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Understanding MPLS Label Operations

 

In the traditional packet-forwarding paradigm, as a packet travels from one switch to the next, an independent forwarding decision is made at each hop. The IP network header is analyzed and the next hop is chosen based on this analysis and on the information in the routing table. In an MPLS environment, the analysis of the packet header is made only once, when a packet enters the MPLS tunnel (that is, the path used for MPLS traffic).

When an IP packet enters a label-switched path (LSP), the ingress provider edge (PE) switch examines the packet and assigns it a label based on its destination, placing the label in the packet’s header. The label transforms the packet from one that is forwarded based on its IP routing information to one that is forwarded based on information associated with the label. The packet is then forwarded to the next provider switch in the LSP. This switch and all subsequent switches in the LSP do not examine any of the IP routing information in the labeled packet. Rather, they use the label to look up information in their label forwarding table. They then replace the old label with a new label and forward the packet to the next switch in the path. When the packet reaches the egress PE switch, the label is removed, and the packet again becomes a native IP packet and is forwarded based on its IP routing information.

This topic describes:

MPLS Label-Switched Paths and MPLS Labels

When a packet enters the MPLS network, it is assigned to an LSP. Each LSP is identified by a label, which is a short (20-bit), fixed-length value at the front of the MPLS label (32 bits). Labels are used as lookup indexes for the label forwarding table. For each label, this table stores forwarding information. Because no additional parsing or lookup is done on the encapsulated packet, MPLS supports the transmission of any other protocols within the packet payload.

Figure 1 shows the encoding of a single label. The encoding appears after data link layer headers, but before any network layer header.

Figure 1: Label Encoding
Label Encoding

Reserved Labels

Labels range from 0 through 1,048,575. Labels 0 through 999,999 are for internal use.

Some of the reserved labels (in the range 0 through 15) have well-defined meanings. The following reserved labels are used by QFX Series and EX4600 devices:

  • 0, IPv4 Explicit Null label—This value is valid only when it is the sole label entry (no label stacking). It indicates that the label must be popped on receipt. Forwarding continues based on the IP version 4 (IPv4) packet.

  • 1, Router Alert label—When a packet is received with a top label value of 1, it is delivered to the local software module for processing.

  • 3, Implicit Null label—This label is used in the signaling protocol (RSVP) only to request label popping by the downstream switch. It never actually appears in the encapsulation. Labels with a value of 3 must not be used in the data packet as real labels. No payload type (IPv4 or IPv6) is implied with this label.

MPLS Label Operations

QFX Series and EX4600 devices support the following MPLS label operations:

  • Push

  • Pop

  • Swap

Note

There is a limit with regard to the number of labels that QFX and EX4600 devices can affix (push operations) to the label stack or remove (pop operations) from the label stack.

  • For Push operations—As many as three labels are supported.

  • For Pop operations—As many as three labels are supported.

The push operation affixes a new label to the top of the IP packet. For IPv4 packets, the new label is the first label. The time to live (TTL) field value in the packet header is derived from the IP packet header. The push operation cannot be applied to a packet that already has an MPLS label.

The pop operation removes a label from the beginning of the packet. Once the label is removed, the TTL is copied from the label into the IP packet header, and the underlying IP packet is forwarded as a native IP packet

The swap operation removes an existing MPLS label from an IP packet and replaces it with a new MPLS label, based on the following:

  • Incoming interface

  • Label

  • Label forwarding table

Figure 2 shows an IP packet without a label arriving on the customer edge interface (ge-0/0/1) of the ingress PE switch. The ingress PE switch examines the packet and identifies that packet’s destination as the egress PE switch. The ingress PE switch applies label 100 to the packet and sends the MPLS packet to its outgoing MPLS core interface (ge-0/0/5). The MPLS packet is transmitted on the MPLS tunnel through the provider switch, where it arrives at interface ge-0/0/5 with label 100. The provider switch swaps label 100 with label 200 and forwards the MPLS packet through its core interface (ge-0/0/7) to the next hop on the tunnel, which is the egress PE switch. The egress PE switch receives the MPLS packet through its core interface (ge-0/0/7), removes the MPLS label, and sends the IP packet out of its customer edge interface (ge-0/0/1) to a destination that is beyond the tunnel.

Figure 2: MPLS Label Swapping
 MPLS Label Swapping

Figure 2 shows the path of a packet as it passes in one direction from the ingress PE switch to the egress PE switch. However, the MPLS configuration also allows traffic to travel in the reverse direction. Thus, each PE switch operates as both an ingress switch and an egress switch.

Penultimate-Hop Popping and Ultimate-Hop Popping

The switches enable penultimate-hop popping (PHP) by default with IP over MPLS configurations. With PHP, the penultimate provider switch is responsible for popping the MPLS label and forwarding the traffic to the egress PE switch. The egress PE switch then performs an IP route lookup and forwards the traffic. This reduces the processing load on the egress PE switch, because it is not responsible for popping the MPLS label.

  • The default advertised label is label 3 (Implicit Null label). If label 3 is advertised, the penultimate-hop switch removes the label and sends the packet to the egress PE switch.

  • If ultimate-hop popping is enabled, label 0 (IPv4 Explicit Null label) is advertised and the egress PE switch of the LSP removes the label.