Route summarisation
When a router or AS is advertising several contiguous routes, then instead of announcing all routes, an AS can send one summary route only.
In this example, 3 prefixes are advertised by AS100. These can be summarised and announced as a single prefix, 172.16.0.0/22.
Summarisation method
One way to summarise prefixes is to:
- Convert them from decimal to binary.
- Select all common bits, they all have. In our example, all first 22 bits are common.
- Using common bits, extract network address, 172.16.0.0 in our case.
- We are left with 10 bits for hosts.
Benefits of summarising
Configuring summarisation
There are 2 methods to summarise in BGP.
- Redistribute a static route. This summary will be announced even if there are no smaller prefixes are not available.
- Aggregate command. Summary is only advertised when there is one or more smaller prefixes in BGP table.
On R1, we create loopbacks10, 20 and 30 to simulate the 3 different networks. Then advertise them in BGP using the network command, as seen in BGP fundamentals.
We add our 3 loopbacks interfaces on R1 and announced them using network statements. On R2, we can see the 3 prefixes received as intended.
Redistribute static
Let’s start with the 1st method and add a static route for 172.16.0.0/22 to null 0, then redistribute static to BGP. On R2, we can see a new summary prefix 172.16.0.0/22.
Aggregate command
We roll back and remove static redistribution, and instead use the aggregate command. R2, will still have a summary prefix 172.16.0.0/22.
What is the point of advertising an aggregated prefix, if the smaller subnet are still received on R2, Wouldn’t be be better to only announce 172.16.0.0/22 and save R2 resources by reducing R2 routing table size ?
To achieve this, we add “summary-only” to the aggregate command.
What is the point of advertising an aggregated prefix, if the smaller subnet are still received on R2, Wouldn’t be be better to only announce 172.16.0.0/22 and save R2 resources by reducing R2 routing table size ?
To achieve this, we add “summary-only” to the aggregate command. When added, R2 receives a single summarised prefix.
There is a lot more to aggregating prefixes, and how route-maps can suppress as set of prefixes and allow others. Another important point is how AS path is presented when the aggregated prefix is originally advertised from a remote AS.
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