Internetwoking is a feature on L2VPN that makes possible to connect an ethernet interface to a frame relay interface for example. Not sure if this is in the scope of the CCIE but I fell on it while studying L2TPv3.

We are going to configure an internetworking between R1 and R9, R1 will be on ethernet and R9 on frame relay.

The topology is still based on the CsC topology with an added router R9 running frame relay with R7.

First step is to configure the frame relay part on R7, this router will act as a frame relay switch for our connection.

R7-PE(config)#frame-relay switching

interface Serial3/0
 no ip address
 encapsulation frame-relay
 clock rate 128000
 frame-relay intf-type dce
end

Ok we’re now a frame relay switch. Lets configure our pseudowire.

pseudowire-class FR-PW
 encapsulation l2tpv3
 interworking ip
 ip local interface Loopback0

The magical option here is the interworking ip this command will enable the translation from FR to Ethernet, well in fact the IP compatibility.

Now on our FR router we have to configure a DLCI mapping to the xconnect

connect FR-L2VPN Serial3/0 901 l2transport
 xconnect 23.99.2.2 19 pw-class FR-PW

The connect option is there to map a source destination pair. Here our incoming interface Serial 3/0, DLCI 901 going to L2VPN.

R7-PE(config)#connect ?
  WORD  Name for this connection

R7-PE(config)#connect FR ?
  Dialer           Dialer interface
  FastEthernet     FastEthernet IEEE 802.3
  GigabitEthernet  GigabitEthernet IEEE 802.3z
  MFR              Multilink Frame Relay bundle interface
  Port-channel     Ethernet Channel of interfaces
  Serial           Serial
  Vlan             Catalyst Vlans
  vc-group         Frame Relay VC group (FRF.5 only)

R7-PE(config)#connect FR serial 3/0 ?
  <16-1007>  Frame Relay DLCI

R7-PE(config)#connect FR serial 3/0 901 ?
  Dialer           Dialer interface
  FastEthernet     FastEthernet IEEE 802.3
  GigabitEthernet  GigabitEthernet IEEE 802.3z
  MFR              Multilink Frame Relay bundle interface
  Port-channel     Ethernet Channel of interfaces
  Serial           Serial
  Vlan             Catalyst Vlans
  l2transport      Layer 2 packet over MPLS config commands

On the other side the config is the same as for Ethernet.

R2-PE(config-pw-class)#do sh run | s pseudowire
pseudowire-class FR-PW
 encapsulation l2tpv3
 interworking ip
 ip local interface Loopback0
R2-PE(config-pw-class)#do sh run int g1/0
Building configuration...

Current configuration : 107 bytes
!
interface GigabitEthernet1/0
 no ip address
 negotiation auto
 xconnect 67.99.7.7 19 pw-class FR-PW
end

And here we are everything is done our tunnel is up.

R2-PE#sh l2tp session all

L2TP Session Information Total tunnels 1 sessions 1

Session id 60463897 is up, tunnel id 1091835524
  Remote session id is 2374063734, remote tunnel id 1647674762
  Locally initiated session
  Unique ID is 4
Session Layer 2 circuit, type is Ethernet, name is GigabitEthernet1/0
  Session vcid is 19
  Interworking type is IP
  Circuit state is UP
    Local circuit state is UP
    Remote circuit state is UP
Call serial number is 398400002
Remote tunnel name is R7-PE
  Internet address is 67.99.7.7
Local tunnel name is R2-PE
  Internet address is 23.99.2.2
IP protocol 115
  Session is L2TP signaled
  Session state is established, time since change 00:39:35
    545 Packets sent, 154 received
    46022 Bytes sent, 12836 received
  Last clearing of counters never
  Counters, ignoring last clear:
    545 Packets sent, 154 received
    46022 Bytes sent, 12836 received
    Receive packets dropped:
      out-of-order:             0
      total:                    0
    Send packets dropped:
      exceeded session MTU:     0
      total:                    0
  DF bit off, ToS reflect disabled, ToS value 0, TTL value 255
  Sending UDP checksums are disabled
  Received UDP checksums are verified
  No session cookie information available
  FS cached header information:
    encap size = 24 bytes
    45000014 00000000 FF7357A8 17630202
    43630707 8D815676
  Sequencing is off
  Conditional debugging is disabled

R7-PE#sh l2tp session all

L2TP Session Information Total tunnels 1 sessions 1

Session id 2374063734 is up, tunnel id 1647674762
  Remote session id is 60463897, remote tunnel id 1091835524
  Remotely initiated session
  Unique ID is 4
Session Layer 2 circuit, type is Frame Relay, name is Serial3/0:901
  Session vcid is 19
  Interworking type is IP
  Circuit state is UP
    Local circuit state is UP
    Remote circuit state is UP
Call serial number is 398400002
Remote tunnel name is R2-PE
  Internet address is 23.99.2.2
Local tunnel name is R7-PE
  Internet address is 67.99.7.7
IP protocol 115
  Session is L2TP signaled
  Session state is established, time since change 00:40:45
    155 Packets sent, 560 received
    12920 Bytes sent, 47282 received
  Last clearing of counters never
  Counters, ignoring last clear:
    155 Packets sent, 560 received
    12920 Bytes sent, 47282 received
    Receive packets dropped:
      out-of-order:             0
      total:                    0
    Send packets dropped:
      exceeded session MTU:     0
      total:                    0
  DF bit off, ToS reflect disabled, ToS value 0, TTL value 255
  Sending UDP checksums are disabled
  Received UDP checksums are verified
  No session cookie information available
  FS cached header information:
    encap size = 24 bytes
    45000014 00000000 FF7357A8 43630707
    17630202 039A9B19
  Sequencing is off
  Conditional debugging is disabled

We can see traffic is flowing and EIGRP is up

R1-CE(config-router)#do sh ip eigrp neig
EIGRP-IPv4 neighbors for process 55
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   78.1.78.9               Gi1/0            157 00:20:34 1016  5000  0  32

R9-CE#ping 12.12.12.12

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.12.12.12, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/64/108 ms