Models for diversity in Direct Link
This document presents common redundant Direct Link configurations which can help you find the model that most closely matches your needs to create the most successful IBM Cloud® Direct Link deployment.
Since customers have the responsibility for creating diversity through their border gateway protocol (BGP) schemas to generate a fully redundant configuration the customer must provision the amount of direct links as per the desired SLA and diversity they require; It is importnat that the provisioned Direct links must be configured using separate, cross-connect routers (XCR) to avoid single points of failure.
The configurations are arranged in increasing levels of complexity and also according to the Direct Link offering.
Key:
- BCR - Back-end customer router
- XCR - Cross-connect router
- NE - Network Edge
Influencing route preference using AS prepends
If the same route prefixes are being advertised to and from IBM Cloud through the BGP session for Direct Link, BGP determines the best path. If the local preference of both paths are equal, such as in the case for Direct Link, the BGP path follows the shortest AS path. Using AS prepends, you can control which path is longest, reducing the priority. The local Autonomous System Number (ASN) on the BGP session is prepended three or more times to the matched route's AS-PATH, artificially increasing that path's length.
The greater the repetition value, the greater the preference is affected. Using three or more AS prepends to your prefixes ensures a significant difference.
You can configure AS prepends when ordering Direct Link Connect, ordering Direct Link Dedicated, or after your direct link is provisioned. A prepend rule is set up with a policy to determine whether the rule will target exported routes (virtual connection routes advertised to on-premises network) or imported routes (on-premises routes being advertised into the IBM Cloud infrastructure), and a repetition value, which determines how many times the local BGP session ASN is applied to routes. The longer the repetition, the less priority the routes have. Optionally, rules have one or more prefixes used to match routes. If provided, only routes matching the prefixes are prepended. Without a prefix, all routes are affected by the prepend rule.
Currently, IBM Cloud Direct Link only supports influencing route preference with AS prepends. Direct Link does not support influencing the route preference with the following BGP route attributes:
- Weight
- Local preference
- Multiple Exit Discriminator (MED)
For more information about prepending route prefixes, see Using AS prepends to manage route priorities and Using AS prepends with VPN connections.
Similar rules apply on the route prefixes that IBM Cloud advertises. The IBM Cloud routers advertise all prefixes that are associated with all applicable Direct Link connections equally (through BGP) with no additional BGP attributes to indicate path preference. You can implement import policies of your choice to prevent asymmetric routing scenarios that align with any existing export policies.
Sample network topologies to achieve diversity
The configurations that are shown in this group assume that all of the assets are located in the same PoP and in the same global market.
Diversity that includes AZs and global routing options
The configurations that are shown in this group offer options for connecting across local availability zones and markets.
Diversity in a local availability zone (AZ)
Diversity in different local markets, with global routing
Achieving redundancy
If you are looking for High Availability (HA), or full redundancy, set up two links into different XCRs in the same data center (for example DAL03). Then, fail over as needed using BGP configurations.
