BlackDiamond X8 Switches

A 40 Gb port must use multiple 20 Gb HGd links to carry its traffic. In the degenerate case, a particular link is always the target of the hash result. With a 40 Gb link hashing into a 20 Gb link, half the traffic is lost. ExtremeXOS attempts to configure the packet hash algorithm to accommodate most common situations; however, some customers may have uncommon situations and may wish to adjust the hash. For this reason, the user can specify values for some of the hash parameters in normal packet hash mode:

Source port hash (cannot be used when 40 Gb ports are in use)
Packet field hash calculation algorithm (CRC or XOR)

The packet hash is configurable.

A Dynamic Load Balance (DLB) algorithm allows distribution of flows to the links in the switch fabric trunk that are currently carrying the least load. DLB uses the packet field hash, but does not use the hash code that is reduced to a number modulo the number of links in the group. Instead the number used is 15 bits for a total of 32 K possible hash codes. This 15-bit hash code indexes into a 32 K-entry “flow table”. Each time an unused entry is allocated to one or more “micro-flows” (i.e., to flows that generate the same 15-bit hash value), a load calculation is performed and the link with the most available bandwidth is assigned to the flow table entry. DLB offers two modes:

Spray mode causes the link assignment to occur on every packet transmission. This is similar to a “round-robin” hash.Every packet goes to the link with the least load, but ordering within flows is not guaranteed. This mode is useful for Bandwidth Management Testing (BMT).
Eligibility mode keeps the link fixed to the flow entry until a 32 ms inactivity timeout occurs. Ordering within flows is guaranteed.

For blades that provide 10 Gb ports only (and for 40 Gb blades that are entirely configured for 10 Gb operation), source port hashing will be used by default.

While source port hashing can be used for non-blocking operation, such operation depends on the switch fabric distribution. For example, suppose three 10 Gb ports on one BDXA-10G48X card hash to the same switch fabric “channel.” Also suppose that the three 10 Gb ports‘ traffic is aggregated into the same 40 Gb port on a different I/O blade. Since each switch fabric channel can only provide 20 Gb of bandwidth to a single 40 Gb port, then the switch fabric channel will try to send 30 Gbps of Ethernet packets to the same 20 Gb switch fabric link, causing congestion in the channel on the FM blade.

To avoid this situation, the user can configure DLB Eligibility mode, which will cause per 10 Gb Ethernet port traffic to be distributed to all switch fabric channels instead of being sent to the same channel.

Use the following command: disable elrp-client [default | source-port | packet {algorithm [crc |xor] | dynamic-mode [spray | eligibility | none]}] {slot slot-number}

Published March 2020prev | next

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