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Chapter 21
Configuring L2VPN Interworking
The interworking function is performed in the same way as for the ATM to Ethernet Port case,
implemented on the PE connected to the ATM attachment VC. The implementation is based on
Multiprotocol Encapsulation over ATM Adaptation Layer 5 (see
For the PE connected to the Ethernet side, one major difference exists due the existence of the VLAN
header in the incoming packet. The PE discards the VLAN header of the incoming frames from the
VLAN CE, and the PE inserts a VLAN header into the Ethernet frames traveling from the MPLS cloud.
The frames sent on the pseudowire (with VC type 5) are Ethernet frames without the VLAN header.
Encapsulation over ATM Adaptation Layer 5, as shown in
Figure 21-6
ATM
CE
ATM to Ethernet—Routed Interworking
To perform routed interworking, both the ATM PE and Ethernet PE routers must be configured.
Figure 21-7
pseudowire is performed on the ATM packets arriving from the ATM CE router.
The address resolution is done at the ATM PE router; it is required when the ATM CE router does an
inverse ARP. It is not required when the ATM CE router is configured using P2P subinterfaces or static
maps.
When packets arrive from the Ethernet CE router, the Ethernet PE router removes the L2 frame tag, and
then 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, the VLAN ID, or port
ID, of the incoming L2 frame. At the ATM PE router, after label disposition, the IP packets are
encapsulated over the AAL5 using routed encapsulation based on RFC 2684.
The address resolution at the Ethernet PE router can be done when the Ethernet CE router configures the
static ARP, or by the proxy ARP on the Ethernet PE. If the proxy ARP is used, the IP address of the
remote CE can be learned dynamically. Cisco IOS Release 12.2(33)XNE supports static and proxy ARP.
Routing protocols need to be configured to operate in the P2P mode on the Ethernet CE.
OL-2226-23
Protocol Stack for ATM to VLAN AToM Bridged Interworking
ATM
ATM
PE
ATM Header
Tunnel Label
LLC
(AA-AA)
LLC(03) OUI (00)
VC Label
OUI
(80-C2)
PID
(00-07)
Control Word
PAD
(00-00)
MAC Header
MAC Header
DA
SA
Type/Length
Type/Length
Remainder of
Remainder of
MAC Frame
MAC Frame
CPCS - UU CPI
Length
CRC
shows the routed interworking between ATM to Ethernet. The IP encapsulation over the
Figure
MPLS Emulated VC of
Tunnel Label
VC Label
Control Word
MAC Header
DA
SA
Type/Length
Remainder of
MAC Frame
Cisco 10000 Series Router Software Configuration Guide
Ethernet/VLAN to ATM AAL5 Interworking
Figure
21-4).
21-6.
Ethernet
Eth
PE
MAC Header
DA
DA
SA
SA
Type/Length
Remainder of
MAC Frame
LAN FCS
VLAN
21-11
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