Policy Framework Configuration Guide - Juniper Networks

Junos 10.4 Policy Framework Configuration Guide
• You can specify what information the routing platform uses for per-flow load balancing
based on port data (instead of on source and destination IP addresses only). For
aggregated Ethernet and aggregated SONET/SDH interfaces, you can load-balance
based on the MPLS label information. For more information, see “Configuring Per-Packet
Load Balancing” on page 143.
• You can also configure per-prefix load balancing, which allows you to configure a hash
value that enables the router to elect a next hop independently of the route chosen by
other routers. For more information, see “Configuring Per-Prefix Load Balancing” on
page 368.
In addition, on the M120, M320, and MX Series routers only, you have the following option:
• You can also configure per-flow load balancing, which allows you to configure the
router to assign a unique, load-balance hash value for each Packet Forwarding Engine
slot. For more information, see “Configuring Per-Flow Load Balancing Based on Hash
Values” on page 369.
Configuring Per-Prefix Load Balancing
By default, the Junos OS uses a hashing method based only on the destination address
to elect a forwarding next hop when multiple equal-cost paths are available. As a result,
when multiple routers or switches share the same set of forwarding next hops for a given
destination, they can elect the same forwarding next hop.
You can enable router-specific or switch-specific load balancing by including a per-prefix
hash value. However, this method applies only to indirect next hops. In other words, when
we have a route with a protocol next hop that is not directly connected, it can be resolved
over a set of equal-cost forwarding next hops. Only in this case, we use the hashing
algorithm to elect a forwarding next hop. An example of this is routes learned from an
IBGP neighbor. The protocol next hop for those routes might not be directly reachable
and would be resolved through some IGP or static routes. The result could be a set of
equal-cost forwarding next hops to reach that protocol next hop. Per-prefix load balancing
thus leads to better utilization of the available links.
To configure per-prefix load balancing, include the load-balance statement at the [edit
forwarding-options] hierarchy level:
[edit forwarding-options]
load-balance {
indexed-next-hop;
per-prefix {
hash-seed number;
}
}
To enable per-prefix load balancing, you must include the hash-seed number statement.
The range that you can configure is 0 (the default) through 65,535. If no hash seed is
configured, the elected forwarding next hop is the same as in previous releases.
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