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    Dynamic Routing Considerations for Service Connections
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    Dynamic Routing Considerations for Service Connections

    Table of Contents

    Dynamic Routing Considerations for Service Connections

    Learn how to route data for service connection traffic.
    Where Can I Use This?
    What Do I Need?
    • Prisma Access (Managed by Strata Cloud Manager)
    • Prisma Access (Managed by Panorama)

    How It Works — Routing Service Connection Traffic

    Prisma Access
     uses BGP for dynamic routing, and uses BGP path selection to install routes in the route table. When
    Prisma Access
     routes traffic to your headquarters or data center using service connections, it uses routing methods that direct that traffic effectively.
    Prisma Access
     uses a default routing model that was designed to fit the majority of network deployments; however, not all organization’s networks are the same. To fit a wider range of deployments,
    Prisma Access
     allows you choose another mode for service connection routing. The following sections describe the BGP routing methods that
    Prisma Access
     uses, along with the factors you need to consider in your organization’s network before changing
    Prisma Access
    ’ default method of service connection routing.
    Changing the
    Prisma Access
     service connection routing method requires a thorough understanding of your organization’s topology and routing devices, along with an understanding of how
    Prisma Access
     routing works as described in this section. We recommend that you read this section carefully before changing the routing method from the default setting.
    Prisma Access
     supports static routing and dynamic routing using BGP for service and remote network connections; this section assumes that you use BGP routing for your
    Prisma Access
     deployments. When you select BGP routing, your organization’s network learns BGP information from
    Prisma Access
    .

    Routing Modes for Service Connections

    You can choose from the following routing modes with
    Prisma Access
    :
    • Default routing—This is the current routing model that
      Prisma Access
       uses.
      Use this routing mode if you want
      Prisma Access
       to use BGP best path-selection mechanisms without adjusting any of the BGP attributes. In this mode,
      Prisma Access
       will honor any attribute advertised by the customer premises equipment (CPE).
    • Hot Potato routing—
      Prisma Access
       hands off the traffic as quickly as it can to your organization’s network.
      Use this routing method if you want your organization’s network to perform the majority of routing decisions.

    Prisma Access
     Default Routing

    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 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 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 :
      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.
    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.

    Hot Potato Routing

    When you select
    Hot Potato Routing
    ,
    Prisma Access
     egresses the traffic bound to service connections/data centers from its internal network as quickly as possible.
    With hot potato routing,
    Prisma Access
     prepends the AS path (AS-PATH) to the BGP prefix advertisements sent from gateways. This prepending is performed when the prefixes are advertised out of the service connection to your organization’s on-premises CPE.
    Prisma Access
     prepends the AS-PATHs so that your CPE gives the correct preference to the primary and secondary tunnels, so that if the primary tunnel goes down, your CPE chooses the secondary tunnel as the backup.
    If you specified a different IP address for the secondary (backup) BGP peer, Prisma Access adds more prepends based on the tunnel type, as shown in the following table.
    Prefix Type
    Service Connection Tunnel Type
    Number of As-Path Prepends
    Total AS-PATHs Seen on the CPE
    Gateway prefixes from primary service connection
    Primary or Secondary tunnel with the same BGP peer IP address
    0
    1
    Gateway prefixes from backup service connection
    Primary or Secondary tunnel with the same BGP peer IP address
    3
    4
    Gateway prefixes from all other service connections
    Primary or Secondary tunnel with the same BGP peer IP address
    6
    7
    Gateway prefixes from primary service connection
    Secondary tunnel with a different BGP peer IP address
    1
    2
    Gateway prefixes from backup service connection
    Secondary tunnel with a different BGP peer IP address
    4
    5
    Gateway prefixes from all other service connections
    Secondary tunnel with a different BGP peer IP address
    7
    8
    Specify a
    Backup Service Connection
     so that
    Prisma Access
     to uses that service connection as the backup when a service connection link fails.
    The following figure shows a hot potato routing configuration for traffic between the US service connection and AS 200, with the EU service connection configured as the
    Backup Service Connection
     of the US connection. Using hot potato routing,
    Prisma Access
     sends the traffic from its closest exit path through the US service connection. The return traffic takes the same path through AS100 because this path has a shorter AS-PATH to the mobile user pool in the US location.
    Prisma Access
     prepends the AS-PATH to its prefix advertisements depending on whether the tunnel is a primary tunnel, a backup tunnel, or not used for either primary or backup.
    Because you have set up a backup service connection, if the link to the US service connection goes down, hot potato routing sends the traffic out using its shortest route through the EU service connection. This routing scenario also applies to networks that use route aggregation.
    You can also use backup service connections for multiple service connections in a single region. The following figure shows a
    Prisma Access
     deployment with two service connections in the North America region. In this case, you specify a
    Backup Service Connection
     of US-E for the US-W service connection, and vice versa, to ensure symmetric routing.

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