OSPF Areas and Routing

OSPF Areas

• All routers in the same OSPF area maintain an identical copy of the Link-State Database (LSDB)
• As the number of network links and routers in an area increases, the area grows in size
• A single area simplifies topology, but it has drawbacks:
  • Complete Shortest Path Tree (SPT) recalculation occurs when a link flaps
  • The LSDB increases in size, consumes more memory, and slows down SPF computation
  • Route summarization does not occur
• Segmenting routers into multiple OSPF areas addresses these issues by keeping LSDBs smaller
• Routers with interfaces in multiple areas have multiple LSDBs, one for each area
• The internal topology of one area is hidden from other areas
• Segmenting into multiple areas reduces LSDB size, speeds up SPT calculations, and decreases LSDB flooding during link flapping
• Routes from one area are not automatically injected into other areas

Area 0: The Backbone Area

• Area 0 is a special area called the backbone
• OSPF uses a two-tier hierarchy where all areas must connect to Area 0
• Area 0 advertises routes to other non-backbone areas
• The backbone design is essential for preventing routing loops

Area Identifier

• The area identifier is a 32-bit field
• Can be formatted in decimal (0-4294967295) or dotted-decimal (0.0.0.0 - 255.255.255.255)
• Routers can form adjacencies even with different format types
• OSPF advertises the area ID in packets

Area Border Routers (ABRs)

• ABRs connect to Area 0 and another OSPF area
• Per Cisco definition and RFC 3509

Confirming OSPF Interfaces

• The show ip ospf interface [brief | interface-id] command displays OSPF-enabled interfaces
• The output includes details like IP address, Designated Router (DR) and Backup Designated Router (BDR) information, and OSPF timers

OSPF Neighbor Adjacency

• The show ip ospf neighbor [detail] command shows the OSPF neighbor table
• The output details neighbor states for each router

Viewing OSPF Installed Routes

• The show ip route ospf command displays OSPF routes installed in the Routing Information Base (RIB)
• The output includes administrative distance (AD) and metric

External OSPF Routes

• An autonomous system boundary router (ASBR) redistributes routes into an OSPF domain
• An ASBR can be any OSPF router, and the ASBR function is independent of the ABR function
• An OSPF domain can have an ASBR without an ABR
• A router can be both an ASBR and an ABR

Nonbroadcast Networks

• The ip ospf network non-broadcast command enables OSPF on nonbroadcast networks
• Requires static neighbor configuration as broadcast discovery is not available
• Can be verified by using the show ip ospf interface command with the Type keyword

Point-to-Point Networks

• A point-to-point network allows only two devices to communicate
• Default OSPF network type for serial interfaces (HDLC or PPP), GRE tunnels, and point-to-point Frame Relay subinterfaces
• Verified by using the show ip ospf interface command with the Type keyword
• Point-to-point OSPF networks do not have a DR

Point-to-Multipoint Networks

• The ip ospf network point-to-multipoint command enables OSPF point-to-multipoint network type
• Supports hub-and-spoke connectivity with the same IP subnet
• Commonly used in Frame Relay and Layer 2 VPN (L2VPN) topologies
• Adds the interface's IP address to the LSDB as a /32 network

Authentication

• Authentication prevents unauthorized access to the OSPF database
• Two types:
  • Plaintext: Uses a shared password
  • Message Digest (MD5): Using a shared secret with MD5 hashing
• Configured on an interface or area-specific level
• The show ip ospf interface command displays the authentication settings for an interface