AS Override for Cloud ROUTER connections

DE-CIX PDM Team Updated by DE-CIX PDM Team

This article explains the AS Override feature for Cloud ROUTER connections, when to use it, and how to enable it.

Please note: AS Override can currently only be set when creating a Cloud ROUTER connection (DirectCLOUD or VirtualPNI). It cannot be toggled on or off for an existing connection during operation. Enabling or disabling AS Override on an already provisioned connection is on our roadmap but not yet available. If you need to change the setting on an existing connection, you currently have to delete and recreate it.

What is AS Override?

When you connect two networks that use the same BGP Autonomous System Number (ASN) through your Cloud ROUTER, BGP's built-in loop prevention will reject the routes. During route exchange, a prefix advertised from network A carries network A's ASN in the AS_PATH. When that prefix reaches network B — which uses the same ASN — network B discards it, because seeing its own ASN in the path looks like a routing loop.

This is a common situation with Oracle Cloud FastConnect: every OCI region uses the same BGP ASN (31898). If you connect two OCI VCNs (for example in different regions) through your Cloud ROUTER, they will fail to exchange prefixes, because both sides advertise routes with ASN 31898.

AS Override solves this. When enabled on a Cloud ROUTER connection, the Cloud ROUTER replaces the neighbor's ASN in the AS_PATH with its own ASN before advertising the route onward. The receiving side no longer sees its own ASN in the path, so it accepts the route.

When do I need it?

Enable AS Override when both ends of a routed path use the same ASN and would otherwise reject each other's routes. Typical scenarios:

  • Connecting two or more OCI VCNs (all using ASN 31898) through a single Cloud ROUTER
  • Connecting any two networks that share an identical ASN and need to exchange routes across the Cloud ROUTER

If the connected networks use different ASNs, you do not need AS Override.

Important: prevent routes from being advertised back

Important: When AS Override is enabled, the loop-prevention safeguard that the shared ASN normally provides no longer applies to that connection. To avoid routes learned from a cloud provider being advertised back toward the same provider, we currently recommend configuring an egress reject policy toward that connection, so that routes learned from an interface are not re-advertised back to it.

You can configure egress policies using Prefix Lists & Policies in Cloud ROUTER.

How to enable AS Override

AS Override is set during the creation of a Cloud ROUTER connection (DirectCLOUD or VirtualPNI).

  1. Start creating your DirectCLOUD or VirtualPNI configuration on your Cloud ROUTER as described in Add a DirectCLOUD configuration or Add a VirtualPNI configuration.
  2. In the connection configuration, enable the AS Override toggle.
  3. Complete the remaining configuration and submit.
  4. On the Cloud ROUTER overview page you can doublecheck if AS Override is enabled or disabled per connection

Further reading

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