Configuring and Troubleshooting Routers

Question 1

Routing Table

Answer 1

• Stores information of other IP networks and hosts in router

Question 2

Routing Entry

Answer 2

Contains:

• Protocol: Source of the route
• Destination: Can be defined by specific host but is usually directed to network ID
• Interface: Local interface to use to forward a packet along the chosen route. Might be IP address of interface or layer 2 interface ID
• Gateway/next hop: IP Address of next router along path to destination

Question 3

Direct Network Route

Answer 3

Category of routing table entry

- For subnets to which router is directly attached

Question 4

Remote Network Routes

Answer 4

Category of routing table entry

- For subnets and IP networks that are not directly attached

Question 5

Host Routes

Answer 5

Category of routing table entry

• For routes to a specific IP address.
• Has /32 network prefix

Question 6

Default Routes

Answer 6

Category of routing table entry

• Used when an exact match for a network or host route is not found
• Most end systems are configured with default route pointing to default gateway
• Also sometime the simplest way for an edge router to forward to ISP router

Question 7

Directly Connected Routes

Answer 7

• IP network or subnet for each active router interface is automatically added to routing table

Question 8

Static Route

Answer 8

• Manually added
• Only changes if edited by administrator
• Can be set tp be persistent or non-persistent (removed from table at reboot)

Question 9

Hop Count

Answer 9

• Each router along path counts as one hop

- Switches do not count as hops

Question 10

TTL

Answer 10

Time to Live

• At each router, TTL IP header field is decreased by 1.
• Could be greater if router is congested
• Nominally the number of seconds a packet can stay on network before being discarded
• In practice, it is maximum hop count
• At 0 packet is discarded
• Prevents badly addressed packets from permanently staying on network

Question 11

Dynamic Routing

Answer 11

• Protocol that uses algorithms and metrics to build routing information
• Stores information about the networks to which the router is connected
• Prioritizes paths

Question 12

Distance Vector

Answer 12

• Algorithm used by routing protocols that select a forwarding path based on next hop router and lowest hop count to destination network

Question 13

Convergence

Answer 13

• Process whereby routers running dynamic routing algorithms agree on network topology
• Important to adapt to changes in network topology
• Network where all routers share same topology is call steady state

Question 14

AS

Answer 14

Autonomous System

- network under administrative control of single owner

Question 15

IGP

Answer 15

Interior Gateway Protocol

- Identifies routes within AS (Autonomous System)

Question 16

EGP

Answer 16

Exterior Gateway Protocol

• Advertises routes between autonomous systems
• Includes a field to communicate the networks autonomous system ID
• Allows network owners to determine if they can use paths through other orgs network

Question 17

RIP

Answer 17

Routing Information Protocol
Type: Distance Vector
Class: IGP
Transport: UDP (Port 520 or 521)

• Maximum hop count 15, limits size of network
• Best suited for small networks with limited failover routes

Question 18

EIGRP

Answer 18

Enhanced Gateway Routing Protocol
Type: Distance Vector/Hybrid
Class: IGP
Transport: Native (Port 88)

• Used Bandwidth and Delay for metric
• Only sends routing information upon topology change
• Faster convergence
• Maximum hops of 255
• Not encapsulated in TCP or UDP

Question 19

OSPF

Answer 19

Open Shortest Path First
Type: Link State
Class: IGP
Transport: Native IP (89)

• Stores complete network topology to asses least cost paths
• Best suited for large orgs with multiple redundant paths
• Makes small, frequent updates for rapid convergence
• Require more CPU and memory

Question 20

BGP

Answer 20

Border Gateway Protocol
Type: Path Vector
Class: EGP
Transport: TCP (Port 179)
- Primarily used on the internet, between ISPs
- Based multiple metrics like hop count, weight preference, origin and community
- Prioritizez stability and can be slow to converge

Question 21

Longest Prefix Match

Answer 21

• Determining factor if router has similar networks in its routing table
• Routes base on longest prefix

Question 22

AD

Answer 22

Administrative Distance
- Used to express relative trustworthiness of protocol supplying route to routers
- Used to break ties in prefix length and provide backup routes
- By default they are: 
Local interface: 0
Static route: 1
BGP: 20
EIGRP: 90
OSPF: 110
RIP: 120
Unknown: 255

Question 23

CIDR

Answer 23

Classless Inter-Domain Routing

• Uses bits normally assigned to network ID to mask complexity of subnet and host addressing
• Also know as supernetting

Question 24

Link State

Answer 24

• Algorithm that allows router to store complete network topology
• Assesses least-cost paths based on this information