iproute2: update ip-l2tp man page

Add documentation about the new l2spec_type parameter for "ip l2tp add

Signed-off-by: James Chapman <jchapman@katalix.com>
diff --git a/man/man8/ip-l2tp.8 b/man/man8/ip-l2tp.8
index 2df7ce2..463878f 100644
--- a/man/man8/ip-l2tp.8
+++ b/man/man8/ip-l2tp.8
@@ -52,6 +52,8 @@
 .RB " ]"
+.RB "[ " l2spec_type " { " none " | " default " } ]"
 .RB "[ " offset
 .RB " ] [ " peer_offset
@@ -239,6 +241,12 @@
 the peer. It tells the local system what cookie value to expect to
 find in received L2TP packets. Default is to use no cookie.
+.BI l2spec_type " L2SPECTYPE"
+set the layer2specific header type of the session.
+Valid values are:
+.BR none ", " udp "."
 .BI offset " OFFSET"
 sets the byte offset from the L2TP header where user data starts in
 transmitted L2TP data packets. This is hardly ever used. If set, the
@@ -368,6 +376,12 @@
 link failures in order to automate tearing down and reestablishing
 dynamic tunnels. If a non-Linux peer supports Hello messages in
 unmanaged tunnels, it must be turned off to interoperate with Linux.
+Linux defaults to use the Default Layer2SpecificHeader type as defined
+in the L2TPv3 protocol specification, RFC3931. This setting must be
+consistent with that configured at the peer. Some vendor
+implementations (e.g. Cisco) default to use a Layer2SpecificHeader
+type of None.
 .BR ip (8)