[BGP] Refactored explanation of communities and AS prepending (#21247)

* revamped bgp routing

* adjusted subheaders

* refined example

Author: marciocloudflare

src/content/partials/networking-services/routing/bgp-peering.mdx

@@ -57,29 +57,47 @@ 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
+## Change route priorities with BGP attributes
 
 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.
+### BGP communities for setting route priority
 
-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 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 priorities are preferred.
 
 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`
+
+- `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 baseMagic 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.
 
+### AS path prepending for adjusting route priority
+
+For each additional mention of the customer ASN in the received AS path an additional `10` is added to the route's base priority. By increasing the priority number, the route is less preferred.
+
+For example, if your ASN is `65000` then the `BGP UPDATE` to Cloudflare will be:
+
+```txt
+# No change to base priority.
+AS_PATH: 65000 65200
+
+# Add 10 to base priority for 1 prepend of 65000
+AS_PATH: 65000 65000 65200
+
+# Add 20 to base priority for 2 prepend of 65000
+AS_PATH: 65000 65000 65000 65200
+```
+
+### How communities and prepends work together
+
+Cloudflare adjusts route priority when using AS prepending with communities. For example, if a route is tagged with `13335:60150`, the base priority is set to `150`. If you prepend your ASN twice, Cloudflare adds `10` for each prepend, increasing the route priority to `180`.
+
 ## BGP timers and settings
 
 Cloudflare uses the timers as described below. These are not configurable: