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Route Aggregation on Border Routers and Transport Gateways
Route Aggregation on Border Routers and Transport Gateways
Table 1. Feature History
Feature Name
Release Information
Description
Route Aggregation on Border Routers and Transport Gateways
Cisco Catalyst SD-WAN Control Components Release 20.11.1
Cisco IOS XE Catalyst SD-WAN Release 17.11.1a
This feature enables you to configure route aggregation on border routers and transport gateways in a Multi-Region Fabric
network environment. For a border router, you can specify whether the route aggregation operates only for the core region,
the router's access region, or both.
Information About Route Aggregation on Border Routers and Transport Gateways
Route aggregation is a method for reducing the number of entries that routers in a network must maintain in routing tables,
for better scaling. In some enterprise settings, the number of routes available to a device might reach the thousands. Storing
this number of entries in the routing tables of devices in the network may require excessive resources on each device, and
can reduce network performance.
To reduce the demand on router resources, and improve network performance, you can configure a router in the network to manage
a range of IP addresses using a networking method called route aggregation, which works as follows:
Configure a router to advertise to the network a single prefix that represents a range of IP addresses. Provide the prefix
in classless inter-domain routing (CIDR) notation. For example, 10.0.0.0/8 includes all addresses from 10.0.0.1 to 10.255.255.254.
Other routers in the network only require a single routing table entry for this prefix, which aggregates a range of IP addresses.
When a router in the network receives traffic for a route represented by an IP address in the aggregated range, the router
directs the traffic to the device handling that range of addresses.
In this manner, route aggregation can significantly reduce the size of routing tables on devices in the network.
Figure 1. Route Aggregation
Note the following behaviors regarding route aggregation:
An aggregating device advertises the aggregate prefix only if a component route within the prefix range is available.
By default, an aggregating router advertises each individual route and the summary prefix. You can use an aggregate-only option
to advertise only the summary prefix and not each individual route. For more information, see Unicast Routing in the Cisco Catalyst SD-WAN Routing Configuration Guide, Cisco IOS XE Release 17.x.
Route Aggregation with Multi-Region Fabric and Transport Gateways
Releases earlier than Cisco IOS XE Catalyst SD-WAN Release 17.11.1a support configuring aggregation for OMP routes. This includes configuring aggregation for edge routers in access regions
of a network using Multi-Region Fabric. Cisco IOS XE Catalyst SD-WAN Release 17.11.1a adds support for using route route aggregation with border routers (with the option to advertise routes to either the access
region or core region) and transport gateways.
When you configure route aggregation on a border router or transport gateway, the aggregation includes even routes that the
device reoriginates. For example, if you configure a border router to aggregate routes defined by the prefix 10.0.0.0/8, the
aggregation applies to the route 10.0.0.20, even if that is a route that the border router is reoriginating.
For more information about route aggregation, see the Configure OMP section of the Cisco Catalyst SD-WAN Routing Configuration Guide, Cisco IOS XE Release 17.x.
Reorigination of Routes by a Border Router
By the nature of the Multi-Region Fabric network topology, a border router functions as a gateway to routers in the access
region that it serves to reach other Multi-Region Fabric regions. To do this, the border router reoriginates routes of devices
in its access region to the core region, and similarly reoriginates routes from the core region to the access region. As the
number of routers in the access or core region grows, the number of routes that the border router reoriginates grows too.
Each route that a border router reoriginates to the core region creates an additional resource demand on all of the border
routers in the core region. The following simplified illustration shows a few important points:
As the number of routes in an access region grows, the number of routes that the border router for that region readvertises
grows.
Each border router for a region reoriginates the same set of routes as other border routers in the region, so the number of
readvertised routes multiplies with each new border router.
As the number of readvertised routes grows, the resource demands grow for the Cisco SD-WAN Controllers serving the core region.
Figure 2. Reorigination of Routes in a Multi-Region Fabric Network
IPv6 Support
Route aggregation supports IPv4 and IPv6 addresses.
Benefits of Route Aggregation on Border Routers and Transport Gateways
Configuring a border router to perform route aggregation reduces the need to readvertise all routes individually in the core
region. Reoriginating fewer individual routes alleviates resource demands on border routers and Cisco SD-WAN Controllers.
Similarly, you can configure route aggregation for an edge router operating as a transport gateway. The benefits do not apply
specifically to the core region, but as with all route aggregation, the result is to reduce demands on router resources by
minimizing the size of routing tables.
Supported Platforms for Route Aggregation on Border Routers and Transport Gateways
An organization using Multi-Region Fabric has a large number of edge routers in two access regions, with two Cisco SD-WAN Controllers managing each access region and the core region. The large number of routes reoriginated by each of two border routers for
each access region strain the resources of the Cisco SD-WAN Controllers for the core region.
Figure 3. Route Aggregation on Border Routers
To reduce the number of routes reoriginated in the core region, configure border routers BR1 and BR2 to aggregate the routes
for the 200+ edge routers in region 1. Configure the border routers BR3 and BR4 to aggregate the routes for the 200+ edge
routers in region 2.
Configure Route Aggregation on Border Routers and Transport Gateways Using Cisco SD-WAN Manager
When configuring route aggregation for a border router, ensure that the VPN template in the procedure that follows is attached
to the device template of a border router. If not, the core and access options that occur in the procedure are not available.
Configure Route Aggregation on Border Routers and Transport Gateways
If you do not have a VPN template attached to the device template of the border router that you are configuring, do the following
to create a VPN template:
From the Cisco SD-WAN Manager menu, choose Configuration > Templates.
Click Feature Templates.
Click Add Template.
Choose the device type of the border router.
Choose the VPN template.
In the Template Name and Description fields, enter a name and description for the template.
Click Save to save the template.
Attach the VPN template to the border router device template for the border router you are configuring, or to the device template
of the transport gateway router you are configuring.
From the Cisco SD-WAN Manager menu, choose Configuration > Templates.
Click Feature Templates.
Locate the VPN template that is attached to the border router you are configuring. Adjacent to the VPN template, click … and choose Edit.
In the Advertise OMP section of the template, click New Advertise OMP.
In the Protocol field, choose Aggregate.
Click Add.
Click New Aggregate.
In the Prefix field, enter the prefix, in CIDR notation, for the range of IP addresses corresponding to routes to aggregate. Example: 10.0.0.0/8
(This step applies only for a border router. It does not apply to a transport gateway edge router.) In the Region field, choose Core or Access to indicate whether the router advertises the prefix of aggregated routes to the core region or the access region that it
serves.
Click Add.
Click Save to save the template.
Configure Route Aggregation on Border Routers and Transport Gateways Using a CLI Template
For more information about using CLI templates, see CLI Add-On Feature Templates and CLI Templates. By default, CLI templates execute commands in global configuration mode.
Note
By default, CLI templates execute commands in global config mode.
For a border router performing route aggregation, we recommend configuring either region core or region access.
Enter OMP configuration mode.
sdwan omp
Enter VRF configuration mode.
address-family {ipv4 | ipv6} vrfvrf-id
Configure aggregation of routes of a specific IP range, using a prefix in CIDR notation. If you are configuring a border router,
you can optionally specify whether the border router advertises the aggregated routes to the core region or its access region.
If you do not specify one, the border router applies the aggregation to both the access region that it serves and the core
region.
Use aggregate-only to advertise only the aggregate prefix and not the component routes included within the range of the prefix.
Border router, which may or may not also be configured as a transport gateway
Note
If you omit region {core | access}, the border router advertises the routes to both its access region and the core region.
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