Cisco (300-410-ENARSI) Exam Questions And Answers page 24

A loopback address does not automatically appear in neighboring routers' routing tables, so it cannot be pinged for network troubleshooting.
A work-around for this problem is to add a network statement under OSPF that advertises the loopback address network so that other routers will know how to reach your loopback.
A loopback address is an IP address assigned to a loopback interface, which is a logical interface on a router that behaves like a physical interface. Their advantage is that, unlike physical interfaces, logical interfaces do not go down.
For example:
Router(config)# interface loopback 0
Router(config-if)# ip address 172.17.1.1 255.255.255.0

In the example, a loopback IP address is used by OSPF to provide its router ID. This type of address is preferred because it is assumed to be more stable than a router ID tied to a physical interface. The traditional problem with a router ID tied to a physical interface is that if the physical interface were to go down, the router would have to change its router ID to some other value. That would cause the OSPF neighbor relationships to reset and change values in the link-state advertisements (LSAs), causing a disruption to the OSPF area.
With this consideration in mind, OSPF is more reliable when using a loopback interface than using a physical interface.
Using a loopback address does not avoid wasting an additional IP address. The address must still be unique.
A loopback interface is always active, and it cannot go "down" as a physical interface can.
Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify OSPF operations
Cisco > IP Routing: OSPF Configuration Guide > Configuring OSPF > Forcing the Router ID Choice with a Loopback Interface

The following command pair should be used to configure successful authentication:
neighbor 192.168.5.2 password routera executed on RouterA
neighbor 192.168.5.3 password routera executed on RouterB

When setting the keys for authentication, the keys must match. The keys do not need to be the names of either router, and should be a combination of letters numbers and symbols. In this example, both keys are set to the value routera.
The following two command pairs are incorrect because the keys do not match:
neighbor 192.168.5.2 password routera executed on RouterA
neighbor 192.168.5.3 password routerb executed on RouterB

and
neighbor 192.168.5.2 password routerb executed on RouterA
neighbor 192.168.5.3 password routera executed on RouterB

If you executed the debug ip bgp command to perform troubleshooting with either of these configurations in place, the error message you would see would be as follows:
%TCP-6-BADAUTH: Invalid MD5 digest from 192.168.5.3 (12293) to 192.168.5.2 (179)

In the error message, the numbers in parentheses are the port numbers used for the attempted communication.
The single commands would be incorrect because the key has only been configured on one end:
neighbor 192.168.5.2 password routera executed on RouterA
and
neighbor 192.168.5.2 password routerb executed on RouterB
If you executed the debug ip bgp command to troubleshoot with either of these configurations in place, you would see the following message:
%TCP-6-BADAUTH: No MD5 digest from 192.168.5.3 (12293) to 192.168.5.2 (179)

Objective:
Layer 3 Technologies
Sub-Objective:
Describe, configure, and verify BGP peer relationships and authentication
Cisco IOS Master Command List, Release 12.4 > l through q > Cisco IOS IP Routing: BGP Command Reference > neighbor password

The eigrp stub command is used to configure a router to send only connected and summary routes to its neighboring router. For example, examine the following output of the show ip route command that was executed on a router configured as a stub router:
router10#show ip route
C 172.16.5.0/24 is directly connected, Serial 0
D 192.168.7.0/24 [90/16523564] via 172.16.4.1, 00:21:20, Serial 1
D 172.16.0.0/16 is a summary, 00:21:23, Null 0
C 172.16.4.0/24 is directly connected, Serial 2

The routes that will be advertised are 172.16.5.0/24, 172.16.4.0/24, and the summary route 172.16.0.0/16. The first two is directly connected routes, and the last is the summary route that is auto configured by the EIGRP process.
When the stub feature is enabled on a router, the router will announce itself as a stub router. Neighbor routers will not query a stub router for alternate routes when a route is lost elsewhere in the network. The EIGRP stub feature works well in hub-and-spoke topologies when the goal is to minimize the amount of EIGRP bandwidth and processing associated with the spoke router. The eigrp stub command has the following syntax:
eigrp stub [receive-only | connected | static | summary]

When you do not specify any keywords with the command, connected and summary are used by default.
• receive-only: Prevents the router from sending any connected or summary routes.
• connected: Instructs the router to send connected routes.
• static: Instructs the router to send static routes that were redistributed by using the redistribute static command.
• summary: Instructs the router to send summary routes.
These parameters can be combined to resolve various problems, as seen in the following image:

Router A is not receiving the route to the 172.16.1.0/16 network because Router B, which stands between Router A and C, is configured with the eigrp stub-receive-only command. This is resulting in hosts from the corporate office being unable to connect to hosts in the 172.16.0.0/16 network. If there were a legitimate reason to keep Router B configured with the eigrp stub-receive-only command, the problem could be solved by executing the following command set on Router A:
routerA(config)# router eigrp 20
routerA(config-router)# ip summary-address eigrp 20 172.16.0.0 255.255.0.0
routerA(config-router)# eigrp stub connected summary

This command set would create a summary address for the 172.16.0.0/16 network and then advertise it to the corporate office as a result of using the eigrp stub connected summary command. The inclusion of the connected parameter ensures that the directly connected networks will also be advertised, to ensure that hosts in the corporate office can reach the 172.16.0.0/16 network.
The eigrp stub static command instructs the router to send static routes that were redistributed by using the redistribute static command. Examine the EIGRP configuration shown below:

ip route 10.4.4.0 255.255.255.0 10.4.3.10
Route eigrp 200
No auto-summary
Redistribute static 1000 1 255 1 1500
Network 10.4.1.0 0.0.0.3.
Network 10.4.2.0 0.0.0.255
Eigrp stub static

With this configuration, the router would not advertise any of the networks defined in the network statements, but would only advertise the static route configured with the line ip route 10.4.4.0 255.255.255.0 10.4.3.10.
Eigrp passive is not a valid Cisco command.
Eigrp stub receive-only will cause the router to not advertise any routes. The router will only receive updates.
Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify EIGRP stubs
Cisco IOS Master Command List, Release 12.4 > e through h > eigrp stub

NAT64 is a solution when IPv6 hosts need to communicate with IPv4-only servers. When the translation occur on the router the IPv4 address 10.0.0.1 will converted to hex as a00:1 and will be attached to the end of the stateful prefix of 3001::/96 that was configured on the router interface connected to the IPv4 server. The result will be 3001::a00:1.
The address will not be 2001::a001. The prefix that will be attached to the hex version of 10.0.0.1 will not be that of the interface fa0/2/7 but will be the prefix that was configured on that interface for nat64 translation which is 3301::/96.
The address will not be 2001::a00:b. That is the IPv6 address on the interface connected to the IPv6 host, but that address is not used for IPv4 to IPv6 communication. A translated address will be generated by converting the IPv4 address of the IPv4 host to hex and attaching it to the IPv6 prefix configured on the interface connected to the IPv4 server.
The address will not be 2001::A00:A. That is the IPv6 address of the IPv6 host. That was statically mapped to 10.0.0.10 in the configuration and as such will be the IPv4 address used by the IPv6 host on the IPv4 side of the router.
Objective:
Infrastructure Services
Sub-Objective:
Describe IPv6 NAT
Stateful Network Address Translation 64 (PDF)

The ICMP path echo operation discovers the path using the traceroute command, and then measures response time between the source router and each intermittent hop in the path. IP SLAs allow users to monitor network performance between Cisco routers or from either a Cisco router to a remote IP device.
The Internet Control Message Protocol (ICMP) Echo Operation measures end-to-end response time between a Cisco router and any IP-enabled device. Response time is computed by measuring the time taken between sending an ICMP echo request message to the destination and receiving an ICMP echo reply. It does not measure hop-by-hop response time.
The UDP Jitter Operation for VoIP is an extension to the current jitter operations with specific enhancements for VoIP. The enhancements allow this operation to calculate voice quality scores and simulate the codec's directly in CLI and the MIB. It does not measure hop-by-hop response time.
The UDP Jitter Operation is designed to measure the delay, delay variance, and packet loss in IP networks by generating active UDP traffic. It does not measure hop-by-hop response time.
Objective:
Infrastructure Services
Sub-Objective:
Describe SLA architecture
Home > Support > Technology support > IP > IP application services > Technology information > Technology white paper > Cisco IOS IP Service Level Agreements User Guide

The interface must be able to support 802.1q encapsulation. The EVN trunk carries the traffic of multiple virtual routing and forwarding (VRF) instances, with the traffic of each instance tagged with an ID called the virtual network tag. Since the VLAN ID field of an 802.1q encapsulated packet is used for this ID, the link must be one that supports 802.1q.
Easy Virtual networking is a technology that allows for the creation of separate networks with separate routing tables and routing instances using the same physical topology. The IP addressing for the networks can even overlap with no problem. The networks are kept separate using the network ID tags in a similar fashion to the way switches keep VLANs separate by using VLAN tags.
An EVN trunk interface cannot also be configured for VRF-Lite. VRF-Lite is an earlier technology that accomplishes the same goal, but lacks the simplicity of EVN.
Neither RIP nor OSPFv3 is supported in Easy Virtual Networking EVN at all.
Objective:
VPN Technologies
Sub-Objective:
Describe Easy Virtual Networking (EVN)
Cisco > Easy Virtual Network Configuration Guide, Cisco IOS XE Release 3S > Overview of Easy Virtual Network

The only path is entered in the routing table as a result of the unequal load balancing configured on the routers:
RA-RB-RE-RI-RK
In EIGRP networks, bandwidth and delay are the default factors for calculating the metric/cost for a given route. Additional factors such as load and reliability can be considered in the computation of the EIGRP metric, as given in the following formula:
Metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]

In this case, only the K3 value has a non-zero value. This implies that only delay is taken into consideration to calculate the metric of the shortest path from 172.16.1.0 network to the WAN. The path with the lowest metric, which is delay in this scenario, is the shortest path, and is therefore entered automatically in the routing table. The total delay and the corresponding metric for the three best paths are given as follows:

In the given table, the path RA-RB-RE-RI-RK has the lowest metric of 14080. This is the shortest path, so it would be entered in the routing table even if variance were not enabled. In this scenario variance is set to 3, which enables unequal load balancing among those paths that have a metric less than three times the least metric for the given route. Three times the least metric in this scenario is 42240 (14080 x 3). This implies that paths between the 172.16.1.0 network and the WAN having a metric less than 42240 participate in the load balancing. On metric values alone, those paths would appear in the routing tables. However, to be eligible to be a feasible successor the reported distance of the path must be less than the feasible distance (current best path). None of the paths, with the exception of RA-RB-RE-RI-RK meet that requirement.
The path RA-RB-RD-RH-RK is not entered in the routing table as a result of the unequal load balancing. The scaled EIGRP delay for this path is 43520 (170 x 256), which is more than three times the least metric available from the 172.16.1.0 network to the WAN (42240). In addition, the reported distance for this path is more than the feasible distance. Therefore, the path RA-RB-RD-RH-RK is not used for balancing the load from the 172.16.1.0 network to the WAN and does not appear in the routing tables.
Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify EIGRP load balancing
Cisco > Support > Technology Support > IP > IP Routing > Design > Design Technotes > How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP? > Document ID: 13677
Cisco > Support > Technology Support > IP > IP Routing > Design > Design Technotes > How Does Load Balancing Work? > Document ID: 5212
Cisco > Support > Technology Support > IP > IP Routing > Technology Information > Technology Whitepaper > Enhanced Interior Gateway Routing Protocol > Document ID: 16406 > Feasible Distance, Reported Distance, and Feasible Successor

The debug command uses a good deal of router CPU time, so you should not initiate this command on an already overloaded router. It often requires the router to do extensive examination of the packets, requiring heavy use of resources at times.
You could also possibly overload the router just with the debug command. If the router is overloaded to the point that it is no longer responding to your EXEC session, you may need to reload the router to stop the debug output.
These additional steps can help to verify proper route redistribution:
• On the router not performing the redistribution, use the show ip route command to see if the redistributed routes are displayed.
• On the router performing the redistribution, use the show ip protocol command to verify the redistribution configuration, and use the show ip route command that to verify the proper routes are there from each routing protocol.
The trace command is used to discover the route that packets take to their destination.
The show ip route command displays the routing table.
The show ip protocols command displays information about each routing protocol running on the router.
Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify manual and autosummarization with any routing protocol
Cisco > Home > Support > Technology Support > Dial and Access > ISDN, CAS > Design > Design Technotes > Important Information on Debug Commands > Document ID: 10374
Cisco > Cisco IOS Debug Command Reference > Using Debug Commands

The IP SLA TCP connect operation is used to gather statistics on connection-oriented services. The tcp-connect 10.0.0.1 23 control disable command specifies the IP address to which the responder should respond, the port number on which to respond and it disables the control protocol normally used to inform the responder to temporarily enable the port specified .by the configuration in the sender. When the responder is a non-Cisco device, a well-known port number must be chosen and the control protocol should be disabled on the responder. When a Cisco device is the responder, then any port number can be chosen and the control protocol should be left enabled.
The frequency 30 command specifies how often the test should occur in seconds. It is not changed in any way as a result of the responder being a non-Cisco device.
The timeout 1000 command specifies in milliseconds the amount of time an IP SLAs operation waits for a response from its request packet. It is not changed in any way as a result of the responder being a non-Cisco device.
The tag FLL-RO command simply applies a user-specified identifier to the IP SLAs operation and is changed in any way as a result of the responder being a non-Cisco device.
Objective:
Infrastructure Services
Sub-Objective:
Configure and verify IP SLA
IP SLAs Configuration Guide, Cisco IOS Release 15M&T > Configuring IP SLAs TCP Connect Operations
Cisco > Cisco IOS IP SLAs Command Reference > tcp-connect

The ipv6 enable command should be used on R1 to enable IPv6. This command automatically provides an IPv6 link-local unicast address for the interface on which IPv6 is being configured. If an explicit IPv6 address were configured on the interface, the command would not be required.
The ipv6 router ospf command should not be used in the configuration because this command allows you to enter the router configuration mode for OSPF for IPv6.
The ipv6 ospf neighbor command is used to configure neighboring routers for OSPF.
The ipv6 ospf cost command should not be added to the configuration because this command allows you to specify the OSPF cost to send packets from a given interface.
Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify OSPF for IPv6
Cisco > Cisco IOS IPv6 Command Reference > ipv6 enable