Chapter 8: OSPF

Question 1

What are the six (6) Feature of OSPF

Answer 1

1. Classless - supports VLSM & CIDR.
2. Efficient – triggered updates (not periodic), uses SPF.
3. Fast convergence
4. Scalable
5. Secure - supports encrypted updates when config’d
6. AD = 110

Question 2

The 3 main components of the OSPF

Answer 2

1. Data Structures
      • Creates & maintains three DBs:
      • Adjacency DB à neighbor table
      • Link-state DB (LSDB) à topology table
      • Forwarding DB à routing table
2. Routing Packets (5 types)
3. Dijkstra’s SPF algorithm

Question 3

What information does OSPF Routers exchange

Answer 3

used to discover neighbours & to exchange router info

Question 4

what does LSA stand for

Answer 4

Link State Advertisment

Question 5

what does the router Exchange Hello Packet do

Answer 5

If neighbour is present, the router establishes a

neighbour adjacency with that neighbour

Question 6

What are six (6) information does router exchange Link State Advertisement

Answer 6

1. LSAs contain the state & cost of each link.
2. Router flood LSAs to adj neighbours.
3. Adj neighbours receiving LSAs immediately flood
   them to other neighbours, until all router in the area
   have all LSAs.
4. Build the topology table based on LSAs.
5. This DB eventually holds all the info about the network.
6. Executes the SPF Algorithm to build tree.

Question 7

What is Administrative distance (AD)

Answer 7

s the trustworthiness (or preference) of the route source. OSPF has a default administrative distance of 110

Question 8

What are the three (3) data structures

8.1.1.3

Answer 8

1. Adjacency database - Creates the neighbor table
2. Link-state database (LSDB) - Creates the topology table
3. Forwarding database - Creates the routing table

Question 9

What are the five (5) Routing Protocol Messages packets

8.1.1.3

Answer 9

1. Hello packet
2. Database description packet
3. Link-state request packet
4. Link-state update packet
5. Link-state acknowledgment packet

Question 10

What is Single-Area OSPF

8.1.1.5

Answer 10

all routers are in one area called the backbone area (area 0).

Question 11

What is Multiarea OSPF

8.1.1.5

Answer 11

is implemented using multiple areas, in a hierarchical fashion. All areas must connect to the backbone area (area 0). Routers interconnecting the areas are referred to as Area Border Routers (ABR).

Question 12

What does Fast Convergence?

8.1.1.6

Answer 12

Quickly Propagates network changes

Question 13

What does Classless support?

8.1.1.6

Answer 13

VSLM and CIDR

Question 14

What security does OSPF use

8.1.1.6

Answer 14

MD5 Authentication

Question 15

What does Adjacency Database?

8.1.1.6

Answer 15

Created the Neighbour table

Question 16

What does Dijksta’s Algorithm do?

8.1.1.6

Answer 16

Used to compute the SPF Cost

Question 17

What does Link-State database do?

8.1.1.6

Answer 17

Creates the Topology Table

Question 18

What is the Encapsulating OSPF Messages process.

8.1.2.1

Answer 18

1. Data Link Ethernet Frame Header - Identifies the destination multicast MAC addresses 01-00-5E-00-00-05 or 01-00-5E-00-00-06.
2. IP Packet Header - Identifies the IPv4 protocol field 89 which indicates that this is an OSPF packet. It also identifies one of two OSPF multicast addresses, 224.0.0.5 or 224.0.0.6.
3. OSPF Packet Header - Identifies the OSPF packet type, the router ID and the area ID.
4. OSPF Packet Type Specific Data - Contains the OSPF packet type information. The content differs depending on the packet type. In this case, it is an IPv4 Header.

Question 19

What are the 7 states of adjacency.

Answer 19

1. Down State
2. Initiation State
3. Two - Way State
4. ExStart State
5. Exchange State
6. Loading State
7. Full State