The existing serial interface on RouterB belongs to the 192.168.16.16/30 IP subnet. The assignable IP addresses in that subnet are 192.168.16.17 through 192.168.16.18. IP address 192.168.16.19 is the broadcast address for the subnet because all host bits are on. To assign the only available address to the serial 0 interface use the following commands:

RouterA(config)# interface serial 0
RouterA(config-if)# ip address 192.168.16.17 255.255.255.252
RouterA(config-if)# no shutdown

The existing Ethernet interface on RouterB belongs to the 192.168.16.65/26 IP subnet. The assignable IP addresses in that subnet are 192.168.16.65 through 192.168.16.126.

[Click on image for larger view.]

Figure 6: IP network 192.168.16.0/24

To assign the same mask on the Ethernet interface on RouterA, requires us to move to an unused subnet such as 192.168.16.128/26. The assignable IP addresses in that subnet are 192.168.16.129 through 192.168.16.190. The IP address 192.168.16.101 belongs to RouterA's subnet. To assign an available address to the Ethernet 0 interface use the following commands:

RouterA(config)# interface ethernet 0
RouterA(config-if)# ip address 192.168.16.129 255.255.255.192
RouterA(config-if)# no shutdown

The version router command selects the RIP version. RIP1 is selected if no version is specified. To specify RIP2, use the 'version 2' command as shown below.

RouterA(config)#router rip
RouterA(config-router)network 192.168.16.0
RouterA(config-router)version 2

RIP2 offers significant enhancements over RIP1 including support for variable-length subnet masks and triggered updates. For more information on RIP Commands, go here.

2. Answer E is correct. RouterA and RouterB do not have the same Dead timer values configured. RouterA and RouterB do not have the same Dead timer values configured.

Routers on a broadcast or non-broadcast multiaccess network will not form a neighbor relationship if any of the following occur:

• The routers have different IP subnet masks configured for the network.
• The routers are configured to use different Hello timer intervals. The Hello timer identifies the number of seconds between sending Hello packets.
• The routers are configured to use different Dead timer intervals. The Dead timer identifies how long an OSPF router waits before declaring that the neighbor is no longer active.

For more information on OSPF, see the Tech document "What does the show ip ospf interface Command Reveal?"

3. Answer D is correct. The server that has been configured with the IP address 172.31.16.32/28 and default gateway 172.31.16.33 is invalid and would cause a communication problem.

This scenario has a router connecting the workstations in the 172.31.16.16/28 subnet to the server in the 172.31.16.32/28 subnet.

Valid host addresses for the 172.31.16.16 255.255.255.240 (also written as 172.31.16.16/28) include 172.31.16.17 through 172.31.16.30. In this scenario, the router's Ethernet 0 interface is assigned IP address 172.31.16.17/28. The 172.31.16.17/28 address is used as the default gateway address for hosts attached to that subnet.

Valid host addresses for the 172.31.16.32 255.255.255.240 (also written as 172.31.16.32/28) include 172.31.16.33 through 172.31.16.46. In this scenario, the router's Ethernet 1 interface is assigned IP address 172.31.16.33/28.

The 172.31.16.33/28 address is used as the default gateway address for hosts attached to that subnet.

The IP address 172.31.16.32/28 represents the subnet address and is invalid for host assignment.

The IP address assigned to the switch in this scenario is also invalid. The switch is assigned the IP address 172.31.16.47/28.

The address 172.31.16.47/28 is the broadcast address for the 172.31.16.32/28 subnet. This invalid address assigned to the switch would affect communications to the switch, but not through the switch.

4. Answer C is correct. The frame relay map statement is incorrect. The frame-relay map statement is incorrect. The wrong DLCI value is used. The frame-relay map statement is to associate the protocol address of the other device with the DLCI used to communicate through the frame-relay network to the other device. The syntax of the frame-relay map command is as follows:

frame-relay map protocol protocol-address dlci [broadcast]

The correct Router1 entry for the Exhibit is as follows:

frame-relay map ip 172.31.16.66 111 broadcast

The IP addresses and the subnet mask values used are valid and will not cause a Frame-relay link problem. A default frame-relay configuration is being used. The default frame-relay encapsulation type is cisco. Since both sides of the connection are Cisco routers, the default encapsulation is valid.

The default LMI type is auto-sensed after IOS release 11.2. Prior to IOS 11.2, the LMI defaults to cisco and may not be compatible with the service provider's frame-relay network switch. Options for the LMI include cisco, ansi, and q933a. A mismatch in the LMI will prevent the Frame-Relay connection from becoming active.

For more information, see frame-relay map.

5. Answer C is correct. Switch3 will be elected the root bridge because it has the lowest bridge ID.

The root bridge is elected based on being the switch or bridge with the lowest bridge ID. The bridge ID is formed from combining the bridge priority and MAC address values. The bridge priority forms the first two hex bytes and the last 6 bytes is the MAC address. The bridge priority is configurable. The default priority is 32768 decimal or 8000 hex. The MAC address is uniquely assigned to each switch. For example, the switch's assigned MAC address is 10005A001111. When the default priority is added to the switch's unique MAC address, the bridge ID becomes 800010005A001111.

It should be apparent that the bridge priority has a major influence in selection of the root bridge. In the question scenario, the bridge priority was 32760. Making its bridge ID less than all other switches.

6. Answer C is correct. OSPF is a link state routing protocol.

Link state routing protocols are named such because instead of sending their routing tables to neighboring routers they send link state advertisements. These adverstisements describe the routers directly connected networks and the state of each link.

Link-state routing protocols (OSPF and NLSP) are more complex in their routing metric and updating strategy. They are ideally suited for larger networks since they update routing tables by sending only link-state changes rather than the entire routing table.

Distance vector routing protocols (RIP and IGRP) determine the direction and distance to any link in the internetwork. RIP has a maximum hop-count limit of 15. A hop count of 16 is considered unreachable. Therefore, RIP is very limited and should not be used in large internetworks. In addition, RIP routing updates take longer since the entire table must be transmitted for the update.

7. The correct subnet mask is 255.255.255.128 or /25.

Since this is a Class B network, you have 16 bits that can be used for subnet and host addresses. You want to use the fewest number of bits possible for host addresses so you will have the greatest number of bits left for defining subnets.

First, determine the number of host bits you will need to use for each subnet. The number of host bits must support 100 hosts per subnet plus 20% growth, or 120 hosts. The number of possible host addresses is found with the formula 2^n-2 = number of hosts, where n is the number of bits used.

2^2-2 = 2
2^3-2 = 6
2^4-2 = 14
2^5-2 = 30
2^6-2 = 62
2^7-2 = 126

Therefore, you will need to use 7 bits for hosts. This will leave 9 bits for defining subnets, and the subnet mask will be

11111111.11111111.11111111.10000000
or
255.255.255.128.

8. 255.255.255.240 or /28

9. 192.168.50.127

10. The two necessary commands are "RouterA(config)#line vty 0 4" and "RouterA(config-line)#access-class 1 in."

To create an access list that specifies that all traffic on the 192.168.15.0-255 is permitted, issue the following command:

RouterA(config)#access-list 1 permit 192.168.15.0 0.0.0.255

The address 0.0.0.255 used in the command is the wildcard mask. Each octet in the wildcard mask can contain either zero or 255. Use zeros in any octets where you want the corresponding octets in the IP address to match exactly. Use 255 in the octets where you want to allow the corresponding octets in the IP address to contain any number. For example, using the IP address 192.168.15.0.0 and wildcard mask 0.0.0.255 would create an access rule that required the first three octets to be 192.168.15 but allowed any number in the fourth octet.

Using the 'any' variable as a subnet mask is an improper syntax for the 'access-list' command.

The syntax for the access-list command is shown below:

Router(config)#access-list (access-list-number) {permit | deny} source [source-mask]

While the access-lists command creates an access list, it is not the only command required to limit traffic to the specified network. After creating the access list, you must issue the 'access-class' command to restrict incoming connections to the vty lines. For example, to apply this access list to the switch's Telnet lines, you would issue the following command:

Router(config-line)#access-class 1 in

The 'access-class' command links an existing access list to a terminal line or a range of terminal lines. In this example, the command links access-list 1 to all vty lines and applies it to incoming sessions. Essentially, this command allows only incoming Telnet connections from the network specified in access list one (208.15.208.0 /24).

Appling the access list using the 'out' paramter with the 'access-class' command would prevent outgoing telnet traffic but would not prevent incoming traffic.

The 'access-group' command is used to apply an access list to an interface. You cannot use this command to apply an access list to the vty lines. You must use the 'access-class' command apply an access-list to the vty lines.

For more information, see the Cisco IOS Master Commands List, Release 12.3(1).

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