Overview of route summarization
Route summary is also called route aggregation, which is to aggregate a group of regular routes into one route, so as to reduce the size of the routing table and optimize the utilization of equipment resources. We call this group of routes before aggregation the fine For routing or detailed routing, the aggregated routing is called summary routing or aggregate routing.
Route summary implementation
Prerequisites for route summary deployment
The prerequisite for a network to deploy summary routes is that the IP addressing and network design in the network are scientific and reasonable. If the network planning is disorderly, the deployment of summary routes is quite difficult or even impossible to implement.
Route summary (1)
In the following network, as far as R1 is concerned, if you want to reach 192.168.1.0/24~192.168.255.0/24 on the right side of R2, there must be a route. If you manually configure static routes for each network segment, it means you need to configure 255, not only the workload is huge, but also the routing table of R1 is extremely bloated.
Routing topology
By creating a summary route ip route-static 192.168.0.0 24 10.1.12.2, you can also pass the default route, but the default route cannot control the routing in more detail.
Route summary (two)
Routing topology
In order to enable R2 to reach the network segment on the left of R1, a summary route is configured for it:
ip route-static 172.16.0.0 24 10.1.12.1
This summary route does serve the purpose of network optimization, but it is too rough, and even includes the network segment on the left side of R3. In this way, the data packets destined for the right side of R3 arrive at R2. It may be forwarded by R2 to R1, resulting in packet loss.
More accurate route summary
The above route summary behavior is not accurate enough. An ideal way is to configure a summary route that just covers all detailed routes for R2 and summarize it accurately.
Implementation method of precise route summary
According to the existing detailed routes 172.16.1.0 to 172.16.31.0 of R1, calculate the most accurate summary route for these detailed routes, or calculate the summary route with the longest mask length.
Implementation steps
1. Convert the destination network addresses of the detailed routing to binary, and then arrange them to find the "same bits" in all the destination network addresses.
Specific process
(1) Write the IP as binary, arrange it as shown in the figure above, and actually only refer to the third 8-bit, only it is changed.
(2) Draw a line. Each digit on the left side of the line must have the same value. The position of this line also indicates the length of the mask of the summary route.
(3) The position of the line in the figure is 19, so the summary route mask is 172.168.0.0/19, this is the most accurate address, in other words, it is the summary address with the longest mask.
Therefore, you can configure R2 as follows:
[R2]ip route-static 172.16.0.0 19 10.1.12.1
[R2]ip route-static 172.16.32.0 19 10.1.23.3
Accurately summarize the network segments on the left of R1 to get the summary address and mask length: 172.16.0.0/19, then configure the corresponding static route summary on R2, set the next hop to R1, and set the network segment on the right of R3 Perform precise summary to get the summarized address and mask length, 172.16.32.0/19, and then configure the corresponding static summary route on R2, and configure the next hop as R3.
Route summary idea
Route summarization is a very important design idea. Usually in the design process of a large and medium-sized network, routing and route optimization must be considered at all times. Among them, route summarization is a very key indicator, not only static routing supports routing Summarization and dynamic routing are basically supported.
Route summarization is actually achieved through flexible operation of the destination network address and network mask. The visual understanding is to replace them with a large network segment that can encompass these small network segments.
Route summary considerations
(1) Configure route summarization to be very cautious and precise, otherwise it may cause routing confusion. As shown in experiment (2), if only 172.16.0.0/24 is configured, data to R3 will be lost.
(2) Route summary is a very important network optimization thinking, but if it is not handled properly, it will bring loops.
Refer to the HCNP routing exchange study guide