w3ch6

OSPF Message Format

• OSPF message format consists of various fields, including:
  • Version Number (set to 2 for OSPF version 2)
  • Type (Hello, Database Description, LSR, LSU, LSAck)
  • Packet Length (in bytes, including the 24-byte header)
  • Router ID (the ID of the router that generated the message)
  • Area ID (an identification of the OSPF area to which the message belongs)
  • Checksum (a 16-bit checksum computed in a manner similar to a standard IP checksum)
  • Authentication Type (plaintext, no authentication, or MD5 type)
  • Authentication (a 64-bit field used for authentication of the message, as needed)
  • Data (contains different information depending on the OSPF packet type)

OSPF Router ID

• OSPF router ID is selected by the router itself when the OSPF process initializes
  • The highest IP address on a loopback interface is selected as the router ID
  • If no loopback interfaces are configured, the highest IP address of running interfaces is selected
  • The router ID uniquely identifies a router within an OSPF domain

OSPF LSA Types

• There are several types of LSAs, including:
  • Type 1 – Router LSA (generated by each router for each area it is located in)
  • Type 2 – Network LSA (generated by the DR)
  • Type 3 – Summary LSA (created by the ABR and flooded into other areas)
  • Type 4 – Summary ASBR LSA (generated by the ABR to include the router ID of the ASBR)
  • Type 5 – External LSA (also known as autonomous system external LSA, generated by the ASBR)
  • Type 6 – Multicast OSPF LSA
  • Type 7 – External LSA (also known as not-so-stubby-area (NSSA) LSA)
  • Type 8 – Used to internetwork OSPF and BGP
  • Type 9, 10, 11 – Designated for future use, for application-specific purposes, used along with MPLS

OSPF Area Types

• There are several types of OSPF areas, including:
  • Standard Area (accepts link updates, summary routes, and external OSPF routes)
  • Backbone Area (also known as transit area, named as Area 0, has all properties of standard area)
  • Stub Area (does not accept external routes, uses default route to reach external routing domains)
  • Totally Stubby Area (does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks)

OSPF Neighbor States

• OSPF neighbor states include:
  • Down (initial state for a neighbor, seen when a working adjacency to a neighbor is torn down or when a manually configured neighbor does not respond to initial Hello packets)
  • Init (this router can hear the other router, but it is not certain whether the other router can hear this router)
  • 2-Way (confirms a bidirectional visibility between the two routers)
  • ExStart (establishes the Master/Slave relationship, determines the Master and Slave roles for each router, and agrees on a common starting sequence number)
  • Exchange (Database Description packets are exchanged between routers, carrying the list of link-state database elements)
  • Loading (neighbor is moved from the Exchange to Loading state after it has advertised the complete list of LSAs and this router needs to download some of the LSAs from the neighbor)
  • Full (stable state between routers that have become fully adjacent, all required LSAs have been downloaded from the neighbor)

OSPF LSDB Exchange Process

• The OSPF LSDB exchange process involves seven stages before two OSPF routers' LSDB gets synchronized
  • Down
  • Init
  • 2-Way
  • ExStart
  • Exchange
  • Loading
  • Full

OSPF Overview

• OSPF (Open Shortest Path First) is a link-state routing protocol used to find the best path between the source and destination router using its own SPF algorithm.
• It is a reliable Interior Gateway protocol for large-scale networks.

OSPF States

• A neighbor is moved to the Full state when all required LSAs have been downloaded from the neighbor, and all missing or outdated LSAs have been acquired.
• The Full state is a stable state between routers that have become fully adjacent.

OSPF LSA Types

• Type 1: Router LSA - generated by each router for each area it is located in.
• Type 2: Network LSA - generated by the DR (Designated Router).
• Type 3: Summary LSA - created by the ABR (Area Border Router) and flooded into other areas.
• Type 4: Summary ASBR LSA - generated by the ABR to inform other routers where to find the ASBR (Autonomous System Boundary Router).
• Type 5: External LSA (Autonomous System External LSA) - generated by the ASBR.
• Type 6: Multicast OSPF LSA
• Type 7: External LSA (Not-So-Stubby Area (NSSA) LSA)
• Type 8: Used to internetwork OSPF and BGP
• Type 9, 10, 11: Designated for future use, for application-specific purposes, used along with MPLS.

OSPF Area Types

• Standard Area: accepts link updates, summary routes, and external OSPF routes.
• Backbone Area (Transit Area): named as Area 0, has all properties of standard area.
• Stub Area: does not accept external routes, uses default route to reach external routing domains.
• Totally Stubby Area: does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks.
• Not So Stubby Area (NSSA): has the properties of stub area, but can contain ASBR.
• Totally Stubby Not So Stubby Area (NSSA): has the properties of totally stub area, but can contain ASBR.

Link-State Routing Protocol Algorithms

• Link-state routing algorithms maintain a complex database of network topology.
• Each router constructs a topological database using all received LSPs (Link-State Packets).
• SPF algorithm is used to compute reachability to networked destinations, and updates the routing table.
• Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the internetwork.
• Each router independently calculates its best paths to all destinations in the network.

OSPF Router Nomenclature

• Backbone Routers: routers A, B are called backbone routers, all their interfaces participate in Area 0.
• Area Border Routers (ABR): routers C, D, E are called ABRs, attach to multiple areas, maintain separate link-state databases for each area.
• Non-Backbone Internal Routers: routers F, G, and H are called non-backbone internal routers, aware of the topology within their respective areas.
• Autonomous System Boundary Router (ASBR): a router that is connected to two or more routing domains, exchanges routing information between different routing protocols.

OSPF Overview

• OSPF (Open Shortest Path First) is a link-state routing protocol used to find the best path between the source and destination router using its own SPF algorithm.
• It is a reliable Interior Gateway protocol for large-scale networks.

OSPF States

• A neighbor is moved to the Full state when all required LSAs have been downloaded from the neighbor, and all missing or outdated LSAs have been acquired.
• The Full state is a stable state between routers that have become fully adjacent.

OSPF LSA Types

• Type 1: Router LSA - generated by each router for each area it is located in.
• Type 2: Network LSA - generated by the DR (Designated Router).
• Type 3: Summary LSA - created by the ABR (Area Border Router) and flooded into other areas.
• Type 4: Summary ASBR LSA - generated by the ABR to inform other routers where to find the ASBR (Autonomous System Boundary Router).
• Type 5: External LSA (Autonomous System External LSA) - generated by the ASBR.
• Type 6: Multicast OSPF LSA
• Type 7: External LSA (Not-So-Stubby Area (NSSA) LSA)
• Type 8: Used to internetwork OSPF and BGP
• Type 9, 10, 11: Designated for future use, for application-specific purposes, used along with MPLS.

OSPF Area Types

• Standard Area: accepts link updates, summary routes, and external OSPF routes.
• Backbone Area (Transit Area): named as Area 0, has all properties of standard area.
• Stub Area: does not accept external routes, uses default route to reach external routing domains.
• Totally Stubby Area: does not accept external routes or summary routes from other areas, uses default route to reach non-local area networks.
• Not So Stubby Area (NSSA): has the properties of stub area, but can contain ASBR.
• Totally Stubby Not So Stubby Area (NSSA): has the properties of totally stub area, but can contain ASBR.

Link-State Routing Protocol Algorithms

• Link-state routing algorithms maintain a complex database of network topology.
• Each router constructs a topological database using all received LSPs (Link-State Packets).
• SPF algorithm is used to compute reachability to networked destinations, and updates the routing table.
• Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the internetwork.
• Each router independently calculates its best paths to all destinations in the network.

OSPF Router Nomenclature

• Backbone Routers: routers A, B are called backbone routers, all their interfaces participate in Area 0.
• Area Border Routers (ABR): routers C, D, E are called ABRs, attach to multiple areas, maintain separate link-state databases for each area.
• Non-Backbone Internal Routers: routers F, G, and H are called non-backbone internal routers, aware of the topology within their respective areas.
• Autonomous System Boundary Router (ASBR): a router that is connected to two or more routing domains, exchanges routing information between different routing protocols.

Link-State Algorithms

• Convergence process can be accelerated with link-state routing protocol, as wait time for timers to expire is eliminated
• In networks with link-state protocols, connectivity between remote areas is not a concern
• The protocol can discriminate between two paths to the same destination and chose the best one

OSPF (Open Shortest Path First)

• OSPF is a link-state routing protocol
• It maintains three tables for efficient routing: Neighbor table, Topology table, and Routing table
• Each OSPF-speaking router is identified by a router ID

OSPF Metric Calculation

• OSPF uses cost as its metric
• Cost is calculated using the formula: 100Mbps / (bandwidth in Mbps)
• The reference bandwidth can be altered by the network administrator

OSPF Tables

• Neighbor table contains details of OSPF neighbors with which the router has built adjacency
• Topology table contains link-state information flooded by all OSPF routers in an area
• Each OSPF router builds an SPF tree and finds the best route to remote networks independently

OSPF Messages

• OSPF message format includes fields such as version number, type, packet length, router ID, area ID, checksum, and authentication
• OSPF message types include Hello, Database description, LSR, LSU, and LSAck

OSPF Router ID

• OSPF router ID is selected as the highest IP address on a router's loopback interface
• If no loopback interfaces are configured, the highest IP address of running interfaces is selected as the router ID
• OSPF router ID can also be configured manually by the network administrator

OSPF Area ID

• OSPF Area ID is a number used to name an area
• It is used to identify an area in an OSPF domain

OSPF DR and BDR Election Process

• OSPF routers connected via a broadcast network elect two routers as DR (Designated Router) and BDR (Backup Designated Router)
• DR and BDR act as a central point for exchanging OSPF information between multiple routers on the same segment
• Election is based on OSPF priority, with the highest priority winning; in case of a tie, the highest router ID wins

OSPF LSDB Exchange Process

• There are seven stages before two OSPF routers' LSDB gets synchronized
• LSDB exchange process involves the exchange of link-state information between OSPF routers

OSPF Area Types

• Not So Stubby Area (NSSA): has the properties of a stub area, but can contain ASBR (Autonomous System Boundary Router)
• Totally Stubby Not So Stubby Area: has the properties of a totally stub area, but can contain ASBR