Understanding Per-Packet Load Balancing

By default, when there are multiple equal-cost paths to the same destination for the active route, Junos OS uses a hash algorithm to choose one of the next-hop addresses to install in the forwarding table. Whenever the set of next hops for a destination changes in any way, the next-hop address is re-chosen using the hash algorithm. Starting in Junos OS Release 18.3R1, for MX series routers, the default behavior for IPv6, GRE, and PPPoE packet hash computation was modified to include the flow-label field for improved load-balancing in certain cases (you can use the no-payload option to revert to the previous method for hash computation). See Understanding the Algorithm Used to Load Balance Traffic on MX Series Routers for details.

You can configure Junos OS so that, for the active route, all next-hop addresses for a destination are installed in the forwarding table. This feature is called per-packet load balancing. The naming may be counter-intuitive. However, Junos per-packet load balancing is functionally equivalent to what other vendors may term per-flow load balancing. You can use load balancing to spread traffic across multiple paths between routers.

Figure 1 shows a simple load balancing scenario. Device R1 is in AS 64500 and is connected to both Device R2 and Device R3, which are in AS 64501. Device R1 can be configured to load balance traffic across the two links.

Figure 1: Simple Load Balancing Scenario

Starting in Junos OS 13.3R3, for MX Series 5G Universal Routing Platforms with modular port concentrators (MPCs) only, you can configure consistent load balancing, which prevents the reordering of all flows to active paths in an equal-cost multipath (ECMP) group when one or more next-hop paths fail. Only flows for paths that are inactive are redirected to another active next-hop path. Flows mapped to servers that remain active are maintained. This feature applies only to external BGP peers.

Starting in Junos OS Release 19.1R1,on QFX10000 switches, you can configure load balancing of IPv4 or IPv6 packets by using GPRS Tunneling Protocol-tunnel endpoint identifier (GTP-TEID) field hash calculations. The GTP-TEID hashing is added to the Layer 2 and Layer 3 field hashing that you have already configured. To enable this feature on QFX10000 switches, configure the gtp-tunnel-endpoint-identifier statement at the [edit forwarding-options enhanced-hash-key family inet] or the [edit forwarding-options enhanced-hash-key family inet6] hierarchy Level. GTP versions 1 and 2 are supported; they support only user data. You must use UDP port number 2152 for both GTP versions.