CCNA3 Mod 2: Single Area OSPFv2 Configuration - Enterprise Networking Security and Automation (ENSA)

This module covers the configuration of single-area OSPF version 2 in point-to-point and broadcast multi-access networks. The lecture will explain OSPF router ID, network types, default route propagation, and verification methods. It builds upon previous CCNA modules, which are recommended for prior viewing.

OSPFv2 is enabled using the `router ospf process-id` command. The process ID is locally significant, a number between 1 and 65535, and it's best practice to use the same ID across all OSPF routers. A 32-bit router ID uniquely identifies an OSPF router for database synchronization and DR/BDR election.

Cisco routers determine the router ID by: first, an explicitly configured ID using `router-id` command (recommended method); second, the highest IPv4 address of any configured loopback interface; and third, the highest active IPv4 address of any physical interface. Loopback interfaces are preferred for stability.

To change an OSPF router ID, the router must be reloaded or the OSPF process reset. Clearing the OSPF process using `clear ip ospf process` is the preferred method, as it allows dynamic updates without a full reload. This command can also be a useful troubleshooting tool.

OSPF interfaces are specified using the `network` command with a network address, wildcard mask, and area ID. For single-area OSPFv2, the area ID must be consistent across all routers, with Area 0 being a best practice for future multi-area expansion. Wildcard masks are typically the inverse of the subnet mask.

Alternatively, OSPF can be configured directly on an interface using the `ip ospf process-id area area-id` command. To prevent excessive OSPF message transmission and improve security on non-neighbor-facing interfaces, the `passive-interface` command is used, allowing advertisements without sending hello packets.

By default, Cisco elects DR/BDR on Ethernet interfaces, even in two-router point-to-point links where it's unnecessary. The `ip ospf network point-to-point` command disables this election. Loopback interfaces, usually advertised as /32 host routes, can be configured as point-to-point to advertise their full network, mimicking a real LAN.

Multi-access OSPF networks elect a Designated Router (DR) to distribute LSAs and a Backup Designated Router (BDR) for redundancy. The DR uses multicast address 224.0.0.5 for all OSPF routers, while DR-others use 224.0.0.6 to communicate with DR/BDR. The DR/BDR election is based on interface priorities and router IDs.

The `show ip ospf interface` command verifies DR/BDR roles and priorities. `show ip ospf neighbor` displays neighbor adjacencies. A 'Full' state indicates full adjacency, while 'Two-Way' is normal for DR-other neighbors in multi-access networks. Adjacency issues often stem from mismatched timers or network types.

DR/BDR election prioritizes interfaces with the highest configured OSPF priority (0-255, default 1). If priorities are equal, the highest router ID wins. The election is not preemptive; a new router with a higher ID won't immediately become DR/BDR. If the DR fails, the BDR becomes the DR, and a new BDR is elected.

The `ip ospf priority` command sets an interface's priority in DR/BDR elections. OSPF uses 'cost' as a metric, where a lower cost denotes a better path. Cost is inversely proportional to bandwidth calculated as `reference bandwidth / interface bandwidth`. Higher bandwidth interfaces typically have lower costs.

Due to integer rounding, high-speed interfaces like Gigabit Ethernet can share the same default OSPF cost. To distinguish them, adjust the `auto-cost reference-bandwidth` on all OSPF routers. Alternatively, `ip ospf cost` can manually set an interface's cost. This is crucial for multi-vendor environments or influencing path selection.

OSPF accumulated cost is the sum of costs from a router to a destination network. `show ip route` verifies the calculated costs for different paths. Simulating link failures (e.g., by shutting down an interface) demonstrates how OSPF automatically recalculates routes and utilizes backup paths via 'failover'.

OSPF Hello packets (multicast 224.0.0.5) are sent every 10 seconds by default. The 'dead interval' is 40 seconds (four times the hello interval), after which a neighbor is declared down if no hello packets are received. These timers must match between neighbors for adjacency formation and can be adjusted (rarely recommended) using `ip ospf hello-interval` and `ip ospf dead-interval`.

To propagate a default route into an OSPF domain, an edge router (ASBR) needs a static default route (e.g., `ip route 0.0.0.0 0.0.0.0 [next-hop/exit-interface]`) and the `default-information originate` command under the OSPF process. This allows other OSPF routers to learn a route to external networks.

Key verification commands include `show ip interface brief` (for interface status/IPs), `show ip route` (for routing table), `show ip ospf neighbor` (for adjacencies), `show ip protocols` (for OSPF process details), `show ip ospf` (for area info/SPF algorithm), and `show ip ospf interface [type/number]` (for specific interface OSPF settings). `show ip ospf interface brief` gives a quick summary of OSPF-enabled interfaces.