CCNP Routing and Switching TSHOOT 300-135 Official Cert Guide (2015)

---

1. Which two data structures does a router use during the packet-forwarding process?

a. Routing table

b. Layer 3 to Layer 2 mapping table

c. Topology table

d. Link-state database

2. Which two data structures reside at the routers data plane?

a. IP routing table

b. ARP cache

c. Forwarding Information Base

d. Adjacency table

3. Which command enables you to verify routes in the FIB?

a. show ip route

b. show ip arp

c. show ip cef

d. show adjacency detail

4. Which are capable of populating a routing protocols data structure, such as the EIGRP topology table? (Choose three answers.)

a. Updates from a neighbor

b. Redistributed routes

c. Interfaces enabled for the routing process

d. Static routes

5. Which of the following has the lowest default administrative distance?

a. OSPF

b. EIGRP (internal)

c. RIP

d. eBGP

6. What is the default administrative distance of an OSPF intra-area route?

a. 90

b. 110

c. 115

d. 120

7. What is the default administrative distance of a static route?

a. 0

b. 1

c. 5

d. 20

8. How can you create a floating static route?

a. Provide the static route with a metric higher than the preferred source of the route

b. Provide the static route with a metric lower than the preferred source of the route

c. Provide the static route with an AD higher than the preferred source of the route

d. Provide the static route with an AD lower than the preferred source of the route

9. What occurs when you create an IPv4 static route with an Ethernet interface designated instead of a next hop IP address?

a. The router ARPs for the MAC address of the directly connected routers IP address.

b. The router forwards the packet with a destination MAC address of FFFF:FFFF:FFFF.

c. The router ARPs for the MAC address of the IP address in the source of the packet.

d. The router ARPs for the MAC address of the IP address in the destination of the packet.

10. What occurs when you create an IPv6 static route with a global unicast address as the next hop?

a. The router ARPs for the MAC address of the directly connected routers IP address.

b. The router forwards the packet with a destination MAC address of FFFF:FFFF:FFFF.

c. The router uses NDP to determine the MAC address associated with the global unicast address.

d. The router uses NDP to determine the MAC address associated with the destination IPv6 address in the packet.

11. Which statements are true about GRE tunnels?

a. The original packet is encapsulated in a GRE header only.

b. The original packet is encapsulated in a GRE header, and then a new IP header is added.

c. The original packet is encapsulated in an IP header, and then a GRE header is added.

d. The original packet header is rewritten by the GRE header.

12. When creating a virtual tunnel interface, what is the default tunnel mode?

a. GRE/IP

b. GRE/IPv6

c. IPv6/IP

d. GRE/Multipoint

13. Which of the following are true about IPsec transport mode? (Choose two answers.)

a. It creates a new tunnel IP packet.

b. It reuses the GRE IP header, which reduces overhead.

c. It encrypts the original IP packet, the GRE header, and the GRE IP header.

d. It encrypts the original IP packet and the GRE header only.

Foundation Topics

Packet-Forwarding Process

When troubleshooting connectivity issues for an IP-based network, the network layer (Layer 3) of the OSI reference model is often an appropriate place to begin your troubleshooting efforts (divide-and-conquer method). For example, if you are experiencing connectivity issues between two hosts on a network, you could check Layer 3 by pinging between the hosts. If the pings are successful, you can conclude that the issue resides at upper layers of the OSI reference model (Layers 4–7). However, if the pings fail, you can focus your troubleshooting efforts on Layers 1–3. If you ultimately determine that there is a problem at Layer 3, your efforts may be centered on the packet-forwarding process of a router.

This section discusses the packet-forwarding process and the commands that you can use to verify the entries in the data structures that are used for this process. It also provides you with a collection of Cisco IOS Software commands that could prove to be useful when troubleshooting-related issues.

Reviewing Layer 3 Packet-Forwarding Process

To review basic routing processes, consider Figure 12-1. In this topology, PC1 needs to access HTTP resources on Server1. Notice that PC1 and Server1 are on different networks. So, the question becomes, how does a packet from a source IP address of 192.168.1.2 get routed to a destination IP address of 192.168.3.2?

Figure 12-1 Basic Routing Topology

Consider the following walkthrough of this process, step by step:

Step 1. PC1 compares its IP address and subnet mask of 192.168.1.2/24 with the destination IP address 192.168.3.2, as discussed in Chapter 9, “Troubleshooting IPv4 Addressing and Addressing Technologies.” PC1 determines the network portion of its own IP address. It then compares these binary bits with the same binary bits of the destination address. If they are the same, the destination is on the same subnet. If they differ, the destination is on a remote subnet. PC1 concludes that the destination IP address resides on a remote subnet in this example. Therefore, PC1 needs to send the frame to its default gateway, which could have been manually configured on PC1 or dynamically learned via Dynamic Host Configuration Protocol (DHCP). In this example, PC1 has a default gateway of 192.168.1.1 (that is, router R1). To construct a proper Layer 2 frame, PC1 needs the MAC address of the frame’s destination, which is PC1’s default gateway in this example. If the MAC address is not in PC1’s Address Resolution Protocol (ARP) cache, PC1 uses ARP to discover it. Once PC1 receives an ARP reply from router R1, PC1 adds router R1’s MAC address to its ARP cache. PC1 now sends its data destined for Server1 in a frame addressed to R1, as shown in Figure 12-2.

Figure 12-2 Basic Routing Step 1

Step 2. Router R1 receives the frame sent from PC1, and because the destination MAC is R1’s, R1 tears off the Layer 2 header and interrogates the IP (Layer 3) header. An IP header contains a time-to-live (TTL) field, which is decremented once for each router hop. Therefore, router R1 decrements the packet’s TTL field. If the value in the TTL field is reduced to zero, the router discards the packet and sends a time-exceeded Internet Control Message Protocol (ICMP) message back to the source. Assuming the TTL is not decremented to zero, router R1 checks its routing table to determine the best path to reach the IP address 192.168.3.2. In this example, router R1’s routing table has an entry stating that network 192.168.3.0/24 is accessible via interface Serial 1/1. Note that ARPs are not required for serial interfaces because these interface types do not have MAC addresses. Therefore, router R1 forwards the frame out of its Serial 1/1 interface, as shown in Figure 12-3, using the PPP Layer 2 framing header.

Figure 12-3 Basic Routing Step 2

Step 3. When router R2 receives the frame, it removes the PPP header, and then decrements the TTL in the IP header, just as router R1 did. Again, assuming the TTL did not get decremented to zero, router R2 interrogates the IP header to determine the destination network. In this case, the destination network of 192.168.3.0/24 is directly attached to router R2’s Fast Ethernet 0/0 interface. Similar to how PC1 sent out an ARP request to determine the MAC address of its default gateway, router R2 sends an ARP request to determine the MAC address of Server1 if it is not already known in the ARP cache. Once an ARP reply is received from Server1, router R2 forwards the frame out of its Fast Ethernet 0/0 interface to Server1, as shown in Figure 12-4.

Figure 12-4 Basic Routing Step 3

The previous steps identified two router data structures:

IP routing table: When a router needed to route an IP packet, it consulted its IP routing table to find the best match. The best match is the route that has the longest prefix. For example, suppose that a router has a routing entry for network 10.0.0.0/8, 10.1.1.0/24, and 10.1.1.0/26. Also, suppose that the router is trying to forward a packet with the destination IP address 10.1.1.10. The router would select the 10.1.1.0/26 route entry as the best match for 10.1.1.10 because that route entry has the longest prefix of /26 (matches the most number of bits).

Layer 3 to Layer 2 mapping table: In the previous figure, router R2’s ARP cache contained Layer 3 to Layer 2 mapping information. Specifically, the ARP cache had a mapping that said a MAC address of 2222.2222.2222 corresponded to an IP address of 192.168.3.2. An ARP cache is the Layer 3 to Layer 2 mapping data structure used for Ethernet-based networks, but similar data structures are used for Multipoint Frame Relay networks and dynamic multipoint virtual private networks (DMVPNs). However, for point-to-point links such as PPP or HDLC, because there is only one other possible device connected to the other end of the link, no mapping information is needed to determine the next-hop device.

Continually querying a router’s routing table and its Layer 3 to Layer 2 mapping data structure (for example, an ARP cache) is less than efficient. Fortunately, Cisco Express Forwarding (CEF), as introduced in Chapter 3, “Troubleshooting Device Performance,” makes lookups much more efficient. CEF gleans its information from the router’s IP routing table and Layer 3 to Layer 2 mapping tables. Then, CEF’s data structures can be referenced when forwarding packets. The two primary CEF data structures are as follows:

Forwarding Information Base (FIB): The FIB contains Layer 3 information, similar to the information found in an IP routing table. In addition, an FIB contains information about multicast routes and directly connected hosts.

Adjacency table: When a router is performing a route lookup using CEF, the FIB references an entry in the adjacency table. The adjacency table entry contains the frame header information required by the router to properly form a frame. Therefore, an egress interface and a next-hop MAC address would be in an adjacency entry for a multipoint interface, whereas a point-to-point interface would require only egress interface information.

As a reference, Figure 12-5 shows the router data structures previously discussed.

Figure 12-5 A Router’s Data Structures

Troubleshooting the Packet-Forwarding Process

When troubleshooting packet-forwarding issues, you will examine a router’s IP routing table. If the traffic’s observed behavior is not conforming to information in the IP routing table, remember that the IP routing table is maintained by a router’s control plane and is used to build the tables at the data plane. CEF is operating in the data plane and uses the FIB. Therefore, you want to view the CEF data structures (that is, the FIB and the adjacency table) that contain all the information required to make packet-forwarding decisions.

Example 12-1 provides sample output from the show ip route [ip_address] command. The output shows that the next-hop IP address to reach an IP address of 192.168.1.11 is 192.168.0.11, which is accessible via interface Fast Ethernet 0/0. Because this information is coming from the control plane, it includes information about the routing protocol, which is OSPF in this case.

Example 12-1 show ip route ip_address Command Output