Technical Documentation

Configuring PIC-Level Symmetrical Hashing for Load Balancing on 802.3ad LAGs for MX Series Routers

Symmetrical hashing for load balancing on an 802.3ad Link Aggregation Group (LAG) is useful when two MX Series routers (for example, Router A and Router B) are connected transparently through Deep Packet Inspection (DPI) devices over a LAG bundle. The DPI devices keep track of traffic flows in both the forward and reverse directions.

If symmetrical hashing is configured, the reverse flow of traffic is also directed through the same child link on the LAG and is bound to flow through the same DPI device. This enables proper accounting on the DPI of the traffic in both the forward and reverse flows.

If symmetrical hashing is not configured, a different child link on the LAG might be chosen for the reverse flow of traffic through a different DPI device. This results in incomplete information about the forward and reverse flows of traffic on the DPI device leading to incomplete accounting of the traffic by the DPI device.

Symmetrical hashing is computed based on fields like source address and destination address. You can configure symmetrical hashing both at the chassis level and the PIC level for load balancing based on Layer 2, Layer 3, and Layer 4 data unit fields for family inet (IPv4 protocol family) and multiservice (switch or bridge) traffic. Symmetrical hashing configured at the chassis level is applicable to the entire router, and is inherited by all its PICs and Packet Forwarding Engines. Configuring PIC-level symmetrical hashing provides you more granularity at the Packet Forwarding Engine level.

For the two routers connected through the DPI devices over a LAG bundle, you can configure symmetric-hash on one router and symmetric-hash complement on the remote-end router or vice-versa.

To configure symmetrical hashing at the chassis level, include the symmetric-hash or the symmetric-hash complement statements at the [edit forwarding-options hash-key family] hierarchy level. For information about configuring symmetrical hashing at the chassis level and configuring the link index, see the JUNOS Network Interfaces Configuration Guide and the JUNOS VPNs Configuration Guide.

Note: On MX Series DPCs, configuring symmetrical hashing at the PIC level refers to configuring symmetrical hashing at the Packet Forwarding Engine level.

To configure symmetrical hashing at the PIC level on the inbound traffic interface (where traffic enters the router), include the symmetric-hash or symmetric-hash complement statement at the [edit chassis fpc slot-number pic pic-number hash-key] hierarchy level:

[edit chassis fpc slot-number pic pic-number hash-key]family multiservice {source-mac;destination-mac;payload {ip {layer-3 (source-ip-only | destination-ip-only);layer-4;}}symmetric-hash {complement;}}
family inet {layer-3;layer-4;symmetric-hash {complement;}}
  • PIC-level symmetrical hashing overrides the chassis-level symmetrical hashing configured at the [edit chassis forwarding-options hash-key] hierarchy level.
  • Symmetrical hashing for load balancing on 802.3ad Link Aggregation Groups is currently supported for the VPLS, INET and bridged traffic only.
  • Any change in the hash-key configuration requires rebooting the FPC for the changes to take effect.
  • Hash key configuration on a PIC or Packet Forwarding Engine can be either in the “symmetric hash” or the “symmetric hash complement” mode, but not both at the same time.

Published: 2010-04-26