Inter-area summarisation — area range, ABR aggregation, prefix-list filtering

Summarisation at the OSPF area boundary is the primary tool the operator has for controlling LSDB size, SPF recalculation cost, and route visibility across an OSPF estate. The mechanism is the area range declaration on the ABR: a prefix / mask the ABR advertises into the rest of the domain as a single Type 3 Summary LSA, replacing the intra-area Type 1 / Type 2 LSAs that would otherwise cross the boundary. This lesson walks through the configuration, the mechanism, and the production impact.

xx-01 and xx-02 covered the area-type filters at the ABR. This lesson covers what the ABR injects into the rest of the domain — the Type 3 Summary LSA — and how the operator can use summarisation to control the routing information that crosses the boundary.

What area range does

The area range declaration has two effects:

1. Suppression of intra-area LSAs. Type 1 (Router LSA) and Type 2 (Network LSA) entries that fall inside the range are NOT advertised across the area boundary. Instead, the ABR generates a single Type 3 Summary LSA for the range.
2. Generation of a Type 3 Summary. The ABR originates a Type 3 Summary LSA with the range’s prefix / mask and the range’s metric. The Type 3 floods through the rest of the OSPF domain.

The effect is that the rest of the OSPF domain sees one prefix (the summary) instead of N prefixes (the individual intra-area prefixes). The cost is that the rest of the domain loses visibility into the specific intra-area topology — it cannot tell which router inside the area originated which prefix, only that the prefix is reachable through the ABR.

flowchart LR
  subgraph "Area 10 — pre-summarisation"
    direction TB
    R10A[R-A<br/>10.1.0.0/24]
    R10B[R-B<br/>10.1.1.0/24]
    R10C[R-C<br/>10.1.2.0/24]
    R10D[R-D<br/>10.1.3.0/24]
  end
  ABR[ABR]
  subgraph "Backbone area 0"
    CORE[Core]
  end
  R10A --- ABR
  R10B --- ABR
  R10C --- ABR
  R10D --- ABR
  ABR --- CORE
  ABR -. "without area range:<br/>4 Type 3 LSAs<br/>10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24" .-> CORE
  ABR -. "with area range 10.1.0.0/22:<br/>1 Type 3 LSA<br/>10.1.0.0/22" .-> CORE

The above diagram shows the mechanism: four intra-area prefixes become either four separate Type 3 LSAs (without summarisation) or one Type 3 LSA (with summarisation). The downstream OSPF domain has fewer LSAs to process and fewer entries in the routing table.

Configuration

The VyOS 1.5 LTS configuration idiom:

set protocols ospf area 10 range 10.1.0.0/22
set protocols ospf area 10 range 10.1.0.0/22 cost 50

The first line declares the range. The second line (optional) sets the metric the ABR attaches to the generated Type 3 Summary LSA. The default metric is the largest intra-area cost of any component prefix in the range.

Additional options:

# Suppress the range entirely (block the prefix from being advertised)
set protocols ospf area 10 range 10.99.0.0/16 not-advertise

# Substitute the range with a different prefix (rare, used for renumbering)
set protocols ospf area 10 range 10.1.0.0/22 substitute 10.2.0.0/22

The not-advertise flag suppresses the range entirely. The matching intra-area prefixes are not advertised across the boundary at all. The substitute flag replaces the range with a different prefix when advertising.

For prefix-list filtering at the area boundary, the operator uses policy:

set policy prefix-list FILTER-OUT-AREA-10 rule 10 action permit
set policy prefix-list FILTER-OUT-AREA-10 rule 10 prefix 10.1.0.0/22
set policy prefix-list FILTER-OUT-AREA-10 rule 10 ge 22
set policy prefix-list FILTER-OUT-AREA-10 rule 10 le 22

set policy prefix-list6 FILTER-OUT-AREA-10-V6 rule 10 action permit
set policy prefix-list6 FILTER-OUT-AREA-10-V6 rule 10 prefix '2001:db8:1::/48'

The prefix-list is then applied to the OSPF redistribution or inter-area filtering. Note: OSPF area filtering on VyOS 1.5 LTS is implemented via area range not-advertise rather than a direct prefix-list filter — see the FRR documentation for the precise mechanism.

The mechanism in detail

When the operator declares area 10 range 10.1.0.0/22, FRR’s ospfd performs the following:

1. The range is registered in the area’s LSDB-construction code.
2. As the ABR receives Type 1 and Type 2 LSAs from Area 10, it classifies each prefix by whether it falls inside the range.
3. Prefixes inside the range are NOT translated to Type 3 Summary LSAs across the ABR boundary. The rest of the domain does not see them individually.
4. The ABR generates ONE Type 3 Summary LSA for the range (if any component prefix is present). The Summary’s prefix / mask is the range; the Summary’s metric is the configured cost (or the default max intra-area cost).
5. The Type 3 Summary LSA is flooded into the non-Area-10 areas the ABR is attached to (typically the backbone).

The ABR continues to advertise the individual prefixes if they fall OUTSIDE the range. A range that covers only part of the intra-area address space leaves the remainder as individual Type 3 LSAs.

sequenceDiagram
  participant R as Intra-area router
  participant ABR as ABR
  participant B as Backbone
  R-->>ABR: Type 1 Router LSA<br/>advertises 10.1.0.0/24
  R-->>ABR: Type 1 Router LSA<br/>advertises 10.1.1.0/24
  R-->>ABR: Type 1 Router LSA<br/>advertises 10.1.2.0/24
  R-->>ABR: Type 1 Router LSA<br/>advertises 10.1.3.0/24
  Note over ABR: range 10.1.0.0/22 configured<br/>all 4 prefixes inside range
  ABR-->>B: Type 3 Summary 10.1.0.0/22<br/>metric 50 (configured)
  Note over B: Rest of domain sees ONE summary

The metric the summary carries

The summary’s metric is either:

• The value configured with set protocols ospf area X range ... cost N.
• The default, which is the maximum intra-area cost of any component prefix in the range. The max-intra-area-cost is the cost the ABR sees in its own SPF calculation for reaching the most distant component prefix.

The metric matters because it is what the downstream OSPF routers see. If the operator wants a specific metric (to influence traffic engineering or to control preference), the explicit cost is the way to set it.

A common pattern is to set the summary cost higher than any specific intra-area prefix. This makes the summary a less-preferred route than the underlying prefix would have been, so if a downstream router learns the same prefix from a different ABR (e.g. a backup path), the specific prefix wins.

The discard route

When the ABR generates a Type 3 Summary for a range, the ABR also installs a discard route (a route to Null0) for the summary in its own routing table. The discard route serves two purposes:

1. Loop prevention. If the ABR receives the summary from a different ABR (e.g. via a backup path), the discard route prevents the ABR from re-advertising the summary back into the area.
2. Recursion safety. If the summary is the only route the ABR has for the prefix range, and the ABR has no specific intra-area prefixes that match the range, the discard route ensures the summary is never used recursively (a router cannot forward to itself).

The discard route is automatic. The operator does not configure it. It appears in the RIB as O 10.1.0.0/22 [110/50] via 0.0.0.0, Null0.

Prefix-list filtering

OSPF does not have a direct “filter routes by prefix-list” interface in the same way BGP does. The mechanism is area range not-advertise — the operator declares a range and marks it not-advertised, and the ABR suppresses it.

For more granular filtering, the operator uses route-maps:

set policy route-map OSPF-OUT-FILTER rule 10 action deny
set policy route-map OSPF-OUT-FILTER rule 10 match ip address prefix-list BLOCK-LIST

set policy route-map OSPF-OUT-FILTER rule 20 action permit

The route-map is applied via FRR’s redistribution mechanism. Note that the practical effect is limited: the primary filter is area range, and prefix-list filtering is mostly used to suppress ranges or to adjust the metric.

How the result is validated

show ip ospf database
show ip ospf border-routers
show ip route ospf
show ip prefix-list
vtysh -c 'show ip ospf'
vtysh -c 'show ip ospf database summary'
vtysh -c 'show ip route ospf'

What the operator looks for:

• show ip ospf database summary lists Type 3 Summary LSAs. The operator checks that the range’s prefix appears as a single Summary with the configured metric.
• show ip route ospf shows the summary installed in the local RIB (the ABR has the discard route for the summary; downstream routers have the summary as a regular OSPF route).
• show ip prefix-list lists the configured prefix-lists with their rule sequences and matches.

How it fails

The production failure modes:

• Summarisation hides a route that should be visible. The downstream domain loses visibility into a specific intra-area prefix because the summary covers it. The fix is to narrow the range (so the prefix falls outside) or to remove the summarisation and accept the LSDB size.
• Summary cost is wrong. The summary carries a cost the operator did not intend. Traffic engineering is off. The fix is to set the explicit cost via range ... cost N.
• ABR does not have the summary in its LSDB. The ABR has been configured with the range, but the LSDB does not have a Summary LSA for the range. The cause is typically that no component prefix is currently inside the range — the ABR does not generate the Summary unless at least one component prefix is present.
• Discard route causes traffic loss. A downstream router sends traffic to the ABR for the summary, and the ABR drops it because no specific intra-area prefix matches. The cause is typically that the summarisation was too aggressive and covers prefixes that are no longer (or not yet) present in the area.

Rollback

# Capture the running configuration before the change
show configuration commands | save /tmp/ospf-range-$(date +%s).txt

# Compare
compare

# Commit with a short confirm window
commit-confirm 5

The rollback for a range change is a simple rollback N and commit. The change is locally scoped (the ABR’s LSDB and the downstream domain’s view of the prefix). The commit-confirm window is useful if the change is on a remote router or if the change covers a critical prefix.

Production discipline

Cross-course references

• The OSPF fundamentals part covers the LSA types (vyos-xviii-03-lsa-types), the area concept (vyos-xviii-04-areas), and the basic OSPF configuration (vyos-xix-03-ospf-area-config).
• The previous lessons (vyos-xx-01-area-types and vyos-xx-02-stub-nssa) covered the area-type filters at the ABR that interact with summarisation (a stub area does not see Type 5 externals, so summarising across a stub is meaningless for external prefixes).
• The route summarisation part (vyos-xxxv-01-aggregation-concept) covers the BGP and static-route analogue. OSPF summarisation is area-scoped; BGP aggregation is AS-scoped; static-route aggregation is a local decision.
• The troubleshooting part (XXII) covers the adjacency failure modes that can surface if summarisation is misconfigured.

Quiz