Frame Relay

Frame Relay can be carried across the MPLS network in two fashions: DLCI-to-DLCI or port-toport. In the DLCI-to-DLCI method, one VC is carried over one pseudowire. In the port-to-port method, all VCs on one port are carried over one pseudowire. You can use both the Frame Relay encapsulation types of Cisco or the IETF (RFC 1490) for the DLCI-to-DLCI and the port-to-port methods.

DLCI-to-DLCI

In the DLCI-to-DLCI method, each VC is carried across the MPLS network over one pseudowire. Frame Relay VCs to pseudowires have a one-to-one mapping. The DLCI-to-DLCI method gives you some flexibility because each VC can be tunneled to different PE routers if needed, which is what you usually need when connecting customer sites with Frame Relay. In the DLCI-to-DLCI method, the transported frame is the Frame Relay payload. The flags and FCS field are stripped off. The Frame Relay header is also stripped off, but the control bits FECN, BECN, DE, and C/R are copied into the control word as the F, B, D, and C bits, respectively. The control word for Frame Relay over MPLS is depicted in Figure 10-10.

Figure 10-10 Control Word Format for Frame Relay over MPLS

01234567890123456789012345678901

01234567890123456789012345678901

Res

F

B

D

C

B

E

Length

Sequence Number

An additional header is added between the control word and the Frame Relay payload indicating the Ethertype. Ethertype is a field that indicates the Layer 2 or Layer 3 protocol of the payload. For example, the Ethertype value of 0x0800 indicates that the payload is IP. The remote PE router receives a labeled packet with the VC label on top. The VC label is looked up and used to indicate the VCID. When the VC label is stripped off, the information inside the control word is used to construct the Frame Relay header before sending the frame toward the CE router. Figure 10-11 shows the DLCI-to-DLCI Frame Relay over MPLS (FRoMPLS) frame format for Cisco encapsulation.

Figure 10-11 DLCI-to-DLCI FRoMPLS Frame Format for Cisco Encapsulation

Tunnel Label

VC Label

Control Word

Ethertype

Payload

NOTE When you use IETF encapsulation instead of Cisco, Frame Relay uses the NLPID field instead of the Ethertype field.

In this mode, you cannot transport the Local Management Interface (LMI) messages across the MPLS network; they are terminated at the PE router. However, when the PE router terminates the LMI messages, the LMI status can dictate the status of the pseudowire. When LMI indicates the presence of a VC, for example, a VC label advertisement is sent to the remote PE router. When the VC is not present, a VC label withdraw is sent.

Port-to-Port Mode (Port Trunking)

Cisco IOS also supports port-to-port FRoMPLS. This means that instead of carrying an individual Frame Relay VC over one pseudowire, you can carry a whole trunk of VCs over one pseudowire, which is what you would do to replace the core of an existing Frame Relay network with MPLS. A many-to-one mapping of VCs to pseudowire exists. On the PE side, the encapsulation that you use to transport Frame Relay in the port-to-port mode is HDLC. Figure 10-12 shows the port-toport FRoMPLS frame format for Cisco encapsulation and a detailed look at the Frame Relay header.

Figure 10-12 Port-to-Port FRoMPLS Frame Format for Cisco Encapsulation

Figure 10-12 Port-to-Port FRoMPLS Frame Format for Cisco Encapsulation

Frame Relay Header

Upper DLCI C/R EA 6 bits 1 bit 1 bit

Lower DLCI FECN BECN DE 4 bits 1 bit 1 bit 1 bit

EA 1 bit

NOTE When you use IETF encapsulation instead of Cisco, Frame Relay uses the NLPID field instead of the Ethertype field.

In port-to-port mode, the pseudowire carries the Frame Relay header—after stripping off the flags and FCS field. In this mode, the LMI messages can be transported transparently across the MPLS network; they appear as regular HDLC frames to the PE routers. The AToM control word cannot hold the Frame Relay control bits (FECN, BECN, DEC/R); they are 0. However, the original Frame Relay control bits are transported transparently as part of the AToM HDLC payload to the remote CE router.

This is a method suited, for example, to reach a remote Frame Relay access site across an MPLS network. Example 10-8 shows the configuration of the CE and PE routers for FRoMPLS in port-to-port mode. For demonstration purposes, two DLCIs were configured on the CE routers toward the PE routers. The PE routers PE1 and PE2 were configured with HDLC as encapsulation on the interfaces toward the CE routers and thus also for HDLC over MPLS.

Example 10-8 Frame Relay over MPLS Port-to-Port Example hostname PE1-ce

interface Serial0/1 no ip address encapsulation frame-relay frame-relay intf-type nni

interface Serial0/1.1 point-to-point ip address 10.100.101.1 255.255.255.0 frame-relay interface-dlci 17

interface Serial0/1.2 point-to-point ip address 10.100.102.1 255.255.255.0 frame-relay interface-dlci 1007

hostname PE1

pseudowire-class one encapsulation mpls

interface Serial0/1/0 no ip address encapsulation hdlc

Example 10-8 Frame Relay over MPLS Port-to-Port Example (Continued)

xconnect 10.200.254.4 200 pw-class one

hostname PE2

pseudowire-class one encapsulation mpls interface Serial4/0/0 no ip address encapsulation hdlc xconnect 10.200.254.1 200 pw-class one

hostname PE2-ce

interface Serial0/0 no ip address encapsulation frame-relay frame-relay intf-type nni

interface Serial0/0.1 point-to-point ip address 10.100.101.2 255.255.255.0 frame-relay interface-dlci 17

interface Serial0/0.2 point-to-point ip address 10.100.102.2 255.255.255.0

frame-relay interface-dlci 1007

PE1#show mpls l2transport vc 200

Local intf Local circuit Dest address VC ID Status

Se0/1/0 HDLC 10.200.254.4 200 UP

PE1#show mpls l2transport vc 200 detail

Local interface: Se0/1/0 up, line protocol up, HDLC up

Destination address: 10.200.254.4, VC ID: 200, VC status: up Preferred path: not configured Default path: active

Tunnel label: 23, next hop 10.200.200.2 Output interface: Et0/0/0, imposed label stack {23 19} Create time: 2d16h, last status change time: 2d15h

Example 10-8 Frame Relay over MPLS Port-to-Port Example (Continued)

Signaling protocol: LDP, peer 10.200.254.4:0 up MPLS VC labels: local 16, remote 19 Group ID: local 0, remote 0 MTU: local 1500, remote 1500 Remote interface description: Sequencing: receive disabled, send disabled Sequence number: receive 0, send 0 VC statistics:

packet totals: receive 36498, send 35542 byte totals: receive 762468, send 842039 packet drops: receive 10, seq error 0, send 0

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