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 ]
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 } ]
[ offset OFFSET ] [ peer_offset OFFSET ]
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 unmanaged L2TPv3 ethernet tunnels. For unman‐
aged tunnels, there is no L2TP control protocol so no userspace daemon is required - tunnels are manually cre‐
ated 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 net‐
works 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 specified, 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 encapsulation is most common. IP encapsulation
uses a dedicated IP protocol value to carry L2TP data without the overhead of UDP. Use IP encapsulation 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 configuring 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 consistency
checks are possible since there is no control protocol used to establish unmanaged L2TP tunnels. Once the vir‐
tual 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.
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_tunnel_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 tunnel_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 tunnel. 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 encapsulation 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 encapsulation is selected.
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 ses‐
sion 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 session_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
Valid values are: none, udp.
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 value must match the peer_offset value used at the peer. Default is 0.
peer_offset OFFSET
sets the byte offset from the L2TP header where user data starts in received L2TP data packets. This is
hardly ever used. If set, the value must match the offset value used at the peer. Default is 0.
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 specified, 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:# 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 exam‐
ple, 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 traffic over the L2TP pseudowire just like
it forwards over any other interface. The bridge learns MAC addresses of hosts attached to each interface 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 recipient, 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 prob‐
lem. 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 connection
tracking (conntrack) or manually load the Linux netfilter degrag modules at each tunnel endpoint.
site-A:# modprobe nf_degrag_ipv4
site-B:# modprobe nf_degrag_ipv4
If L2TP is being used over IPv6, use the IPv6 degrag module.
INTEROPERABILITY
Unmanaged (static) L2TPv3 tunnels are supported by some network equipment equipment 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 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.
SEE ALSO
ip(8)
AUTHOR
James Chapman <[email protected]>
iproute2 19 Apr 2012 IP-L2TP(8)
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