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Chapter 21 Configuring L2VPN Interworking
Ethernet/VLAN to Frame Relay Interworking
Frame Relay to Ethernet—Routed Interworking
To perform routed interworking, both the FR PE and Ethernet PE routers must be configured. When FR
packets arrive from the FR CE router, the FR PE router removes the frame relay header. The FR PE router
forwards the IP packet to the egress PE router using IPoMPLS encapsulation over the pseudowire. At
the Ethernet PE router, after label disposition, the IP packets are encapsulated with an MAC rewrite.
The address resolution is done at the FR PE router and is required when the FR CE router does an inverse
ARP. It is not required when the the FR CE router is configured using P2P subinterfaces or static maps.
For packets arriving from the Ethernet CE router, the Ethernet PE router removes the L2 framing. The
Ethernet PE router forwards the IP packet to the egress PE router, using IPoMPLS encapsulation over
the pseudowire. The Ethernet PE router makes the forwarding decision based on the L2 circuit ID
(VLAN ID or port ID) of the incoming L2 frame. At the FR PE router, after label disposition, the IP
packets are encapsulated over FR, using routed encapsulation based on RFC 2427.
The address resolution is also done at the Ethernet PE router by configuring static ARP on the Ethernet
CE router, or by implementing proxy ARP on the Ethernet PE router. If a proxy ARP is used, the IP
address of the remote CE router can be learned dynamically, or can be statically configured in the PE
router. Cisco IOS Release 12.2(33)XNE supports static and proxy ARP. Routing protocols need to be
configured to operate in a point-to-point mode on the Ethernet CE router.
Packets that arrive either from the FR CE or the Ethernet CE router with unsupported translations are
dropped.
Cisco IOS Release 12.2(33)XNE supports both Cisco and IETF Frame Relay.
Figure 21-16
shows an example of the protocol stacks at the PEs when IPv4 PDUs are exchanged using
routed interworking. Other than IPv4, the Prot_Type field is used while carrying L3 packets.
Figure 21-16
Protocol Stacks for FR to Ethernet Routed Interworking
277386
FR
CE
FR
PE
Eth
PE
Eth
CE
Frame Relay
Ethernet
MPLS Emulated VC of type TBD
Bridge Ethernet 802.3
802.1Q Encap
MPoMPLS Encap
Q.922 Address
I
Pv4 Header
I
Pv4 Header
Control Word
Tunnel Label
VC Label
Type/Length
VLAN Tag
Data
Control(03) NLP
I
D(00)
(00-00)
Prot. Type
(08-00)
MAC Header
DA
SA
LAN FCS
FCS
Data
I
Pv4 Header
I
Pv4 Header
Control Word
Tunnel Label
VC Label
(00-00)
Prot. Type
(08-00)
Data
Data
