OSPF Area Configuration¶

To configure area-specific settings in OSPF, start in config-ospf mode and use the area <area-id> command to enter config-ospf-area mode.

tnsr(config-ospf)# area <area-id>
tnsr(config-ospf-area)#

config-ospf-area mode contains the following commands:

authentication:

Enables authentication for this area. Communication from peers must contain the expected authentication information to be accepted, and outgoing packets will have authentication information added.

When present on its own, the authentication mechanism used is simple passwords. Authentication passwords are configured in OSPF Interface Configuration mode using the authentication-key command.

message-digest:: When present, enables MD5 HMAC authentication for this area. Much stronger authentication than simple passwords. The key is configured in OSPF Interface Configuration mode using the message-digest-key command.

default-cost <cost>:

Sets the cost applied to default route summary LSA messages sent to stub areas.

export-list <acl-name>:

Uses the given ACL to limit Type 3 summary LSA messages for intra-area paths that would otherwise be advertised. This behavior only applies if this router is the ABR for the area in question.

filter-list (in|out) prefix-list <prefix-list-name>:

Similar to export-list and import-list but uses prefix lists instead of ACLs, and can work in either direction.

import-list <acl-name>:

Similar to export-list, but for routes announced by other routers into this area.

nssa [(no-summary|translate (always|candidate|never))]:

Configures this area as a Not-so-Stubby Area (NSSA), which does not contain external links but may contain static routes to non-OSPF destinations (See Area Types for more information on area types and behaviors.

no-summary:

When present, the area will instead of considered an NSSA Totally Stub area (Area Types).

translate (always|candidate|never):

Configures NSSA-ABR translations, for converting between Type 5 and Type 7 LSAs.

always:: Always translate messages.
candidate:: Participate in NSSA-ABR candidate elections. Currently the default behavior.
never:: Never translate messages.

range <prefix> [cost <val>|not-advertise|substitute <sub-prefix>]:

Configure summarization of routes inside the given prefix. Instead of Type 1 (Router) and Type 2 (Network) LSAs, it creates Type 3 Summary LSAs instead.

cost <val>:: Apply the specified cost to summarized routes for this prefix.
not-advertise:: Disable advertisement for this prefix.
substitute <sub-prefix>:: Instead of advertising the first prefix, advertise this prefix instead.

shortcut (default|disable|enable):

For use with abr-type shortcut (OSPF Server Configuration), this advertises the area as capable of supporting ABR shortcut behavior (draft-ietf-ospf-shortcut-abr-02).

stub [no-summary]:

Configure this area as a Stub Area (Area Types).

no-summary:: When present, the area will instead be considered a Totally Stub Area (Area Types).

virtual-link <router-id>:

Configures a virtual link in this area between this router and the specified router. Both this router and the target router must be ABRs, and both must have a link to this (non-backbone) area. Additionally, the virtual link must be added on both ends. This command enters config-ospf-vlink mode which has a subset of commands available similar to OSPF Interface Configuration. The available commands are authentication, authentication-key, dead-interval, hello-interval, message-digest-key, retransmit-interval, and transmit-delay. The usage of these commands is explained in OSPF Interface Configuration.

The virtual link is used to exchange routing information directly between the routers involved, and can be used to deliver traffic via the peer if necessary. Such a relationship may be necessary to nudge traffic from an ABR with a single undesirable link to another ABR with a faster link to a common remote destination, when the path would otherwise be selected because it is shorter.