Junos® OS Ethernet Interfaces Configuration ... - Juniper Networks

Configuring Symmetric Load Balancing on an 802.3ad LAG on MX Series Routers
Copyright © 2012, Juniper Networks, Inc.
You can specify whether symmetric hash or complement hash is done for load-balancing
traffic. To configure symmetric hash, use the symmetric-hash statement at the [edit
forwarding-options hash-key family inet] hierarchy level. To configure symmetric hash
complement, use the symmetric-hash complement statement and option at the [edit
forwarding-options hash-key family inet] hierarchy level.
These operations can also be performed at the PIC level by specifying a hash key. To
configure a hash key at the PIC level, use the symmetric-hash or symmetric-hash
complement statement at the [edit chassis hash-key family inet] and [edit chassis hash-key
family multiservice] hierarchy levels.
Consider the example in Figure 14 on page 125.
Figure 14: Symmetric Load Balancing on an 802.3ad LAG on MX Series
Routers
A
MX-series router
i1
i2
i3
Intelligent Devices
A
A
B
MX-series router
B
B
Forward traffic
Reverse traffic
Router A is configured with symmetric hash and Router B is configured with symmetric
hash complement. Thus, for a given flow fx, post hash computation is from Router A to
Router B through i2. The reverse traffic for the same flow fx is from Router B to Router A
through the same i2 device as its hashing (done after swapping source and destination
fields) and returns the same link index; since it is performed on the interchanged source
and destination addresses.
However, the link chosen may or may not correspond to what was attached to the DPI.
In other words, the hashing result should point to the same links that are connected, so
that the traffic flows through the same DPI devices in both directions. To make sure this
happens, you need to also configure the counterpart ports (ports that are connected to
same DPI-iN) with the identical link index. This is done when configuring a child-link into
the LAG bundle. This ensures that the link chosen for a given hash result is always the
same on either router.
Note that any two links connected to each other should have the same link index and
these link indices must be unique in a given bundle.
Chapter 4: Configuring Aggregated Ethernet Interfaces
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