WeOS
5.29.0

# OSPF Route Summarization and Filtering HowTo

## About

This document aims to provide example OSPF use-cases where we configure OSPF areas to utilise route summarisation and filtering, to reduce the amount of routing information that the ABR propagates to the rest of the OSPF network.

For an overview of all available configuration settings for OSPF refer to the configuration guide:

- OSPF Configuration Guide

## Introduction

In another howto, an example was provided how the amount routing information each router in an OSPF network need to handle can be reduced by utilising OSPF areas.

In addition to this the ABR can be configured to further limit the routing information that is propagated from an area into the rest of the OSPF network. This can be done by using route summarisation and route filtering.

Note

Being able to efficiently utilise this approach requires that the network have been segmented in a efficient manner.

The following example use-cases are presented in this document:

- Case 1: Route Summarization

- Case 2: Route Filtering

## Case 1: Route Summarization

In this use-case an example will be provided how routing information from different areas can be reduced even more by utilising route summarisation. Route summarisation can be applied on ABRs, before route information is distributed into the other areas in the network.

What route summarisation entails, is that multiple different devices inside an OSPF area can be represented by a single route to devices outside the area. The reason the network segmentation is important in order to efficiently execute route summarisation, is because it is based on subnet sizing.

                                    . . . . . . . . . . . . . . . . . . . . . . . . .
                                    .  Area 0.0.0.1                                 .
                                    .                 .-------.   | 172.16.20.0/24  .
                                    .  172.16.1.0/24  |       |   | 172.16.21.0/24  .
    . . . . . . . . . . . . . .     .  .--------------+  R2   +-->| 172.16.22.0/24  .
    .     Area 0.0.0.0        .     .  |              |       |   | 172.16.23.0/24  .
    .                         .     .  |              '-------'   | 172.16.24.0/24  .
    . .-------.              .-------. |                                            .
    . |       |              |       +-'                                            .
    . |  R1   +--------------+  ABR  |                                              .
    . |       | 10.0.1.0/24  |       +-.                                            .
    . '-------'              '-------' |                                            .
    .                         .     .  |              .-------.   | 192.168.20.0/24 .
    .                         .     .  |              |       |   | 192.168.21.0/24 .
    . . . . . . . . . . . . . .     .  '--------------+  R3   |-->| 192.168.22.0/24 .
                                    .  192.168.1.0/24 |       |   | 192.168.23.0/24 .
                                    .                 '-------'   | 192.168.24.0/24 .
                                    .                                               .
                                    . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 1: Example of an OSPF network segmented into two areas, with route summarisation used on the ABR.

In Figure 1, an example of a OSPF network segmented into two different areas can be seen. Route summarisation will be added on the `ABR` router in order to represent the additional networks that can be accessed from router `R2` and router `R3`, as a single route for each of the routers.

The additional networks for `R2` can be summarised with `172.16.16.0/20` and the networks for `R3` can be summarised with `192.168.16.0/20`.

### Configuration

The route summarisation only needs to be configured on the ABR, since the routes are summarised when they are advertised into the backbone area:

    ABR:/#> configure
    ABR:/config/#> router
    ABR:/config/router/#> ospf
    ABR:/config/router/ospf/#> area 1
    ABR:/config/router/ospf/area-0.0.0.1/#> range 172.16.16.0/20 advertise
    ABR:/config/router/ospf/area-0.0.0.1/#> range 192.168.16.0/20 advertise
    ABR:/config/router/ospf/area-0.0.0.1/#> leave
    ABR:/#>

### Resulting Routes

Without any route summarisation on the ABR these are the routes that R1 will be assigned by OSPF:

    R1:/#> show ip route
    S - Static | C - Connected | K - Kernel route  | > - Selected route
    O - OSPF   | R - RIP       | [Distance/Metric] | * - FIB route

    O   10.0.1.0/24 [110/10] is directly connected, vlan1, 00:12:01
    C>* 10.0.1.0/24 is directly connected, vlan1
    O>* 172.16.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:08:31
    O>* 172.16.20.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.21.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.22.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.23.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.24.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 192.168.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:08:31
    O>* 192.168.20.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.21.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.22.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.23.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.24.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06

After routes summarisation have been applied on the `ABR` the routes on `R1` are assigned like this:

    R1:/#> show ip route
    S - Static | C - Connected | K - Kernel route  | > - Selected route
    O - OSPF   | R - RIP       | [Distance/Metric] | * - FIB route

    O   10.0.1.0/24 [110/10] is directly connected, vlan1, 01:59:19
    C>* 10.0.1.0/24 is directly connected, vlan1
    O>* 172.16.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:01:43
    O>* 172.16.16.0/20 [110/30] via 10.0.1.2, vlan1, 00:01:43
    O>* 192.168.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:01:43
    O>* 192.168.16.0/20 [110/30] via 10.0.1.2, vlan1, 00:01:43

As we can see the routes towards `172.16.20.0/24`, `172.16.21.0/24`, `172.16.22.0/24`, `172.16.23.0/24`, `172.16.24.0/24` are now represented by `172.16.16.0/20`.

In the same manner, the routes towards `192.168.20.0/24`, `192.168.21.0/24`, `192.168.22.0/24`, `192.168.23.0/24`, `192.168.24.0/24` are now represented by `192.168.16.0/20`.

## Case 2: Route Filtering

In addition to being able to summarise routes, they can also be filtered out in order to not be advertised outside of the area.

Consider the same setup as in Case 1. Instead of summarising the routes towards the networks `R1` and `R2` are connected to, they will be filtered out, since we do not want that routing information in any other areas of the OSPF network.

### Configuration

The route filtering needs to be configured on the ABR between the area and the backbone:

    ABR:/#> configure
    ABR:/config/#> router
    ABR:/config/router/#> ospf
    ABR:/config/router/ospf/#> area 1
    ABR:/config/router/ospf/area-0.0.0.1/#> range 172.16.16.0/20 not-advertise
    ABR:/config/router/ospf/area-0.0.0.1/#> range 192.168.16.0/20 not-advertise
    ABR:/config/router/ospf/area-0.0.0.1/#> leave
    ABR:/#>

### Resulting Routes

Without the route filtering, the routes on `R1` will look like this:

    R1:/#> show ip route
    S - Static | C - Connected | K - Kernel route  | > - Selected route
    O - OSPF   | R - RIP       | [Distance/Metric] | * - FIB route

    O   10.0.1.0/24 [110/10] is directly connected, vlan1, 00:12:01
    C>* 10.0.1.0/24 is directly connected, vlan1
    O>* 172.16.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:08:31
    O>* 172.16.20.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.21.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.22.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.23.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 172.16.24.0/24 [110/30] via 10.0.1.2, vlan1, 00:03:46
    O>* 192.168.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:08:31
    O>* 192.168.20.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.21.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.22.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.23.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06
    O>* 192.168.24.0/24 [110/30] via 10.0.1.2, vlan1, 00:00:06

After the route filtering have been applied on the `ABR`, the routes on `R1` will look like this:

    R1:/#> show ip route
    S - Static | C - Connected | K - Kernel route  | > - Selected route
    O - OSPF   | R - RIP       | [Distance/Metric] | * - FIB route

    O   10.0.1.0/24 [110/10] is directly connected, vlan1, 02:55:46
    C>* 10.0.1.0/24 is directly connected, vlan1
    O>* 172.16.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:01:50
    O>* 192.168.1.0/24 [110/20] via 10.0.1.2, vlan1, 00:01:50

As can be seen, none of the networks connected to `R2` and `R3` have any routes towards them propagated outside of their area.

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