Routing policies and BGP communities

AWS Direct Connect applies inbound (from your on-premises data center) and outbound (from your AWS Region) routing policies for a public AWS Direct Connect connection. You can also use Border Gateway Protocol (BGP) community tags on routes advertised by Amazon and apply BGP community tags on the routes you advertise to Amazon.

Public virtual interface routing policies

If you're using AWS Direct Connect to access public AWS services, you must specify the public IPv4 prefixes or IPv6 prefixes to advertise over BGP.

The following inbound routing policies apply:

• You must own the public prefixes and they must be registered as such in the appropriate regional internet registry.

• Traffic must be destined to Amazon public prefixes. Transitive routing between connections is not supported.

• AWS Direct Connect performs inbound packet filtering to validate that the source of the traffic originated from your advertised prefix.

The following outbound routing policies apply:

• AS_PATH and Longest Prefix Match are used to determine the routing path. AWS recommends advertising more specific routes using AWS Direct Connect if the same prefix is being advertised to both the Internet and to a public virtual interface.

• AWS Direct Connect advertises all local and remote AWS Region prefixes where available and includes on-net prefixes from other AWS non-Region points of presence (PoP) where available; for example, CloudFront and Route 53.

  Note

  • Prefixes listed in the AWS IP address ranges JSON file, ip-ranges.json, for the AWS China Regions are only advertised in the AWS China Regions.

  • Prefixes listed in the AWS IP address ranges JSON file, ip-ranges.json, for the AWS Commercial Regions are only advertised in the AWS Commercial Regions.

  For more information about the ip-ranges.json file, see AWS IP address ranges in the AWS General Reference.

• AWS Direct Connect advertises prefixes with a minimum path length of 3.

• AWS Direct Connect advertises all public prefixes with the well-known NO_EXPORT BGP community.

• If you advertise the same prefixes from two different Regions using two different public virtual interfaces, and both have the same BGP attributes and longest prefix length, AWS will prioritize the home Region for outbound traffic.

• If you have multiple AWS Direct Connect connections, you can adjust the load-sharing of inbound traffic by advertising prefixes with the same path attributes.

• The prefixes advertised by AWS Direct Connect must not be advertised beyond the network boundaries of your connection. For example, these prefixes must not be included in any public internet routing table.

• AWS Direct Connect keeps prefixes advertised by customers within the Amazon network. We do not re-advertise customer prefixes learned from a public VIF to any of the following:

  • Other AWS Direct Connect customers

  • Networks that peer with the AWS Global Network

  • Amazon's transit providers

Public virtual interface BGP communities

AWS Direct Connect supports scope BGP community tags to help control the scope (Regional or global) and route preference of traffic on public virtual interfaces. AWS treats all routes received from a public VIF as if they were tagged with the NO_EXPORT BGP community tag, meaning only the AWS network will use that routing information.

Scope BGP communities

You can apply BGP community tags on the public prefixes that you advertise to Amazon to indicate how far to propagate your prefixes in the Amazon network, for the local AWS Region only, all Regions within a continent, or all public Regions.

AWS Region communities

For inbound routing policies, you can use the following BGP communities for your prefixes:

• 7224:9100—Local AWS Regions

• 7224:9200—All AWS Regions for a continent:

  • North America-wide

  • Asia Pacific

  • Europe, the Middle East and Africa

• 7224:9300—Global (all public AWS Regions)

Note

If you do not apply any community tags, prefixes are advertised to all public AWS Regions (global) by default.

Prefixes that are marked with the same communities, and have identical AS_PATH attributes are candidates for multi-pathing.

The communities 7224:1 – 7224:65535 are reserved by AWS Direct Connect.

For outbound routing policies, AWS Direct Connect applies the following BGP communities to its advertised routes:

• 7224:8100—Routes that originate from the same AWS Region in which the AWS Direct Connect point of presence is associated.

• 7224:8200—Routes that originate from the same continent with which the AWS Direct Connect point of presence is associated.

• No tag—Routes that originate from other continents.

Note

To receive all AWS public prefixes do not apply any filter.

Communities that are not supported for an AWS Direct Connect public connection are removed.

NO_EXPORT BGP community

For outbound routing policies, the NO_EXPORT BGP community tag is supported for public virtual interfaces.

AWS Direct Connect also provides BGP community tags on advertised Amazon routes. If you use AWS Direct Connect to access public AWS services, you can create filters based on these community tags.

For public virtual interfaces, all routes that AWS Direct Connect advertises to customers are tagged with the NO_EXPORT community tag.

Private virtual interface and transit virtual interface routing policies

If you're using AWS Direct Connect to access your private AWS resources, you must specify the IPv4 or IPv6 prefixes to advertise over BGP. These prefixes can be public or private.

The following outbound routing rules apply based on the prefixes advertised:

• AWS evaluates the longest prefix length first. AWS recommends advertising more specific routes using multiple Direct Connect virtual interfaces if the desired routing paths are meant for active/passive connections. See Influencing Traffic over Hybrid Networks using Longest Prefix Match for more information.

• Local preference is the BGP attribute recommended to use when desired routing paths are meant for active/passive connections and the prefix lengths advertised are the same. This value is set per Region to prefer AWS Direct Connect Locations that have the same associated AWS Region using the 7224:7200—Medium local preference community value. Where the local Region is not associated with the Direct Connect location, it is set to a lower value. This applies only if no local preference community tags are assigned.

• AS_PATH length can be used to determine the routing path when the prefix length and local preference are the same.

• Multi-Exit Discriminator (MED) can be used to determine the routing path when prefix length, local preference, and AS_PATH are the same. AWS does not recommend using MED values given their lower priority in evaluation.

• AWS will load-share across multiple transit or private virtual interfaces when prefixes have the same length and BGP attributes.

Private virtual interface and transit virtual interface BGP communities

When an AWS Region routes traffic to on-premises locations via Direct Connect private or transit virtual interfaces, the associated AWS Region of the Direct Connect location influences the ability to use equal-cost multi-path routing (ECMP). AWS Regions prefer Direct Connect locations in the same associated AWS Region by default. See AWS Direct Connect Locations to identify the associated AWS Region of any Direct Connect location.

When there are no local preference community tags applied, Direct Connect supports ECMP over private or transit virtual interfaces for prefixes with the same length, AS_PATH length, and MED value over two or more paths in the following scenarios:

• The AWS Region sending traffic has two or more virtual interface paths from locations in the same associated AWS Region, whether in the same or different colocation facilities.

• The AWS Region sending traffic has two or more virtual interface paths from locations not in the same Region.

Fore more information, see How do I set up an Active/Active or Active/Passive Direct Connect connection to AWS from a private or transit virtual interface?

Note

This has no effect on ECMP to an AWS Region from on-premises locations.

To control route preferences, Direct Connect supports local preference BGP community tags for private virtual interfaces and transit virtual interfaces.

Local preference BGP communities

You can use local preference BGP community tags to achieve load balancing and route preference for incoming traffic to your network. For each prefix that you advertise over a BGP session, you can apply a community tag to indicate the priority of the associated path for returning traffic.

The following local preference BGP community tags are supported:

• 7224:7100—Low preference

• 7224:7200—Medium preference

• 7224:7300—High preference

Local preference BGP community tags are mutually exclusive. To load balance traffic across multiple AWS Direct Connect connections (active/active) homed to the same or different AWS Regions, apply the same community tag; for example, 7224:7200 (medium preference) across the prefixes for the connections. If one of the connections fails, traffic will be then load balance using ECMP across the remaining active connections regardless of their home Region associations . To support failover across multiple AWS Direct Connect connections (active/passive), apply a community tag with a higher preference to the prefixes for the primary or active virtual interface and a lower preference to the prefixes for the backup or passive virtual interface. For example, set the BGP community tags for your primary or active virtual interfaces to 7224:7300 (high preference) and 7224:7100 (low preference) for your passive virtual interfaces.

Local preference BGP community tags are evaluated before any AS_PATH attribute, and are evaluated in order from lowest to highest preference (where highest preference is preferred).