Multi-area OSPF design

When a local change creates work for distant routers

A company's OSPF network has grown across several sites. In one area, routers share detailed connection records, so a branch link change can trigger topology processing elsewhere in that area. The useful question is not whether a particular router count is too large. It is whether flooding, database size or route calculation is becoming a problem, and whether the sites have clear boundaries for summarisation.

An area is a group of OSPF interfaces that shares detailed topology information. A link-state advertisement (LSA) is a stored record describing links or reachability. The link-state database (LSDB) holds those records. Shortest path first (SPF) is the calculation a router uses to find paths through its area.

Multi-area OSPF gives each area its own detailed topology. Area 0 is the backbone joining the other areas. An area border router (ABR) participates in the backbone and another area, calculates reachability and advertises eligible networks between them.

Areas, ABRs and the backbone

Our small example keeps five routers so you can follow the boundaries. R1's user LAN is 172.16.1.0/24; three additional test networks complete 172.16.0.0/24 through 172.16.3.0/24. Traffic to the partner network 10.50.0.0/16 crosses R2, R3, R4 and R5. R5 learns that external network through EIGRP (Enhanced Interior Gateway Routing Protocol), a different routing protocol, and imports it into OSPF.

Importing routes from another source is redistribution. A router performing it is an autonomous system boundary router (ASBR). ABR means a boundary between OSPF areas; ASBR means a boundary between OSPF and another route source. A router can perform both jobs.

The point-to-point router links are R1–R2 10.1.12.0/30, R2–R3 10.0.23.0/30, R3–R4 10.0.34.0/30 and R4–R5 10.2.45.0/30. On each link the left router uses .1 and the right router .2. Router IDs run from 1.1.1.1 to 5.5.5.5; these identify OSPF routers, not the address of every port.

A router can have several roles: R3 is internal and backbone; R5 is internal and an ASBR.

What splitting into areas buys you

• Less remote topology detail. Type 1 router records and type 2 shared-network records stay in their area. R1 learns other areas' networks through type 3 summary LSAs. It can also hold external-route records in a normal area.
• Smaller scope for topology calculation. Area 2's detailed link change is not flooded as a router LSA into area 1. A changed inter-area cost, lost prefix or external path can still require processing elsewhere. Areas reduce work; they do not guarantee that every remote change is invisible.
• Summarisation. An ABR can replace several prefixes with one range. If at least one qualifying component remains and the advertised summary cost is unchanged, losing another component need not change that summary outside the area.

Three rules about area 0

1. Every area must touch area 0 through an ABR.
2. Area 0 must be contiguous: one connected piece.
3. Use the backbone hierarchy for routes between non-backbone areas.

Inside an area OSPF is link-state; between areas the ABRs exchange "network X at cost Y", which is distance-vector behaviour. Forcing everything through one backbone is what keeps that loop-free.

Building an ABR on Cisco IOS

These are Cisco IOS/IOS XE style examples, starting in global configuration mode. Addresses, interface state and basic connectivity must already work. R2 Gi0/1 faces R1 in area 1; R2 Gi0/0 faces R3 in area 0. Both neighbouring ends of a link must agree on its area. No separate abr command is needed.

Inter-area summarisation

The four /24 networks form the contiguous range 172.16.0.0/22, mask 255.255.252.0. The 1 in area 1 range names their source area. It does not put the summary in area 1 or change the original interface masks.

External routes are summarised on the ASBR instead, with summary-address, covered in External routes.

Summarisation only pays off when addressing was planned per area. A site that borrowed a subnet from another site's block leaves a hole the summary can't cover cleanly, and that is usually what stops a tidy area range in an older network.

Virtual links: patching an area that misses area 0

Suppose a new area 3 is attached only to R5. It doesn't touch area 0, which breaks rule 1. A virtual link through area 2 makes R5 act as if it had an interface in area 0:

Checking the ABR and its summary

The following metrics assume outgoing cost 1 on every participating interface, including R1's LAN and test networks. R1's loopbacks use point-to-point OSPF type so their configured /24 masks are advertised. With default loopback behaviour you would instead need to account for /32 host routes.

R2#show ip ospf | include border|normal
 It is an area border router
 Number of areas in this router is 2. 2 normal 0 stub 0 nssa

R2#show ip route ospf | include 172.16
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
O        172.16.0.0/22 is a summary, 00:01:05, Null0
O        172.16.0.0/24 [110/2] via 10.1.12.1, 00:20:13, GigabitEthernet0/1
O        172.16.1.0/24 [110/2] via 10.1.12.1, 00:20:13, GigabitEthernet0/1
O        172.16.2.0/24 [110/2] via 10.1.12.1, 00:20:13, GigabitEthernet0/1
O        172.16.3.0/24 [110/2] via 10.1.12.1, 00:20:13, GigabitEthernet0/1

R3#show ip route ospf | include 172.16
O IA     172.16.0.0/22 [110/3] via 10.0.23.1, 00:01:05, GigabitEthernet0/0

Short lab: compare the range with its components

Start with the five-router lab above and no area range. On R3, record the four /24 inter-area routes. Add the shown area 1 range on R2, then repeat the route check after convergence. R3 should learn the /22 at cost 3, while R2 retains the more specific /24 routes at cost 2.

On R2, remove the range with no area 1 range 172.16.0.0 255.255.252.0 under process 1. Confirm R3 learns the component /24s again. Test an assigned host before and after each change; a route-table count alone does not verify delivery.

Symptoms of a broken area design

• An area has no routes to the rest of the network. It doesn't touch area 0, or its ABR has no working adjacency in area 0.
• Two parts of area 0 can't reach each other although there is a path through another area: area 0 is split. Fix the topology, or bridge it with a virtual link.
• The summary is not advertised. area range was configured on a router that isn't the ABR for that area, or none of the component networks exists.
• An area mismatch log on a link: the two ends of a link put it in different areas.

Design mistakes with areas

• Thinking areas are assigned per router. They are assigned per interface; the link is in one area.
• Connecting two non-backbone areas directly and expecting routes to flow between them.
• Using area range on the ASBR for external routes (that is summary-address).
• Building a virtual link through a stub area.

Multi-area rules to remember

Test yourself on areas and ABRs

Sources: RFC 2328, areas and summary LSAs; Cisco areas and virtual links; Cisco area range command reference. Outputs are illustrative, not captures from a tested router image.