Route Summarization
Route summarization creates a single route whose numeric range, as implied by the prefix/prefix
length, is larger than the one or more smaller component routes. For example, 10.1.0.0/16 is a
summary route that includes component subnets 10.1.1.0/24, 10.1.4.132/30, and any other subnets
with the range 10.1.0.0 through 10.1.255.255.
The following list details some of the key features that the three IGPs covered in this book have in
common with regard to how route summarization works (by default):
■ The advertised summary is assigned the same metric as the currently lowest-metric
component subnet.
■ The router does not advertise the component subnets.
■ The router does not advertise the summary when its routing table does not have any of the
component subnets.
■ The summarizing router creates a local route to the summary, with destination null0, to
prevent routing loops
NOTE I use the term component route to refer to a route whose range of IP addresses is a
subset of the range specified by a summary route; however, I have not seen this term in other
reference materials from Cisco.
Route Summarization 327
■ Summary routes reduce the size of routing tables and topology databases, indirectly
improving convergence.
■ Summary routes decrease the amount of specific information in routing tables, sometimes
causing suboptimal routing.
Figure 10-7 depicts the suboptimal-routing side effect when using route summarization. It also
depicts the effect of using a summary to null0 on the summarizing router.
Figure 10-7 Route Summarization Suboptimal Routing and Routing to Null0
In Figure 10-7, R4 learned two paths to summary route 10.0.0.0/8, and picked the route through
R3 based on the metric. Because R4 does not have a route for 10.2.2.0/24, R4 then sends any
packets to that subnet based on its route to network 10.0.0.0/8, through R3. So, although subnets
like 10.2.2.0/24 may be topologically closer to R4 through R1, R4 sends the packets via the scenic,
suboptimal route through R3.
Also note that R4’s summary route to 10.0.0.0/8 matches packets for which the component subnet
does not exist anywhere in the network. In that case, routers like R4 forward the packets based on
the larger summary, but once the packet reaches the router that created the summary, the packet is
discarded by the summarizing router due to its null route. For instance, Figure 10-7 shows R4
forwarding a packet destined to 10.3.3.1 to R3. R3 does not have a more specific route than its
route to 10.0.0.0/8, with next-hop interface null0. As a result, R3 discards the packet.
The sections that follow provide a few details about summarization with each routing protocol.
R3 R4
R1
Subnets That All
Begin with 10.1
R3 Routing Table:
10.1.1.0/24…
10.1.2.0/24…
10.2.2.0/24…
10.0.0.0/8, to null0
R4 Routing Table:
10.0.0.0/8, to R3
Subnets That All
Begin with 10.2
Subnet 10.2.2.0/24
Packet to 10.3.3.1
Packet to 10.2.2.1
Summary 10.0.0.0/8
Summary 10.0.0.0/8
Both packets match my
best route to 10.0.0.0/8,
which points to R3
• Packet to 10.3.3.1 matches
route to Null0 – Discard
• Packet to 10.2.2.1 matches
specific route – Forward
Represents Routing Updates
Represents Example Packets