doc: add ldp information over transport connection

transport connection prefers ipv6, unless ipv4 is explicitly configured
on ldp.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

diff --git a/doc/user/ldpd.rst b/doc/user/ldpd.rst
index 8d88ef176bca40200f639aedf33b503717d268ce..85d280343d68661fa20e53efa1d99a8cc9a07539 100644 (file)
--- a/doc/user/ldpd.rst
+++ b/doc/user/ldpd.rst
@@ -151,6 +151,16 @@ LDP Configuration
    HOLDTIME value ranges from 1 to 65535 seconds. Default value is 15 seconds.
    That value is added as a TLV in the LDP messages.
 
+.. index:: [no] dual-stack transport-connection prefer ipv4
+.. clicmd:: [no] dual-stack transport-connection prefer ipv4
+
+   When *ldpd* is configured for dual-stack operation, the transport connection
+   preference is IPv6 by default (as specified by :rfc:`7552`). On such
+   circumstances, *ldpd* will refuse to establish TCP connections over IPv4.
+   You can use above command to change the transport connection preference to
+   IPv4. In this case, it will be possible to distribute label mappings for
+   IPv6 FECs over TCPv4 connections.
+
 .. _show-ldp-information:
 
 Show LDP Information