Prisma Access Default Routing
Table of Contents
Prisma Access Default Routing
How Prisma Access default routing works for service connections.
The following figure shows an example of Prisma Access
routing service connection traffic in default routing mode. The
organization’s network has three separate networks in three data
centers and does not have a backbone connecting the networks. In
default routing mode, mobile user pools are advertised equally on
the three networks, as shown at the bottom of the figure.
Note that, when Prisma Access advertises mobile user routes,
it divides the subnets
into Class C /24 address blocks before advertising them;
thus, it advertises the /20 mobile user subnets in chunks of /24
as prefixes are consumed by the gateways.
Make a note of how Prisma Access uses BGP route advertisements:
- Prisma Access does not adjust the default BGP attributes for mobile user advertised routes (Prisma Access adds its AS number to the route advertisements).
- Prisma Access advertises mobile user routes in blocks of /24 subnets and adds BGP community values in the routes it advertises through the service connection. The following figure shows a mobile user deployment with three service connections and three different IP address blocks specified for the mobile user IP address pool: 192.168.64.0/20 for the Asia, Australia & Japan region, 192.168.72.0/20 for the Africa, Europe & Middle East region, and 192.168.48.0/20 for the North America & South America region. Prisma Access divides these routes into block of /24 and advertises them with an Prisma Access’ AS number of 65534, but also appends the BGP community values to the advertisements (Z for Asia, Y for EU, and X for US). Those routes are shown in the middle of the figure. In this way, you can differentiate service connections in your network, even though Prisma Access assigns the same AS number to them.
You can view the community string by selecting and
find the Community field in the Peer tab.
The following figure shows a more common network with a full-mesh
eBGP backbone. The figure shows the routes that Prisma Access has
learned from your organization’s network on the top right. Note
the extra routes that Prisma Access has learned through the Prisma
Access backbone (iBGP) and your organization’s backbone (eBGP).
For traffic between mobile users in the North America
& South America region (US in the diagram) and the
data center in your organization’s Africa, Europe &
Middle East region (EU in the diagram), Prisma Access
chooses the path through the EU service connection because it prefers
routes with a shorter AS-PATH.
In deployments with a full-mesh eBGP backbone, asymmetry can
arise when Prisma Access cannot reach a particular data center due
to an ISP/CPE failure at the customer’s data center. The following
figure shows what could happen when the link to the EU service connection
goes down. Your network detects the link failure and builds a new
route table for AS 200. Traffic from the US service connection to
AS 200 uses the path through AS 100 because the eBGP route for your
backbone between AS 200 and AS 100 is preferred to the iBGP route
between service connections EU and US. However, return traffic is
not guaranteed through the same path because the on-premises CPE
can choose either path (shown in red) to return the traffic.
The previous examples show a network whose routes have not been
aggregated (that is, you have not performed route summarization
before you send the BGP route advertisements to Prisma Access).
The following example shows a network that summarizes its routes
to 10.0.0.0/8 before sending to Prisma Access. If you select default
routing, this configuration can lead to asymmetric routing issues,
because Prisma Access cannot determine the correct return path from
the summarized routes.
If your Prisma Access deployment has Remote
Networks, Palo Alto Networks does not recommend the use of route
summarization on Service Connections. Route summarization on service
connections is for Mobile Users deployments only.
If you use route aggregation for mobile users, we strongly recommend
that you enable
hot potato routing
instead
of default routing, where Prisma Access hands off the traffic as
quickly as possible to your organization’s network; in addition,
we recommend that you select a Backup SC as
described in the following section for each service connection to
have a deterministic routing behavior.