Find the summary prefix for contiguous networks, configure a static summary with Null0 and know when summarising is unsafe.
Intermediate · 9 min read
Technical definition
Route summarization is replacing several contiguous network routes with one route that covers them all. It is also called route aggregation or supernetting, and it uses a shorter prefix length than the routes it replaces.
Why summarise
Smaller routing tables use less memory and make lookups faster.
Less update traffic and less SPF work: a flapping network inside the summary is hidden from the rest of the network.
Simpler design: each site or region advertises one prefix.
Finding the summary: the shared bits
Route summarisation: find the bits that all routes share
Rule: the networks must be contiguous and the count must be a power of two that starts on the right boundary. 192.168.0.0/22 covers 192.168.0.0 to 192.168.3.255 exactly.
List the networks and write the changing octet in binary.
Count the bits from the left that are the same in every network.
That count is the summary prefix length. Keep the shared bits, set the rest to 0, for the network address.
A second example
Summarise 172.16.32.0/24 to 172.16.47.0/24. The third octets are 32 to 47, which is 00100000 to 00101111 in binary. The first four bits (0010) are shared, so the prefix is 16 + 4 = 20 and the summary is 172.16.32.0/20 (mask 255.255.240.0).
When it does not work
Which groups of /24 networks summarise cleanly
On small screens, swipe sideways or focus the diagram and use the arrow keys.
A summary must cover only networks you own, or you risk attracting traffic you cannot deliver. Plan address blocks so that each site starts on a power-of-two boundary.
Configuring a static summary
Example commands and output on this page were written from Cisco IOS documentation and generally described behaviour. They were not captured from a live device, so prompts and details can differ on your platform and version.
! On the router that owns the four networks (R1)
ip route 192.168.0.0 255.255.252.0 Null0
! On the upstream router (R2), pointing at R1
ip route 192.168.0.0 255.255.252.0 10.0.12.1
R1 discards anything in the summary that it has no specific route for. R2 needs only one route instead of four.
Longest prefix match keeps this safe: the specific /24 routes on R1 still win for real networks, and the Null0 summary catches only what is left.
Summarising in routing protocols
Where each protocol summarises
On small screens, swipe sideways or focus the diagram and use the arrow keys.
OSPF summarises only at area borders and at ASBRs, not on ordinary links inside an area.
IPv6 summaries
The same rule applies: 2001:db8:acad:0::/64 to 2001:db8:acad:3::/64 share all but the last two bits of the subnet ID, so the summary is 2001:db8:acad::/62.
Test yourself
Predict · scenario 1
Which summary covers 10.1.4.0/24, 10.1.5.0/24, 10.1.6.0/24 and 10.1.7.0/24 exactly?
Predict · scenario 2
Why does a router that owns a summary add a route to Null0 for it?
Predict · scenario 3
Which statement is true about summarising 192.168.1.0/24 through 192.168.4.0/24?
FAQ
What is the difference between summarisation, aggregation and supernetting?
They describe the same idea. Supernetting is the older name for combining classful networks, aggregation is the term used with BGP and IPv6, and summarisation is the common term in IGP design.
Why point a summary route at Null0?
A summary covers addresses you may not own yet. If the router has no more specific route, a packet to such an address would follow the summary or a default route and could loop. A route to Null0 discards it instead.
Can I summarise any group of networks?
Only if they are contiguous and align on a power-of-two boundary. Four /24s starting at 192.168.0.0 can be summarised to a /22; four starting at 192.168.1.0 cannot, because the smallest single prefix that covers them is wider.