Configuring OSPF Multi-Area Adjacency

OSPF multi-area adjacency allows one link to be shared by multiple OSPF areas.

Usage Scenario

In OSPF, intra-area links take precedence over inter-area links. Therefore, even if a high-speed link exists between two areas, a low-speed intra-network link rather than the high-speed link is used to transmit inter-area traffic. To address this problem, run the ospf enable multi-area command to enable OSPF on a multi-area adjacency interface so that a link can be shared by multiple areas.

Among the OSPF features supported by an OSPF multi-area adjacency interface, some can be inherited from the main interface, whereas others need to be configured independently. For details, see Table 1.

**Table 1** OSPF features supported by OSPF multi-area adjacency interfaces
OSPF Feature	Description	Related Configuration Links
BFD for OSPF	BFD for OSPF can be inherited from main interfaces, except for the configuration that disables BFD from an interface. This configuration needs to be configured independently for OSPF multi-area adjacency interfaces.	Disabling an OSPF Multi-Area Adjacency Interface from Creating BFD Sessions
OSPF fast convergence	OSPF multi-area adjacency interfaces support OSPF fast convergence which needs to be configured independently for these interfaces and cannot be inherited from their main interfaces.	Configuring Fast Convergence for a Multi-Area Adjacency Interface
IGP-LDP synchronization	OSPF multi-area adjacency interfaces support IGP-LDP synchronization which needs to be configured independently for these interfaces and cannot be inherited from their main interfaces.	Configuring LDP-IGP Synchronization
OSPF IP FRR	OSPF multi-area adjacency interfaces support OSPF IP FRR. OSPF IP FRR can be inherited from main interfaces, except for the configuration that disables FRR from a specified OSPF interface. This configuration needs to be configured independently for OSPF multi-area adjacency interfaces.	Disabling OSPF IP FRR on an OSPF Multi-Area Adjacency Interface
OSPF neighbor relationship flapping suppression	OSPF multi-area adjacency interfaces support OSPF neighbor relationship flapping suppression which needs to be configured independently for these interfaces and cannot be inherited from their main interfaces.	Configuring Neighbor Relationship Flapping Suppression on an OSPF Multi-Area Adjacency Interface
OSPF flush LSA source tracing	OSPF multi-area adjacency interfaces support OSPF flush LSA source tracing which can be inherited from their main interfaces.	Configuring OSPF Flush LSA Source Tracing
OSPF TE	OSPF multi-area adjacency interfaces inherit TE information from their main interfaces.	Configuring IGP TE (OSPF or IS-IS)

Pre-configuration Tasks

Before configuring OSPF multi-area adjacency, complete the following tasks:

Configure a link layer protocol.
Assign an IP address to each interface to ensure reachability between neighboring nodes at the network layer.
Configure basic OSPF functions.

Enabling OSPF on a Multi-Area Adjacency Interface: Enabling OSPF on a multi-area adjacency interface is the prerequisite for configuring basic multi-area adjacency functions.

Configuring a Cost for a Multi-Area Adjacency Interface: Configuring a cost for a multi-area adjacency interface can control route selection.

Configuring an Authentication Mode for a Multi-Area Adjacency Interface: Configuring an authentication mode for a multi-area adjacency interface improves OSPF network security.

Configuring Fast Convergence for a Multi-Area Adjacency Interface: Configuring fast convergence for a multi-area adjacency interface improves network performance.

Configuring Neighbor Relationship Flapping Suppression on an OSPF Multi-Area Adjacency Interface: Neighbor relationship flapping suppression can be configured on an OSPF multi-area adjacency interface to delay OSPF neighbor relationship reestablishment or set the link cost to the maximum value in case of neighbor relationship flapping.

Disabling an OSPF Multi-Area Adjacency Interface from Creating BFD Sessions: After BFD for OSPF is configured, all interfaces on which neighbor relationships are in Full state in the OSPF process create BFD sessions. You can disable an interface from creating BFD sessions as required.

Disabling OSPF IP FRR on an OSPF Multi-Area Adjacency Interface: OSPF IP fast reroute (FRR) can be disabled on an OSPF multi-area adjacency interface that is connected to a link carrying key services. This prevents FRR from calculating the link as the backup link and minimizes any potential impact on the services.

Configuring a Fallback Cost for an Eth-Trunk Multi-Area Adjacency Interface: Configuring a fallback cost for an Ethernet multi-area adjacency interface helps control route selection.

Maintaining OSPF on a Multi-Area Adjacency Interface

Verifying the Configuration of OSPF Multi-Area Adjacency: After configuring OSPF multi-area adjacency, verify the configuration.