doc: Clarify what is supported directly in PIM documentation

The FRR community keeps getting asked about what is supported or not.
Try to clarify in an additional spot what is and what is not supported.
Where people interested in using PIM might have a chance at actually
seeing the notification.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

diff --git a/doc/user/pim.rst b/doc/user/pim.rst
index 9267095b3e116cd6401739ec1e5ddb7476813f63..6bda692607e1aad251a3c12f6748e4906b3d2eaf 100644 (file)
--- a/doc/user/pim.rst
+++ b/doc/user/pim.rst
@@ -11,6 +11,13 @@ vrf aware and can work within the context of vrf's in order to
 do S,G mrouting.  Additionally PIM can be used in the EVPN underlay
 network for optimizing forwarding of overlay BUM traffic.
 
+.. note::
+
+   On Linux for PIM-SM operation you *must* have kernel version 4.18 or greater.
+   To use PIM for EVPN BUM forwarding, kernels 5.0 or greater are required.
+   OpenBSD has no multicast support and FreeBSD, NetBSD and Solaris only
+   have support for SSM.
+
 .. _starting-and-stopping-pimd:
 
 Starting and Stopping pimd