Control Word Functions

The control word has the following five functions:

■ Pad small packets

■ Carry control bits of the Layer 2 header of the transported protocol

■ Preserve the sequence of the transported frames

■ Facilitate the correct load balancing of AToM packets in the MPLS backbone network

■ Facilitate fragmentation and reassembly

Pad Small Packets

Sometimes the transmitted labeled AToM packet is smaller than the required minimum length for the specific encapsulation type. An example is Ethernet, where the required minimum length of the frame is 64 bytes. If the AToM packet does not meet this minimum length, the frame is padded to meet the minimum length on the Ethernet link. Because the MPLS header (the label stack) has no length field that indicates the length of the frame, the control word holds a length field indicating the length of the frame. If the received AToM packet on the egress PE router has a control word with a length field that is not 0, the router knows that padding was added and can correctly remove the padding before forwarding the frame.

Carry Control Bits of the Layer 2 Header of the Transported Protocol

The second reason for a control word is the carrying of the control bits that are set to certain Layer 2 protocol flags. The flags that are copied to the control word depend on the carried protocol. For Frame Relay, the forward explicit congestion notification (FECN), backward explicit congestion notification (BECN), discard eligible (DE), and command/response (C/R) bits are carried in the control word as control bits. For ATM, the Explicit Forward Congestion Indication (EFCI) and cell loss priority (CLP) bit can be copied to the control word. For the PPP and HDLC protocols, no control bits are copied, but the fragmentation bits can be set.

Preserve the Sequence of the Transported Frames

The third reason for the presence of the control word is to carry the sequence number. With this sequence number, the receiver can detect out-of-sequence packets. The first packet sent onto the pseudowire has a sequence number of 1 and increments for each subsequent packet by 1 until it reaches 65,535, at which point it wraps around back to 1.

If such out-of-sequence packets are detected, they are dropped. Reordering for out-of-sequence AToM packets is not done in Cisco IOS on the PE routers.

In Cisco IOS, sequencing is disabled by default. You can enable it under the pseudowire-class configuration. You can use a pseudowire-class in Cisco IOS to specify AToM-specific configuration. You can enable sequencing in the transmit or receive direction or both, as shown in Example 10-2.

Example 10-2 Enabling Sequencing

pseudowire-class one encapsulation mpls sequencing both

PE1#show mpls l2transport vc 100 detail

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

Destination address:, VC ID: 100, VC status: up Output interface: Et0/0/0, imposed label stack {19 23} Preferred path: not configured Default path: active

Tunnel label: 23, next hop Create time: 00:00:01, last status change time: 00:00:00 Signaling protocol: LDP, peer up MPLS VC labels: local 22, remote 19 Group ID: local 0, remote 0 MTU: local 1500, remote 1500 Remote interface description: Sequencing: receive enabled, send enabled VC statistics:

packet totals: receive 153, send 12352 byte totals: receive 14984, send 1259492 packet drops: receive 0, send 0

Facilitate the Correct Load Balancing of AToM Packets in the MPLS Backbone Network

The fourth reason for the control word is the correct load balancing of AToM packets in the MPLS network. Routers can perform MPLS payload inspection to determine what the payload is. Based on the type of payload, the router can decide how to load-balance the traffic. Some implementations—such as Cisco IOS—look at the first nibble behind the label stack. If that nibble is a 4, the router assumes that this is an IPv4 packet, because the first nibble of an IPv4 packet is always 4. However, sometimes the first nibble is 4 but the MPLS payload is not an IPv4 packet. For example, an Ethernet frame might start with 4. The insertion of the control word between the label stack and the MPLS payload provides certainty that the first nibble after the MPLS label stack is not 4. The generic control word—as depicted in Figure 10-7—starts with a nibble with value 0, and the control word used for Operation and Maintenance (OAM) data starts with a nibble with value 1. Refer to Chapter 14, "MPLS Operation and Maintenance," to see a second definition of the control word, where the first nibble is always 1.

Facilitate Fragmentation and Reassembly

The fifth reason for the control word is the correct functioning of the fragmentation and reassembly of the frames. The PE routers can indicate whether they support fragmentation by signaling it when they advertise the VC label, or you can configure them to do fragmentation and reassembly. If the PE routers do support fragmentation, they can correctly reassemble received fragments if the control word is present. In the control word, the fragmentation is performed by using the Beginning (B) and Ending (E) bits. Fragmentation and reassembly uses the sequence number in the control word to indicate a group of fragments. You can see in Figure 10-7 where the bits B and E and the sequence number are put in the control word.

If the combined label stack, AToM payload, and pseudowire header (like the control word) result in an AToM packet whose MTU is bigger than the MTU in the MPLS network, the packet is dropped somewhere in the network. P routers do not support fragmenting AToM packets.

On the other hand, the AToM packet might be forwarded if it is fragmented and reassembled. The fragmentation of the AToM packet occurs on the ingress PE router, whereas the reassembly occurs on the egress PE router of the MPLS network.

Table 10-3 shows the values of the B and E bits and their meanings. Table 10-3 B and E Bit Values

B and E Bit



Indicates that the entire (unfragmented) payload is carried in a single packet


Indicates the packet carrying the first fragment


Indicates the packet carrying the last fragment


Indicates a packet carrying an intermediate fragment

Fragmentation can affect the performance, so avoid it whenever possible. You can avoid fragmentation by carefully selecting the MTU and MPLS MTU values on the links in the MPLS backbone, as described in the next section.

MPLS MTU in the MPLS Backbone

In AToM networks, if the two PE routers are not directly connected, at least two labels are added to each frame that is transported across the pseudowire. This increases the packet size with at least 8 bytes. Another 4 bytes are added if the control word is used. The payload that is transported with AToM is not just an IP packet as with MPLS VPN, but a Layer 2 frame. Therefore, when calculating the biggest possible AToM packet, you must add the number of bytes for the Layer 2 frame header. Take Ethernet over MPLS as an example. If the transported frame is an Ethernet II frame carrying an IP packet of 1500 bytes, the AToM payload is 1514 bytes. The added 14 bytes are the 6 bytes for Source and Destination MAC Address each and the 2 bytes for the Ethertype. If, however, the transported frame is 802.1Q, you need to add another 4 bytes for the VLAN tag. If the transported frame is a QinQ frame, you need to add another 8 bytes.

To ensure that fragmentation is not needed, check that the MTU of the core links is bigger than the maximum-sized transported AToM packets. If this is not the case, you can increase the MTU for MPLS packets with the interface command mpls mtu. MTU problems surface regularly where Ethernet links exist in the MPLS core. The default MPLS MTU value is the default MTU value for that link. On an Ethernet, this is 1500 bytes. Therefore, assuming that the transported Layer 2 payload is 1500 bytes and the default MTU or MPLS MTU value remains unchanged, you will run into trouble. If you take an IP packet of 1500 bytes that is transported across an Ethernet over MPLS (EoMPLS) pseudowire with one VLAN header, the MPLS packet is 1530 bytes long if the control word is used.

The 1530 bytes in this example break up as follows:

■ 8 bytes for the two MPLS labels

■ 4 bytes for the control word

■ 4 bytes for the Ethernet VLAN header

■ 14 bytes for the transported Ethernet II frame header (no FCS)

Thus, the MPLS MTU on all MPLS interfaces in the core network should be at least 1530 bytes to avoid fragmentation. The alternative is to use path MTU discovery throughout the network.

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  • LEON
    What is the correct sequence of the following operations of MPLS?
    1 year ago

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