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Document iBGP in README.md

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@ -241,6 +241,27 @@ session with your route collector over IPv6, using multiprotocol BGP to export
routes for both IPv4 and IPv6 in a single session, using `add paths` to also
all routes instead of the best routes available.
### iBGP
There is no true convention regarding iBGP usage since it's strictly internal
to an AS, so this filter library will not attempt to make any assumption about
how you might use iBGP. This means that by default, any route received from
iBGP will not be exported by functions like `export_cone` or
`export_to_downstream`. This avoids potential accidents like internal traffic
engineering hijacks from being exported to the Internet and causing a major
incident.
To export an iBGP route to downstreams, set `export_downstream = 1;` in the
import filter when importing the iBGP route.
To export an iBGP route to upstreams, create a new `export_*` function that
returns `true` for the subset of routes you wish to export, such as
`export_ibgp`. Then, you can write your export clause as follows:
```
export where export_cone([PEER_ASN]) || export_ibgp();
```
## BGP communities
The following large informational communities are implemented by default: