IP-L2TP(8) Linux IP-L2TP(8)
NAME
ip-l2tp - L2TPv3 static unmanaged tunnel configuration
SYNOPSIS
ip [ OPTIONS ] l2tp { COMMAND | help }
ip l2tp add tunnel
remote ADDR local ADDR
tunnel_id ID peer_tunnel_id ID
[ encap { ip | udp } ]
[ udp_sport PORT ] [ udp_dport PORT ]
[ udp_csum { on | off } ]
[ udp6_csum_tx { on | off } ]
[ udp6_csum_rx { on | off } ]
ip l2tp add session [ name NAME ]
tunnel_id ID session_id ID peer_session_id ID
[ cookie HEXSTR ] [ peer_cookie HEXSTR ]
[ l2spec_type { none | default } ]
[ seq { none | send | recv | both } ]
ip l2tp del tunnel tunnel_id ID
ip l2tp del session tunnel_id ID session_id ID
ip l2tp show tunnel [ tunnel_id ID ]
ip l2tp show session [ tunnel_id ID.B ] [ session_id ID ]
NAME := STRING
ADDR := { IP_ADDRESS | any }
PORT := { NUMBER }
ID := { NUMBER }
HEXSTR := { 8 or 16 hex digits (4 / 8 bytes) }
DESCRIPTION
The ip l2tp commands are used to establish static, or so-called unman-
aged L2TPv3 ethernet tunnels. For unmanaged tunnels, there is no L2TP
control protocol so no userspace daemon is required - tunnels are manu-
ally created by issuing commands at a local system and at a remote peer.
L2TPv3 is suitable for Layer-2 tunneling. Static tunnels are useful to
establish network links across IP networks when the tunnels are fixed.
L2TPv3 tunnels can carry data of more than one session. Each session is
identified by a session_id and its parent tunnel's tunnel_id. A tunnel
must be created before a session can be created in the tunnel.
When creating an L2TP tunnel, the IP address of the remote peer is spec-
ified, which can be either an IPv4 or IPv6 address. The local IP address
to be used to reach the peer must also be specified. This is the address
on which the local system will listen for and accept received L2TP data
packets from the peer.
L2TPv3 defines two packet encapsulation formats: UDP or IP. UDP encapsu-
lation is most common. IP encapsulation uses a dedicated IP protocol
value to carry L2TP data without the overhead of UDP. Use IP encapsula-
tion only when there are no NAT devices or firewalls in the network
path.
When an L2TPv3 ethernet session is created, a virtual network interface
is created for the session, which must then be configured and brought
up, just like any other network interface. When data is passed through
the interface, it is carried over the L2TP tunnel to the peer. By con-
figuring the system's routing tables or adding the interface to a
bridge, the L2TP interface is like a virtual wire (pseudowire) connected
to the peer.
Establishing an unmanaged L2TPv3 ethernet pseudowire involves manually
creating L2TP contexts on the local system and at the peer. Parameters
used at each site must correspond or no data will be passed. No consis-
tency checks are possible since there is no control protocol used to es-
tablish unmanaged L2TP tunnels. Once the virtual network interface of a
given L2TP session is configured and enabled, data can be transmitted,
even if the peer isn't yet configured. If the peer isn't configured, the
L2TP data packets will be discarded by the peer.
To establish an unmanaged L2TP tunnel, use l2tp add tunnel and l2tp add
session commands described in this document. Then configure and enable
the tunnel's virtual network interface, as required.
Note that unmanaged tunnels carry only ethernet frames. If you need to
carry PPP traffic (L2TPv2) or your peer doesn't support unmanaged L2TPv3
tunnels, you will need an L2TP server which implements the L2TP control
protocol. The L2TP control protocol allows dynamic L2TP tunnels and ses-
sions to be established and provides for detecting and acting upon net-
work failures.
ip l2tp add tunnel - add a new tunnel
tunnel_id ID
set the tunnel id, which is a 32-bit integer value. Uniquely
identifies the tunnel. The value used must match the peer_tun-
nel_id value being used at the peer.
peer_tunnel_id ID
set the peer tunnel id, which is a 32-bit integer value assigned
to the tunnel by the peer. The value used must match the tun-
nel_id value being used at the peer.
remote ADDR
set the IP address of the remote peer. May be specified as an
IPv4 address or an IPv6 address.
local ADDR
set the IP address of the local interface to be used for the tun-
nel. This address must be the address of a local interface. May
be specified as an IPv4 address or an IPv6 address.
encap ENCAP
set the encapsulation type of the tunnel.
Valid values for encapsulation are: udp, ip.
udp_sport PORT
set the UDP source port to be used for the tunnel. Must be
present when udp encapsulation is selected. Ignored when ip en-
capsulation is selected.
udp_dport PORT
set the UDP destination port to be used for the tunnel. Must be
present when udp encapsulation is selected. Ignored when ip en-
capsulation is selected.
udp_csum STATE
(IPv4 only) control if IPv4 UDP checksums should be calculated
and checked for the encapsulating UDP packets, when UDP encapsu-
lating is selected. Default is off.
Valid values are: on, off.
udp6_csum_tx STATE
(IPv6 only) control if IPv6 UDP checksums should be calculated
for encapsulating UDP packets, when UDP encapsulating is se-
lected. Default is on.
Valid values are: on, off.
udp6_csum_rx STATE
(IPv6 only) control if IPv6 UDP checksums should be checked for
the encapsulating UDP packets, when UDP encapsulating is se-
lected. Default is on.
Valid values are: on, off.
ip l2tp del tunnel - destroy a tunnel
tunnel_id ID
set the tunnel id of the tunnel to be deleted. All sessions
within the tunnel must be deleted first.
ip l2tp show tunnel - show information about tunnels
tunnel_id ID
set the tunnel id of the tunnel to be shown. If not specified,
information about all tunnels is printed.
ip l2tp add session - add a new session to a tunnel
name NAME
sets the session network interface name. Default is l2tpethN.
tunnel_id ID
set the tunnel id, which is a 32-bit integer value. Uniquely
identifies the tunnel into which the session will be created. The
tunnel must already exist.
session_id ID
set the session id, which is a 32-bit integer value. Uniquely
identifies the session being created. The value used must match
the peer_session_id value being used at the peer.
peer_session_id ID
set the peer session id, which is a 32-bit integer value assigned
to the session by the peer. The value used must match the ses-
sion_id value being used at the peer.
cookie HEXSTR
sets an optional cookie value to be assigned to the session. This
is a 4 or 8 byte value, specified as 8 or 16 hex digits, e.g.
014d3636deadbeef. The value must match the peer_cookie value set
at the peer. The cookie value is carried in L2TP data packets and
is checked for expected value at the peer. Default is to use no
cookie.
peer_cookie HEXSTR
sets an optional peer cookie value to be assigned to the session.
This is a 4 or 8 byte value, specified as 8 or 16 hex digits,
e.g. 014d3636deadbeef. The value must match the cookie value set
at the peer. It tells the local system what cookie value to ex-
pect to find in received L2TP packets. Default is to use no
cookie.
l2spec_type L2SPECTYPE
set the layer2specific header type of the session.
Valid values are: none, default.
seq SEQ
controls sequence numbering to prevent or detect out of order
packets. send puts a sequence number in the default layer2spe-
cific header of each outgoing packet. recv reorder packets if
they are received out of order. Default is none.
Valid values are: none, send, recv, both.
ip l2tp del session - destroy a session
tunnel_id ID
set the tunnel id in which the session to be deleted is located.
session_id ID
set the session id of the session to be deleted.
ip l2tp show session - show information about sessions
tunnel_id ID
set the tunnel id of the session(s) to be shown. If not speci-
fied, information about sessions in all tunnels is printed.
session_id ID
set the session id of the session to be shown. If not specified,
information about all sessions is printed.
EXAMPLES
Setup L2TP tunnels and sessions
site-A:# ip l2tp add tunnel tunnel_id 3000 peer_tunnel_id 4000 \
encap udp local 1.2.3.4 remote 5.6.7.8 \
udp_sport 5000 udp_dport 6000
site-A:# ip l2tp add session tunnel_id 3000 session_id 1000 \
peer_session_id 2000
site-B:# ip l2tp add tunnel tunnel_id 4000 peer_tunnel_id 3000 \
encap udp local 5.6.7.8 remote 1.2.3.4 \
udp_sport 6000 udp_dport 5000
site-B:# ip l2tp add session tunnel_id 4000 session_id 2000 \
peer_session_id 1000
site-A:# ip link set l2tpeth0 up mtu 1488
site-B:# ip link set l2tpeth0 up mtu 1488
Notice that the IP addresses, UDP ports and tunnel / session ids are
matched and reversed at each site.
Configure as IP interfaces
The two interfaces can be configured with IP addresses if only IP data
is to be carried. This is perhaps the simplest configuration.
site-A:# ip addr add 10.42.1.1 peer 10.42.1.2 dev l2tpeth0
site-B:# ip addr add 10.42.1.2 peer 10.42.1.1 dev l2tpeth0
site-A:# ping 10.42.1.2
Now the link should be usable. Add static routes as needed to have data
sent over the new link.
Configure as bridged interfaces
To carry non-IP data, the L2TP network interface is added to a bridge
instead of being assigned its own IP address, using standard Linux util-
ities. Since raw ethernet frames are then carried inside the tunnel, the
MTU of the L2TP interfaces must be set to allow space for those headers.
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add br0 type bridge
site-A:# ip link set l2tpeth0 master br0
site-A:# ip link set eth0 master br0
site-A:# ip link set br0 up
If you are using VLANs, setup a bridge per VLAN and bridge each VLAN
over a separate L2TP session. For example, to bridge VLAN ID 5 on eth1
over an L2TP pseudowire:
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add brvlan5 type bridge
site-A:# ip link set l2tpeth0.5 master brvlan5
site-A:# ip link set eth1.5 master brvlan5
site-A:# ip link set brvlan5 up
Adding the L2TP interface to a bridge causes the bridge to forward traf-
fic over the L2TP pseudowire just like it forwards over any other inter-
face. The bridge learns MAC addresses of hosts attached to each inter-
face and intelligently forwards frames from one bridge port to another.
IP addresses are not assigned to the l2tpethN interfaces. If the bridge
is correctly configured at both sides of the L2TP pseudowire, it should
be possible to reach hosts in the peer's bridged network.
When raw ethernet frames are bridged across an L2TP tunnel, large frames
may be fragmented and forwarded as individual IP fragments to the recip-
ient, depending on the MTU of the physical interface used by the tunnel.
When the ethernet frames carry protocols which are reassembled by the
recipient, like IP, this isn't a problem. However, such fragmentation
can cause problems for protocols like PPPoE where the recipient expects
to receive ethernet frames exactly as transmitted. In such cases, it is
important that frames leaving the tunnel are reassembled back into a
single frame before being forwarded on. To do so, enable netfilter con-
nection tracking (conntrack) or manually load the Linux netfilter defrag
modules at each tunnel endpoint.
site-A:# modprobe nf_defrag_ipv4
site-B:# modprobe nf_defrag_ipv4
If L2TP is being used over IPv6, use the IPv6 defrag module.
INTEROPERABILITY
Unmanaged (static) L2TPv3 tunnels are supported by some network equip-
ment vendors such as Cisco.
In Linux, L2TP Hello messages are not supported in unmanaged tunnels.
Hello messages are used by L2TP clients and servers to detect link fail-
ures in order to automate tearing down and reestablishing dynamic tun-
nels. 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 con-
sistent with that configured at the peer. Some vendor implementations
(e.g. Cisco) default to use a Layer2SpecificHeader type of None.
SEE ALSO
ip(8)
AUTHOR
James Chapman <jchapman@katalix.com>
iproute2 19 Apr 2012 IP-L2TP(8)
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