1. CCNP Routing Semester 5
Chapter 4
OSPF

2. Using OSPF in a Single Area
OSPF is more complex than RIP. To harness its power, it is important to spend time in the analysis and design of the network. Sometimes, it is necessary to redesign the addressing scheme to support the hierarchical structure that OSPF requires.

3. Hello Packet Fields Field Router ID Hello/Dead Intervals Neighbor
Area ID
Function
Identifies the router within the AS
Hello maintains presence of router in its neighbor databases; works like a keepalive
Another router with whom updates will be exchanged to synchronize database
Hello packet must come from a router within same area to be valid

4. Hello Packet Fields Field Function Used to manually select DR and BDR
Router Priority
DR IP
Address
BDR IP
Authentication
Password
Stub Area Flag
Function
Used to manually select DR and BDR
Used to allow the router to create unicast traffic to DR
Allows router to create unicast traffic to the BDR
Used for security
Identifies which type of LSA will be transmitted and accepted

5. OSPF Packets Hello Protocol Database Descriptor
Used to find neighbors and to determine designated router and backup designated router – continued propagation of Hello protocol maintains the transmitting router in the topology database of those that hear the message
Used to send summary info to neighbors to synchronize topology databases

6. OSPF Packets Link State Request Link State Update Acknowledge
Works as a request for more detailed info which is sent when router receives a database descriptor that contains new info
Works as LSA packet issues in response to request for database info in LSA request packet
Acknowledges link-state update

7. OSPF Databases Adjacency Database Link-State (topological database)
Forwarding
(routing table)
List of all neighbor routers to which a router has established bidirectional communication. Unique for each router
List of info about all other routers in the network. This database shows the network topology. All routers within an area have identical link-state databases
List of routes generated when an algorithm is run on the link-state database. Each router’s routing table is unique and holds info on how / where to send packets

8. Types of OSPF Networks Type Broadcast Multiaccess Nonbroadcast
Point-to-Point
Point-to-Multipoint
Determining Characteristic
Ethernet, Token Ring, or FDDI
Frame Relay, X.25, SMDS
PPP, HDLC
Configured by an administrator DR
Election?
Yes No

9. Designated Router (DR)
For every multiaccess IP network, one router will be elected the DR. This DR has two main functions: first, to become adjacent to all other routers on the network, and second, to act as a “spokesperson” for the network. As spokesperson, the DR sends other IP networks network LSAs that list all local routers. Because the DR becomes adjacent to all other routers on the IP network, it is the focal point for collecting routing information (LSAs).

10. Backup Designated Router (BDR)
The DR could represent a single point of failure, so a second router is elected as the BDR to provide fault tolerance. Thus, the BDR must also become adjacent to all routers on the network and must serve as a second focal point for LSAs. But, unlike the DR, the BDR is not responsible for updating the other routers or sending network LSAs. Instead, the BDR keeps a timer on the DR’s update activity to ensure that it is operational. If the BDR does not detect activity from the DR before the timer expires, the BDR takes over the role of DR and a new BDR is elected.

11. Steps in the Operation of OSPF
OSPF routers progress through five distinct
steps of operation:
Establish router adjacencies
Elect a DR and BDR (if necessary)
Discover routes
Select the appropriate routers to use
Maintain routing information

12. Message-Digest Authentication
Router(config)#ip ospf message-digest-key key-id md5 [encryption-type] password
key_id An identifier in the range 1 to 255, which allows for multiple keys. The key ID configuration on each router must match to authentication
md5 A required value that specifies the MD5 algorithm will be used
encryption-type Optional value that specifies the type of encryption to use (0 to 7). Type 7 indicates Cisco proprietary encryption. Type 0 is the default
password An alphanumeric password to be used as the message-digest key

13. Learning a New Route (2500 Series)
Router takes first entry from update – first network with info about state of its link
Router verifies that type of LSA is one that can be accepted by this router
Router issues a lookup to its topological database when the LSA is valid
Entries NOT in topological database will be flooded immediately out all OSPF interfaces except receiving interface
Further questions are required when LSA entry is in topological database

14. Learning a New Route (2500 Series)
If sequence numbers are the same, router calculates checksum for LSAs and uses the LSA with the higher checksum
When checksum numbers are the same, router checks MaxAge field to ascertain which is more recent update
Router determines whether the LSU has arrived outside the wait period before another computation is allowed
When new LSA entry passes the tests, it is flooded out all OSPF interfaces except for the receiving interface

15. Learning a New Route (2500 Series)
Current copy replaces old LSA entry. If there was no entry, current copy is placed in database
Received LSA is acknowledged
If LSA entry was in database, but LSA just received has an older sequence number, the process asks whether info in database is the same
If info is different and new LSA has an older sequence number, receiving router discards the LSA update and issues its own LSA
After initial flood, updates are sent only when there are changes in the area or when the 30 min. timer goes off

16. Glossary Adjacency Area Autonomous System
Formed when two neighboring routers have exchanged info and share the same topology table. Database are synchronized and see same networks
A group of routers that share the same area ID. Each router in the area has the same topology table. Each router is an internal router. The area is defined on an interface basis in the OSPF configuration
Routers that share the same routing protocol within the same organization

17. Glossary Backup Designated Router (BDR) Cost
Backup to the designated router in case the DR fails. BDR performs none of the DR functions while the DR is operating correctly
A metric for OSPF. Not defined in standard with a value. Cisco uses default of inverse of bandwidth – higher the speed, lower the cost. Can be overridden with manual configuration – do only with full knowledge of network

18. Glossary Database Descriptor (DDP) Designated Router (DR)
Referred to as DBD – database descriptor packet – packets exchanged between neighbors during the exchange state. DDPs contain LSAs which describe the links of every router in the neighbor’s topology table
Responsible for making adjacencies with all neighbors on a multiaccess network such as Ethernet or FDDI. DR represents the multiaccess network in that it ensures every router on the link has the same topology database

19. Glossary Exchange State ExStart init
Method by which two neighboring routers discover the map of the network. When these routers become adjacent, they must first exchange DDPs to ensure they have the same topology table
State in which the neighboring routers determine the sequence number of the DDPs and establish the master/slave relationship
State in which hello packet has been sent from the router, which is waiting for reply to establish two-way communication

20. Glossary Internal Router Link-State Adv LSA Database
Router that has all its interfaces in the same area
Packet describing router’s links and state of those links
Topology map. Map of every router, its links, state of the links. Also has a map of every network and every path to each network

21. Glossary Link-State Request LSR Update LSU Neighbor Table
Router requests additional info when LSA entry is not present or is older than the DDP when comparing the topology database
Update sent in response to the LSR – it is the LSA that was requested
Router on the same link with whom routing info is exchanged
Table built from hello received from neighbor

22. Glossary Priority SPF Tree Topology Table
Cisco tool by which designated router can be manually elected or prevented from taking part in a DR/BDR election
Tree of topological network. Drawn after SPF algorithm has been run. Algorithm prunes database of alternative paths and creates loop-free shortest path to all networks
Same as link-state database

23. More Glossary Flood Fully Adjacent Loading State Loopback Interface
Setup Script
Two-way State
Refers to network info – sent to every device in domain
When routing tables of two neighbors are fully synchronized
State where router will request more detail using LSR
Virtual interface that does not exist physically. If it doesn’t exist, it can’t go down
Question and answer dialogue offered by Cisco router
State during process in which two routers are creating an adjacency. This is stage before routing info is exchanged