CCNP Routing Semester 5 Chapter 4 OSPF

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.

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

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

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

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

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

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

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).

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.

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

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

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

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

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

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

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

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

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

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

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

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

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