[PATCH pve-docs v3 8/8] sdn: add bgp fabric section

[Hannes Laimer <h.laimer@proxmox.com>]

---
 pvesdn.adoc | 81 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/pvesdn.adoc b/pvesdn.adoc
index 8f955e8..27a9d75 100644
--- a/pvesdn.adoc
+++ b/pvesdn.adoc
@@ -835,6 +835,87 @@ NOTE: The dummy interface will automatically be configured as `passive`. Every
 interface which doesn't have an ip-address configured will be treated as a
 `point-to-point` link.
 
+[[pvesdn_bgp]]
+BGP
+~~~
+
+BGP (Border Gateway Protocol) can be used as an eBGP unnumbered fabric. Each
+node has its own Autonomous System Number (ASN) and peers with its neighbors
+over physical interfaces without requiring IP addresses on the fabric links.
+
+Configuration options:
+
+[[pvesdn_bgp_fabric]]
+On the Fabric
+^^^^^^^^^^^^^
+
+IPv4 Prefix:: Optional IPv4 CIDR network range (e.g., 192.0.2.0/24) used to
+verify that all router-IDs in the fabric are contained within this prefix.
+
+IPv6 Prefix:: Optional IPv6 CIDR network range (e.g., 2001:db8::/64) used to
+verify that all node IPv6 addresses in the fabric are contained within this
+prefix.
+
+BFD:: Enable Bidirectional Forwarding Detection on all peering sessions in this
+fabric. BFD provides fast failure detection for links between nodes.
+
+NOTE: If neither prefix is configured, each node must specify an explicit
+Router ID instead. This is useful when the loopback addresses are already
+managed by another fabric running on the same nodes.
+
+[[pvesdn_bgp_node]]
+On the Node
+^^^^^^^^^^^
+
+Options that are available on every node that is part of a fabric:
+
+Node:: Select the node which will be added to the fabric. Only nodes that
+are currently in the cluster will be shown.
+
+ASN:: BGP Autonomous System Number for this node. It is recommended to use
+private ASN numbers (64512-65534 for 16-bit, 4200000000-4294967294 for 32-bit).
+
+IPv4:: A unique IPv4 address for this node, also used as the BGP router-id.
+Required when the fabric has an IPv4 prefix configured; each node in the same
+fabric must have a different address.
+
+IPv6:: IPv6 address for dual-stack fabrics. If only an IPv6 address is
+configured (without an IPv4 address), the BGP router-id is automatically
+derived from it via a FNV-1a hash.
+
+Router ID:: Explicit BGP router-id for this node. Required when the fabric
+has no prefix configured; otherwise takes precedence over the router-id
+derived from the IPv4/IPv6 address.
+
+Interfaces:: Specify the interfaces used to establish peering connections with
+other BGP nodes. These interfaces run BGP unnumbered (no IP address assignment
+needed). When a node IP is configured, a dummy "loopback" interface is
+automatically created with that IP.
+
+NOTE: Unlike OSPF and OpenFabric, BGP unnumbered interfaces do not need IP
+addresses. Peering is established using IPv6 link-local addresses automatically.
+
+NOTE: In an eBGP unnumbered fabric, directly peering nodes must have different
+ASNs for BGP sessions to establish.
+
+[[pvesdn_bgp_evpn]]
+Using BGP Fabrics with EVPN
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When configuring an EVPN controller, a BGP fabric can be selected as the
+underlay instead of manually specifying peer addresses. The EVPN overlay
+sessions will run as iBGP, using the EVPN controller's ASN for the router
+process. The per-node fabric ASN is automatically applied via `local-as` on
+the underlay neighbor group.
+
+This means the EVPN controller ASN and the per-node fabric ASNs should be
+different. For example, with three nodes using ASNs 65001, 65002, and 65003
+for the underlay, the EVPN controller could use ASN 65000 for the overlay.
+
+NOTE: Using a BGP fabric for an EVPN underlay requires the fabric to have a
+prefix and each node to have an IP configured, since EVPN uses the node IP
+as the VTEP address.
+
 [[pvesdn_config_ipam]]
 IPAM
 ----
-- 
2.47.3