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[BGP] Added AS prepending to route prioritization #20939

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merged 4 commits into from
Mar 19, 2025
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[BGP] Added AS prepending to route prioritization #20939

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added as prepending
marciocloudflare Mar 19, 2025
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marciocloudflare Mar 19, 2025
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marciocloudflare Mar 19, 2025
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Update src/content/partials/network-interconnect/bgp-peering.mdx
marciocloudflare Mar 19, 2025
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23 changes: 23 additions & 0 deletions src/content/partials/network-interconnect/bgp-peering.mdx
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Expand Up @@ -57,6 +57,29 @@ When BGP advertises a route, it is automatically added to the Magic routing tabl

Additionally, when multiple BGP routes exist with the same prefix length and priority, traffic is distributed across them using <a href={props.ecmp}>equal-cost multi-path (ECMP) routing</a>.

### BGP communities and AS prepending

Cloudflare supports traffic engineering via BGP communities and AS prepending. You can use these traffic routing techniques to set route priorities and perform traffic engineering across multiple interconnects.

The default BGP route priority is `100`. This base priority can be adjusted using communities. For example, when a route is tagged with the community `13335:60010` its priority is set to `10`. This makes it a higher priority than the default of `100` because lower numeric values are preferred. This operates similarly to local preference communities. However, unlike local preference, these combine numerically with AS prepends. This means that for each AS prepending — where the customer ASN is added multiple times to the AS Path — an additional `10` is added to the route's base priority. By increasing the priority number, the route is less preferred.

In summary:

- Default BGP priority is `100`
- You can use communities to influence routing priorities
- When using AS prepending with communities, Cloudflare combines both values to determine the new route priority. For example, if a route is tagged with `13335:60010` the base priority is set to `10`. If you prepend your ASN twice, Cloudflare adds `10` for each prepend resulting in a final route priority of `30`.

The community values supported for setting base route priority are:
- `13335:60010` — set base Magic route priority to `10`
- `13335:60050` — set base Magic route priority to `50`
- `UNSET` — set base Magic route priority to `100`
- `13335:60150` — set base Magic route priority to `150`
- `13335:60200` — set base Magic route priority to `200`
- `13335:60901` — set base Magic route priority to `501000`
- `13335:60902` — set base Magic route priority to `1001000`

It is considered a misconfiguration to set multiple base priority communities in the same prefix update message. In this situation the highest priority (lowest integer value) is preferred.

## BGP timers and settings

Cloudflare uses the timers as described below. These are not configurable:
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