adjacency to be formed and maintained, both interfaces must agree on the following:
— The same MTU.
— Both are Level 1. If both are Level 1, they must be in the same area.
— Both are Level 2.
— At least one is Level 1-2.
— The authentication must be the same.
— The Hello timers (including the holddown timer) must match. If one router has a Hello timer of 40 seconds, the defaults on the other router would time out the holddown timer and purge the LSP, resulting in a ﬂapping link and endless SPF calculations.
15. Explain briefly why two routers cannot have the same system ID within the area.
The system ID is the unique identiﬁer for the area. The ﬁrst part of the address is a very long area address, of which only the last six octets are available for addressing the router or host.
16. What does TLV stand for? Briefly explain its purpose.
TLV is the same as CLV, but some literature refers to the variable length ﬁelds as Type/Length/ Value in accordance to the IP terminology. Although the IS-IS PDUs are ﬁxed, the TLV ﬁelds are variable length and can expand as needed. This design allows great ﬂexibility and movement to develop in step with technological advances. The development of TLV code 128 extended IS to carry integrated IS-IS.
17. How many link-state databases does a Level 1-2 router have?
A Level 1-2 router has two link-state databases, one for the Level 1 routes and the other for the Level 2 routes. A separate SPF algorithm is run for each database.
18. Integrated IS-IS packets run directly on top of which layer? IS-IS packets run directly on top of the data-link layer.
19. What is the NET address associated with in the IS-IS addressing scheme?
The NET address is associated with the end system, but not with a process on the end system. The address is that of an entire system, as opposed to an interface on the system, as is the case with IP. Because the NET (unlike the NSAP) does not identify a process, the address is that of a transitional or intermediate system. Therefore, the NET address is associated with the router or IS and is the destination address of the next hop in the life of a routed or routing packet.
20. Describe briefly the main characteristics of a Level 1 IS.
Characteristics of a Level 1 IS include the following:
— An intra-area router.
— Similarity to an OSPF stub router.
— Knowledge of the network limited to the area.
— A link-state database with all the routing information for the area.
— The address of the closest Level 2 router to which to send trafﬁc destined for another area.
— Neighbors must be in the same area.
— A DIS is elected on LANs.