GNS3 - VitualBox Part 6: Stub, Totally Stubby and Not-So-Stubby OSPF Areas

An explanation of the various types of OSPF Stub Areas and demonstrations of how to configure Stub, Totally Stubby and Totally NSSA Areas.

 

Introduction

OSPF Stub Areas restrict the number and types of routing table updates -- Link State Advertisements -- they accept, limiting the amount of overall network topology detail they must maintain in their routing tables and link state databases.  A common (but by no means mandatory) implementation of Stub Areas is an OSPF Area that has only one ABR for all traffic between the Area and the rest of the Autonomous System.

OSPF Link State Advertisement Types

OSPF routers maintain a routing table and link state database by communicating topology information using Link State Advertisements (LSAs).  There are different types of LSAs; this article examines the six relevant to Stub Areas.

Type 1 - Router LSA

Type 1 - Router LSA - the router announces its presence and lists the links and metrics to other routers or networks in (and only within) its own area.  The link-state ID of the Type 1 LSA is the originating router ID.

Type 2 - Network LSA

Type 2 - Network LSA - the Area's designated router (DR) on a broadcast segment (e.g. Ethernet) lists the router IDs in broadcast domain. Type 2 LSAs are flooded across their own area only. The link-state ID of the Type 2 LSA is the IP interface address of the DR.

Type 3 - Summary LSA

Type 3 - Summary LSA - an Area Border Router (ABR) summarizes information about an Area and sends it to other attached Areas.  Summarization provides scalability by aggregating contiguous networks information into a single address/mask entry.  For instance, all of the individual networks in the 172.16.0.0 - 172.16.31.255 range may be summarized as 172.16.0.0 / 255.240.0.0 The link-state ID is the destination network number for Type 3 LSAs.

Type 4 - ASBR-Summary LSA

Type 4 - ASBR-Summary LSA - support Type 5 - External LSAs (see below).  Type 5 External LSAs list routes imported from external systems and are flooded to all areas in the OSPF Domain.  However, accurate next-hop information may not be available for other areas in the OSPF Domain with the Type 4 ASBR-Summary LSA which includes information specific to the ASBR advertising the routes.  The link-state ID is the router ID of the described ASBR for Type 4 LSAs.

Type 5 - External LSA

Type 5 - External LSA - provide routes imported into OSPF from other routing protocols. They are flooded to all areas unchanged (except stub and NSSA areas).  The link-state ID of the Type 5 LSA is the external network number.

Type 7 - NSSA External LSAs

Type 7 - NSSA External LSAs are specific to Not-So-Stubby-Areas (NSSA), which -- like Stub and Totally Stubby Areas -- do not receive external routes from ABRs.  Unlike Stub Areas, NSSA Areas may have links to external routing protocols. NSSA Areas use Type 7 LSAs to tell their ABRs about these external routes, which the Area Border Router then translates to type 5 external LSAs and floods as normal to the rest of the OSPF network.

OSPF Stub Area Types

The illustration above depicts the important differences between the four types of Stub Areas, the types of LSAs they use and relations to external routing protocols.  The most important concepts of stubby areas is understanding how they treat Type 3 Summary and Type 5 External LSAs.  In the NSSA area types, Type 7 NSSA External LSAs operate only within the NSSA Area, but are translated at border routers into Type 5 External LSAs.

Stub Areas

Stub Areas receive route summaries from the ABR via Type 3 Summary LSAs, providing information about networks in other Areas in the Autonomous System.  They also receive a default route.  They do not receive Type 5 External LSAs that provide information about routes external to the OSPF Autonomous System.

Totally Stubby Areas

Totally Stubby Areas receive neither Type 5 External LSAs nor Type 3 Summary LSAs from the ABR and lack both routes external to the OSPF Autonomous System and summary information about the networks in other Areas in the Autonomous System.  They are only aware of routes within their own Area and reach others through a default route, typically handled by the ABR.

Not-So-Stubby Areas

Not-So-Stubby Areas (NSSAs) differ from Stub and Totally Stubby Areas by including external routes (in the illustration above injected from EIGRP by and ASBR in the NSSA).  They are stubby with respect to the OSPF Autonomous System n that they do not receive Type 5 External LSAs but do receive Type 3 Summary LSAs from the ABR.  They differ significantly from Stub and Totally Stubby Areas in that they use Type 7 LSAs to advertise their external routes to the ABR, which then floods them to the OSPF Autonomous System as Type 5 External LSAs.  NSSAs differ from the other three types of Stub Areas in that they do not automatically assign default routes; these must be manually configured on the ABR.

Totally Not-So-Stubby Areas

Totally Not-So-Stubby Areas (Totally NSSAs) operate much like NSSAs with two important differences:  they receive neither Type 3 Summary nor Type 5 External LSAs (and thus do not have external routes) and do automatically assign default routes.

Configuring Stub and Totally Stub Areas

The illustrations at the beginning of this article depicts a network with several potential Stub Areas.  One, however, is not:  10.64.0.0.  This OSPF Area is a Transit Area providing a virtual link (see Part 5 of this series) to Backbone Area 0.0.0.0 for Area 10.72.0.0.  Area 10.72.0.0 is a WAN Border Router (10.64.0.0-10.72.0.0) serving Cisco 2691 Branch Office routers.  This is a good candidate for limiting the detail of routing information it maintains (the Link State Databases) by configuring it as a Stub or Totally Stubby Area.

Each router in the area must be configured with the following OSPF statement to create a Stub Area:

area 10.72.0.0 stub

This statement blocks Type 5 - External LSAs from Area 10.72.0.0.  However, notice the topology: there are no external routing protocols injecting routes into the OSPF Autonomous System.  Specifically, the ASBRs are simply Debian Linux Gateways with an external static route to the wireless router.  This will be apparent when comparing the Links State Databases on routers in Area 10.72.0.0 before and after -- there is no change.

Reducing the size of the Link State Databases in Area 10.72.0.0 in this system requires configuring it as a Totally Stubby Area.  Each router in the area must be configured with the following OSPF statement to create a Totally Stubby Area:

area 10.72.0.0 stub no-summary

This statement also blocks Type 3 - Summary LSAs from Area 10.72.0.0.  Now the change in this system is noticeable -- summary routes to other OSPF Areas are no longer listed in the Link State Databases, which now contain only routes within Area 10.72.0.0 and a default route.

The video below illustrates configuring area 10.72.0.0 as a Stub and then Totally Stubby Area.

Configuring Totally NSSA Areas

NSSA and Totally NSSA Areas are a difficult topic.  A simplified topology is warranted for clarity.  The illustration aboce is the topology that will be used in this section.

NSSA and Totally NSSA Areas are stubby with respect to OSPF, but connected to external routes and routing protocols.  Since Stub and Totally Stubby Areas do not accept Type 5 External LSAs, such an area would no longer inject external routes into OSPF.  NSSA and Totally NSSA Areas handle exteral routes using Type 7 NSSA External LSAs.  These LSAs inject external routes from the NSSA ASBR into the NSSA Area; the NSSA ABR then translates the Type 7 NSSA External LSAs into Type 5 External LSAs and floods the OSPF AS with the external routes.

Redistributing OSPF and External Routes

The first step in configuring a NSSA or Totally NSSA Area is to assure the external routing protocol and OSPF are exchanging information correctly.  The ASBR in this example (COU_ASBR) is already running OSPF and RIP.  It is the only router in the topology that is aware of all routes because it is not configured to redistribute.  The following commands enable redistribution of OSPF routes into RIP and RIP routes into OSPF:

router ospf 1
redistribute rip subnets

router rip
redistribute ospf 1 metric 1

Once configured, the router redistributes the routes.  Checking OSPF routers, RIP routes 192.168.x.x are now present.  Checking RIP routers, the OSPF route 10.0.0.0/8 is now present.

Creating the Totally NSSA Area 10.0.0.0

On all routers in the 10.0.0.0 Area (COU_ABR, COU_ASBR and COU_IR), issue the command:

area 10.0.0.0 nssa no-summary

This command drops Type 3 Summary and Type 5 External LSAs while creating Type 7 NSSA LSAs for the connected RIP External System.  IRs now have no OSPF Summary or External routes, but do have Intra Area and RIP External routes.  The ABR and ASBR manage redistribution of RIP External routes into OSPF and OSPF routes into RIP.

Creating the Stub Area 10.64.0.0 and Totally Stubby Area 10.128.0.0

The two normal Areas 10.64.0.0 and 10.128.0.0 maintain OSPF Summary Routes (10.x.x.x) and RIP External Routes (192.168.x.x).  Converting them to Stub and Totally Stubby Areas further summarizes routing information.

Convert Area 10.64.0.0 into a Stub Area by issuing, on all area routers, the command:

area 10.64.0.0 stub

Once converted, the Stub Area no longer accepts Type 5 External LSAs (originating at the COU_ABR router) and rely upon the default gateway (PHL_ABR) to maintain external routes.  Although the external routes are no longer maintained by IRs, you may still ping addresses in the RIP External System (192.168.x.x) and get responses.

The Stub Area still maintains Intra Area routes and summary routes (10.x.x.x) for OSPF AS Areas.


Convert Area 10.128.0.0 into a Totally Stubby Area by issuing, on all area routers, the command:

area 10.128.0.0 stub no-summary

Once converted, the Totally Stubby Area no longer accepts Type 3 Summary and Type 5 External LSAs (originating at the COU_ABR router) and rely upon the default gateway (PHL_ABR) to all but Intra Areal routes.  Although the routes are no longer maintained by IRs, you may still ping addresses in the OSPF AS (10.x.x.x) and RIP External System (192.168.x.x) and get responses.

The video below illustrates configuring Area 10.0.0.0 as Totally NSSA, Area 10.64.0.0 as Stub and 10.128.0.0 as Totally Stubby.