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Feature Description - MPLS 01

NE05E and NE08E V300R003C10SPC500

This is NE05E and NE08E V300R003C10SPC500 Feature Description - MPLS

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Entropy Label

Entropy Label

An entropy label is not assigned through protocol negotiation and is not used to forward packets. The only function of the entropy label is to improve load balancing performance. The entropy label is generated based on IP on the ingress. The entropy label value cannot be set to a reserved label value in the range of 0 to 15. It extends the LDP and RSVP protocols and uses a set of mechanisms to improve load balancing.


With the increasing growth of user networks and the extending scope of network services, load-balancing techniques are used to improve bandwidth between nodes. When LSPs are used in load balancing, transit nodes (P) obtain IP content carried in MPLS packets as a hash key. If a transit node cannot obtain MPLS packets, the transit node can only use the top label in the MPLS label stack as a hash key. The top MPLS labels do not elaborate forwarding layer protocols in packets, to the top layer and cannot be used to perform load balancing based on protocols, resulting in load imbalance. Per-flow load balancing can be used, which compromises terminal service experience and results in user complaints. To address the problems, the entropy label capability can be configured to improve load balancing performance.


The entropy label is generated by the ingress completely for load balancing purpose. To help the egress distinguish the entropy label generated by the ingress and application labels, an identifier label of 7 is added before an entropy label in the MPLS label stack.

Figure 4-39 Load balancing performed among transit nodes

The ingress generates an entropy label and encapsulates it into the MPLS label stack. If packets are not encapsulated with MPLS labels on the ingress, the ingress easily obtains IP or Layer 2 protocol data as a hash key. If the ingress identifies the entropy label capability enabled for tunnels, the ingress uses IP information carried in packets to compute an entropy label, adds in to the MPLS label stack, and advertises it to the transit node (P). The P uses the entropy label as a hash key to load-balance traffic and does not need to parse IP data inside MPLS packets.

The entropy label is negotiated using LDP or RSVP for load balancing. The entropy label applies to the L2VPN and L3VPN.

The entropy label is pushed into packets on the ingress and removed by the egress. Therefore, the egress needs to notify the ingress of the support for the entropy label capability.

In Figure 4-39, the implementation is as follows:
  • Egress: If the egress can parse an entropy label, the egress extends a Label Mapping message by adding an entropy label capability TLV into the message. The egress sends the message to notify upstream nodes including the ingress of the local entropy label capability.
  • Transit node: sends a Label Mapping message to upstream nodes to transparently transmit the downstream node's entropy label capability. If load balancing is enabled, the Label Mapping messages sent by the transit node carry the entropy label capability TLV only if all downstream nodes have the capability. If a transit node does not identify the entropy label capability TLV, the transit node transparently transmits the TLV by undergoing the unknown TLV process.
  • Ingress: determines whether to add an entropy label into packets to improve load balancing based on the entropy label capability advertised by the egress.

Usage Scenarios

  • In Figure 4-39, entropy labels apply when load balancing is performed among transit nodes.
  • In Figure 4-40, the whole LSP obtains the entropy label capability only if both the primary and backup LSPs have the entropy label capability. An LDP session is established between each pair of directly connected devices, including P1 through P4. On P1, for the LSP to P3, the primary path is P1–>P3 and the backup path is P1–>P2–>P4–>P3. On P2, for the LSP to P3, the primary path is P2–>P4–>P3 and the backup path is P2–>P1–>P3. In this example, the entropy label capability is enabled on P3, and P3 sends Label Mapping messages carrying the entropy label capability to P1 and P4. Upon receipt of the message, P1 does not send a Label Mapping message carrying the entropy label capability to P2 because the path P1–>P2 does not have the entropy label capability. When path information is queried on P1, the LSP to P3 does not have the entropy label capability. After P2 receives a Label Mapping message carrying the entropy label capability sent by P4, P2 finds that the path P2–>P1 does not have the entropy label capability. When path information is queried on P2, the LSP to P3 does not have the entropy label capability, either.
    Figure 4-40 Special scenario


Entropy labels help decrease load imbalance.

Updated: 2019-01-14

Document ID: EDOC1100058933

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