Prisma Access Default Routing

Prisma Access

Prisma Access Default Routing

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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:
    for the
    Asia, Australia & Japan
    for the
    Africa, Europe & Middle East
    region, and
    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
    , but also appends the BGP community values to the advertisements (
    for Asia,
    for EU, and
    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
Cloud Services
Network Details
Service Connection
Show BGP Status
and find the
field in the
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 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.

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