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Ch. 16 Configuring IPv4 Addresses and Routes

Terms in this set (42)

A PC opens a command prompt and uses the ipconfig command to see that the PC's IP address and mask are 192.168.4.77 and 255.255.255.224. The user then runs a test using the ping 192.168.4.117 command. Which of the following answers is the most likely to happen?

a. The PC sends packets directly to the host with address 192.168.4.117.

b. The PC sends packets to its default gateway.

c. The PC sends a DNS query for 192.168.4.117.

d. The PC sends an ARP looking for the MAC address of the DHCP server.
B (The PC sends packets to its default gateway.) . PCs use two-option logic: send local packets (destined for hosts in the same subnet) directly, and send remote packets (destined for hosts in other subnets) to the default gateway/router. In this case, the PC's own IP address is 192.168.5.77, with mask 255.255.224.0, so it is in subnet 192.168.4.64/27. This subnet has a range of addresses from 192.168.4.64 to 192.168.4.95, including the subnet and broadcast address. As a result, the PC sends the packet to its default gateway. As for the other incorrect answers, if the ping command had used a host name, it would have first asked a DNS server to resolve the name. Also, the PC already has an IP address, so DHCP would not be needed.
Router R1 lists a route in its routing table. Which of the following answers list a fact from a route, that the router then compares to the packet's destination address? (Choose 2 answers.)

a. Mask

b. Next-hop router

c. Subnet ID

d. Outgoing interface
A (Mask) and C (Subnet ID) . The route defines the group of addresses represented by the route using the subnet ID and mask. The router can use those numbers to find the range of addresses that should be matched by this route. The other two answers list facts useful when forwarding packets that happen to match the route.
Router 1 has a Fast Ethernet interface 0/0 with IP address 10.1.1.1. The interface is connected to a switch. This connection is then migrated to use 802.1Q trunking. Which of the following commands could be part of a valid configuration for Router 1's Fa0/0 interface? (Choose two answers.)

a. interface fastethernet 0/0.4

b. dot1q enable

c. dot1q enable 4

d. trunking enable

e. trunking enable 4

f. encapsulation dot1q 4
A (interface fastethernet 0/0.4) and F (encapsulation dot1q 4 ) . Of all the commands listed, only the two correct answers are syntactically correct router configuration commands. The command to enable 802.1Q trunking is encapsulation dot1q vlan_id.
A router is configured with the no ip subnet-zero global configuration command. Which of the following interface subcommands would not be accepted by this router?

a. ip address 10.1.1.1 255.255.255.0

b. ip address 10.0.0.129 255.255.255.128

c. ip address 10.1.2.2 255.254.0.0

d. ip address 10.0.0.5 255.255.255.252
C (ip address 10.1.2.2 255.254.0.0) . With the no ip subnet-zero command configured, the router will not allow any interfaces to be configured with an IP address in the zero subnet. Of the listed answers, subnet 10.0.0.0 255.254.0.0 is a zero subnet, with a range of addresses from 10.0.0.1 to 10.1.255.254. The ip address 10.1.2.2 255.254.0.0 command would be rejected.
A Layer 3 switch has been configured to route IP packets between VLANs 1, 2, and 3, which connect to subnets 172.20.1.0/25, 172.20.2.0/25, and 172.20.3.0/25, respectively. The engineer issues a show ip route command on the Layer 3 switch, listing the connected routes. Which of the following answers lists a piece of information that should be in at least one of the routes?

a. Interface Gigabit Ethernet 0/0.3

b. Next-hop router 172.20.4.1

c. Interface VLAN 2

d. Mask 255.255.255.0
C (Interface VLAN 2) . The configuration of the Layer 3 switch's routing feature uses VLAN interfaces, with the interface number matching the VLAN ID. The matching connected routes, like all connected IP routes, will list the interfaces but not a next-hop IP address. The three connected routes will list the VLAN interfaces 1, 2, and 3, respectively.
An engineer configures a static IPv4 route on Router R1. Which of the following pieces of information should not be listed as a parameter in the configuration command that creates this static IPv4 route?

a. The destination subnet's subnet ID

b. The next-hop router's IP address

c. The next-hop router's neighboring interface

d. The subnet mask
C (The next-hop router's neighboring interface) . The ip route command can refer to the IP address of the next-hop router or to the local router's interface. It also refers to the subnet ID and matching subnet mask, defining the range of addresses matched by the route.
Which of the following commands correctly configures a static route?

a. ip route 10.1.3.0 255.255.255.0 10.1.130.253

b. ip route 10.1.3.0 serial 0

c. ip route 10.1.3.0 /24 10.1.130.253

d. ip route 10.1.3.0 /24 serial 0
A (ip route 10.1.3.0 255.255.255.0 10.1.130.253) . The correct syntax lists a subnet number, then a subnet mask in dotted-decimal form, and then either an outgoing interface or a next-hop IP address.
List the steps taken by a host when forwarding IP packets.
Step 1. If the destination is local, send directly:
A. Find the destination host's MAC address. Use the already-known Address Resolution Protocol (ARP) table entry, or use ARP messages to learn the information.
B. Encapsulate the IP packet in a data link frame, with the destination data link address of the destination host.
Step 2. If the destination is not local, send to the default gateway:
A. Find the default gateway's MAC address. Use the already-known ARP table entry, or use ARP messages to learn the information.
List the steps taken by a router when forwarding IP packets.
1. For each receive data link frame, choose whether or not to process the frame. process it if :
A. The frame has no errors (per the data link trailer Frame Check Sequence or FCS, Field)
B. The frame's destination data link address is the router's address (or an appropriate multicast or broadcast address).
2. If choosing to process the frame at step 1, deencapsulate the packet from inside the data link frame.
3. Make a routing decision. To do so, compare the packet's destination IP address to the routing table and find the route that matches the destination address. This route identifies the outgoing interface of the router and possibly the next-hop router.
4. Encapsulate the packet into a data link frame appropriate for the outgoing interface. when forwarding out LAN interfaces, use ARP as needed to find the next devices MAC address.
5. Transmit the frame out the outgoing interface, as listed int he matched IP route.
List three common source from which routers build IP routes
Connected routes: Added because of the configuration of the ip address interface subcommand on the local router.
Static routes: Added because of the configuration of the ip route global command on the local router.
Routing protocols: Added as a function by configuration on all routers, resulting in a process by which routers dynamically tell each other about the network so that they all learn routes.
List the rules regarding when a router creates a connected route
The interface is in a working state---in other words, the interface status n the show interfaces command lists a line status of up and a protocol status of up.
The interface has an IP address assigned through the ip address interface subcommand.
List three options for connecting a router to each VLAN
Use a router, with one router LAN interface and cable connected to the switch for each and every VLAN (typically not used).

Use a router, with a VLAN trunk connecting to a LAN switch.

Use a Layer 3 switch.
Checklist configuration 802.1Q
Step 1. Create a unique subinterface for each VLAN that needs to be routed (interface (type number.subint).
Step 2. Enable 802.1Q, and associate one specific VLAN witht he subinterface in subinterface config mode (encapsulation dot1q vlan_id)
Step 3. Configure IP settings (address and mask) in subinterface config mode (ip address address mask)
802.1 native VLAN configuration checklist
Configure the ip address command on the physical interface, but without an exapsulation command; the router considers this physical interface to be using the native VLAN.

Configure the ip address command on a subinterface, and use the encapsulation...native subcommand.
List Layer 3 switching configuration
Step 1. Enable hardware support for IPv4 routing. For ecample, on 2960 switches configure the sdm prefer lanbase-routing global command and reload the switch.
Step 2. Enable IPv4 routing globally (ip routing)
Step 3. Create VLAN interfaces for each VLAN for which the Layer 3 switch is routing packets (interface vlan vlan_id).
Step 4. Configure an IP address and mask on the VLAN interface (in interface configuration mode for that interface), enabling IPv4 on that VLAN interface (ip address address mask).
Step 5. If the switch defaults to place the VLAN interface in a disabled (shutdown) state, enable the interface (no shutdown).
Default gateway/router
On an IP host, the IP address of some router to which the host sends packets when the packet's destination address is on a subnet other than the local subnet.
ARP
Address Resolution Protocol. An internet protocol used to map an IP address to a MAC address. Defined in RFC 826
ARP table
A list of IP addresses of neighbors on the same VLAN, along with their MAC addresses, as kept in memory by hosts and routers.
Routing table
A list of routes in a router, with each route listing the destination subnet and mask, the router interface out which to forward packets destined to that subnet, and as needed, the next-hop router's IP address.
Next-hop router
In an IP route in a routing table, part of a routing table entry that refers to the next IP router (by IP address) that should receive packets that match the route.
Outgoing interface
In an IP route in a routing table, part of a routing table entry that refers to the loal interface out which the local router should forward packets that match the route.
Subinterface
is a logical interface that uses the "parent" physical interface for actually moving the data.
VLAN interface
A configuration conecept inside Cisco switches, used as an interface between IOS running on the switch and a VLAN supported inside the switch, so that the switch can assign an IP address and send IP packets into that VLAN.
Layer 3 switch
also known as a multilayer switch is a LAN switch that can also perform Layer 3 routing functions. The name comes from the fact that this device makes forwarding decisions based on logic from multiple OSI layers (Layers 2 and 3).
Cisco Express Forwarding (CEF)
A method of internal processing on Cisco routers, meant to make the routing process very efficient, doing so by caching IP routes in a table that can be searched very quickly, and by remembering data link headers rather rather than building them for every packet that is forwarded.
connected route
On a router, an IP route added to the routing table when the router interface is both up and has an IP address configured. The route is for the subnet that can be calculated based on the configured IP address and mask.
Static route
An IP route on a router created by the user configuring the details of the route on the local router.
Default route
On a router, the route that is considered to match all packets that are not otherwise matched by some more specific route.
Zero subnet
For every classful IPv4 network that is subnetted, the one subnet whose subent number has all binary 0s in the subnet part of the number. In decimal, the zero subnet can be easily identified because it is the same number as the classful network number.
ip address (ip-address mask [secondary])
Interface subcommand that assigns interface's IP address and optionally makes the address a secondary address
Interface (type number.subint)
Global command to create a subninterface and to enter configuration mode for that subinterface
encapsulation dot1q (vlan-id [native])
A subinterface subcommand that tells the router to use 802.1Q trunking. For a particular VLAN, and with the native keyword, to not encapsulate in a trunking header.
encapsulation isl (vlan-identifier)
A subinterface subcommand that tells the router to use ISL trunking for a particular VLAN.
sdm prefer lanbase-routing
A command on Cisco switches that enables the switch to support IP routing if configured.
[no] ip routing
Global command that enables (ip routing) or disables (no ip routing) the routing of IPv4 packets on a router or Layer 3 switch
interface vlan (vlan_id)
Global command on a Layer 3 switch to create a VLAN interface and to enter configuration mode for that VLAN interface
[no] ip subnet zero
Global command that allows (ip subnet-zero) or disallows (no ip subnet-zero) the configuration of an interface IP address in a zero subnet.
ip route (prefix mask {ip-address | interface-type interface number} [distance] [permanent]
Global configuration command that creates a static route.
Show up route
Lists the router's entire routing table.
Show up route [connected | static | ospf]
Lists a subset of the IP routing table.
Show ip route (ip address)
Lists detailed information about the route that a router matches for the listed IP address.
Show VLANS
Lists VLAN configuration and statistics for VLAN trunks configured on routers.
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