Configuring Per-Packet Load Balancing
For the active route, when there are multiple equal-cost paths to the same destination, by default, the Junos OS chooses in a random fashion one of the next-hop addresses to install into the forwarding table. Whenever the set of next hops for a destination changes in any way, the next-hop address is chosen again, also in a random fashion.
You can configure the Junos OS so that, for the active route, all next-hop addresses for a destination are installed in the forwarding table. This is called per-packet load balancing. You can use load balancing to spread traffic across multiple paths between routing devices. The behavior of the per-packet load-balancing function varies according to the version of the Internet Processor ASIC in the routing device.
On routing devices with an Internet Processor I ASIC, when per-packet load balancing is configured, traffic between routing devices with multiple paths is spread in a random fashion across the available interfaces. The forwarding table balances the traffic headed to a destination, transmitting packets in round-robin fashion among the multiple next hops (up to a maximum of eight equal-cost load-balanced paths). The traffic is load-balanced on a per-packet basis.
 | Note: Per-packet load distribution uses a hashing algorithm that distributes packets over equal-cost links. The algorithm is designed to distribute packets to prevent any single link from being saturated. However, per-packet load balancing offers no guarantee of equal distribution of traffic over equal-cost links, nor does it guarantee that increasing the number of Internet flows creates a better hash distribution. |
On routing devices with the Internet Processor II ASIC and T Series Internet Processor II ASIC, when per-packet load balancing is configured, traffic between routing devices with multiple paths is divided into individual traffic flows (up to a maximum of 16Â equal-cost load-balanced paths). Packets for each individual flow are kept on a single interface. To recognize individual flows in the transit traffic, the routing device examines each of the following:
- Source IP address
- Destination IP address
- Protocol
- Source port number
- Destination port number
- Source interface index
- Type of service (ToS)
The routing device recognizes packets in which all of these parameters are identical, and it ensures that these packets are sent out through the same interface. This prevents problems that might otherwise occur with packets arriving at their destination out of their original sequence.
| Note: Load balancing is not supported on management and internal Ethernet (fxo) interfaces because this type of interface cannot handle the routing process. On fxp interfaces, you cannot configure multiple next hops and enable load balancing. |
The following steps shows how to configure per-packet load balancing:
- Define a load-balancing routing policy by including one
or more policy-statement statements at the [edit policy-options] hierarchy level, defining an action of load-balance per-packet: policy-statement policy-name {from {match-conditions;route-filter destination-prefix match-type <actions>;prefix-list name;}then {load-balance per-packet;}}
- Apply the policy to routes exported from the routing table
to the forwarding table. To do this, include the forwarding-table and export statements:
Note: You cannot apply the export policy to VRF routing instances.
For a list of hierarchy levels at which you can include these statements, see the statement summary section for these statements.
| Note: Specify all next-hops of that route, if more than one exists, when allocating a label corresponding to a route that is being advertised. |
| Note: Configure the forwarding-options hash key for MPLS to include the IP payload. |
Examples: Configuring Per-Packet Load Balancing
Perform per-packet load balancing for all routes:
Perform per-packet load balancing for only a limited set of routes:
