Chapter 14: Routing Concepts

Question 1

Primary functions of a router

Answer 1

Determine best path to forward packets based on info in routing table, forward packets toward their destination and connect multiple networks

Question 2

Best path in routing table is also known as

Answer 2

The longest match

Question 3

What is the longest match

Answer 3

The route in the routing table that has the greatest number of far-left matching bits with the dst IP address of the packet. Greatest number of far-left matches = preferred route

Question 4

How does router learn about remote networks?

Answer 4

• Static Routes - added to routing table when a route is manually configured
• Dynamic routing protocols - added when routing protocols dynamically learn about the remote network; includes EIGRP and OSPF

Question 5

What’s default route and its IPv4 and IPv6 add?

Answer 5

specifies next-hop router to use when routing table doesn’t contain specific route that matches the dst IP add. IPv4 route entry of 0.0.0.0/0 and IPv6 route entry of ::/0

Question 6

What is used to determine the minimum number of far-left bits that must match btwn the IP add of the packet and the dst network (prefix) for a route to be used?

Answer 6

prefix length of the dst network in the routing table entry

Question 7

What happens to a packet when there are no matching route entry in the routing table and no default route?

Answer 7

Packet is dropped

Question 8

What happens after router determines the best path to forward packet?

Answer 8

Router now must determine how to encapsulate the packet in appropriate data link frame type and forward it out the correct egress interface

Question 9

How are packets with dst IP on Directly Connected Network encapsulated?

Answer 9

With Ethernet Frame. Router needs to determine the dst MAC address associated with dst IP add of the packet.

Question 10

Where is the next-hop router address found?

Answer 10

Route Entry

Question 11

What type of dst address is used when forwarding packets over serial connection? why?

Answer 11

Layer 2 broadcast dst add. Because Serial Connections don’t have MAC addresses.

Question 12

Three packet forwarding mechanisms supported by routers

Answer 12

Process switching.
Fast switching.
Cisco Express Forwarding (CEF).

Question 13

Process switching

Answer 13

Every packet that arrives on an interface is forwarded to the control plane where the CPU matches the dst add with an entry in its routing table. Older mechanism.

Question 14

Fast switching

Answer 14

Similar to process switching. Except includes fast-switching cache where next-hop info is stored. So only the first packet of a flow is sent to control plane and added. The next packets of that flow are quickly processed based on the info in cache.

Question 15

Cisco Express Forwarding (CEF)

Answer 15

Default Cisco IOS packet-forwarding mechanism.
Forwarding Information Base (FIB) and adjacency table are built after the network has converged. Packets are processed quickly in the data plane.

Question 16

If a router needs to send an ARP Request for the IPv4 address in one of its route entries, this means the packet will be forwarded:

Answer 16

to a next-hop router

Question 17

Common routing table code: L

Answer 17

Identifies the address assigned to a router interface

Question 18

Common routing table code: C

Answer 18

Identifies a directly connected network

Question 19

Common routing table code: S

Answer 19

Identifies a static route created to reach a specific network

Question 20

Common routing table code: O

Answer 20

Identifies a dynamically learned network from another router using OSPF routing protocol

Question 21

Common routing table code: *

Answer 21

This route is a candidate for a default route

Question 22

Routing Table Principles

Answer 22

• every router makes its decision alone, based on the info in its own routing table
• routing table info of one router does not necessarily match routing table of another router
• routing info about a path does not provide return routing info

Question 23

Routing Table Entry Information: Administrative distance

Answer 23

identifies the trustworthiness of the route source. Lower the value = preferred route source

Question 24

Routing Table Entry Information: Metric

Answer 24

Identifies value assigned to reach the remote network. Lower values = preferred routes

Question 25

Routing Table Entry Information: Route timestamp

Answer 25

Identifies how much time has passed since the route was learned

Question 26

Prefix length for local IPv4 and IPv6 in Directly Connected networks?

Answer 26

/32 and /128

Question 27

Pros and cons of static routes

Answer 27

Pros
- improved security and resource efficiency
- less bandwidth and no CPU cycles are used to calculate and communicate routes
Cons
- routes are manually configured and not automatically updated

Question 28

Primary uses of static routing:

Answer 28

• provides ease of routing table maintenance in smaller networks
• uses single default route to represent a path to any network that does not have a more specific match with another route in the routing table
• routes to and from stub networks (stub network is network accessed by a single route and router has only one neighbor)

Question 29

Advantages of dynamic routing protocols:

Answer 29

• ability to select a best path

- ability to automatically discover a new best path when there is a change in topology

Question 30

Two activities that dynamic routing protocol performs:

Answer 30

Network discovery and maintaining routing tables

Question 31

Network Discovery

Answer 31

ability of a routing protocol to share info about the networks that it knows about with other routers that are also using the same routing protocol

Question 32

When router is given a choice btwn static route or an OSPF route, which one is used?

Answer 32

Static route takes precedence because it has an administrative distance of 1 while OSPF has AD of 110

Question 33

Which route entry would be used for a packet with a dst IP add that matches an IP add of one of the router interfaces?

Answer 33

L for local

Question 34

Static routes are commonly used in these scenarios:

Answer 34

• as default route forwarding packets to a service provider
• for routes outside the routing domain and not learned by the dynamic routing protocol
• when network admin wants to explicitly define path for a specific network
• routing btwn stub networks

Question 35

Dynamic routing protocols are commonly used in the following scenarios:

Answer 35

• networks consisting of more than just a few routers
• when a change in network topology requires the network to auto determine another path
• for scalability

Question 36

Main components of dynamic routing protocols include:

Answer 36

• data structures (info kept in RAM)
• routing protocol messages
• algorithms

Question 37

What metric is used by OSPF to find best path?

Answer 37

“Cost” is based on cumulative bandwidth from source to destination. Faster links are assigned lower costs .

Question 38

What metric is used by EIGRP to find best path?

Answer 38

Metric is based on slowest bandwidth and delay values. Could also include load and reliability into the metric calculations

Question 39

What metric is used by RIP to find best path?

Answer 39

“Hop count”; each router along the path adds a hop to the count. 15 max hops allowed.

Question 40

Equal cost load balancing

Answer 40

When router has two or more paths to dst with equal cost metrics, then the router forwards the packets using both paths equally.

Question 41

Two common types of static routes in routing table?

Answer 41

• default static route

- static route to a specific network