User Manual

Features Overview and ConfigurationRev 2.3-1.0.1

Mellanox Technologies

connected through the loop. As such, the UPDN routing algorithm should be send if the subnet is

not a pure Fat Tree, and one of its loops may experience a deadlock (due, for example, to high

pressure).

The UPDN algorithm is based on the following main stages:

1. Auto-detect root nodes - based on the CA hop length from any switch in the subnet, a statisti-

cal histogram is built for each switch (hop num vs number of occurrences). If the histogram

reflects a specific column (higher than others) for a certain node, then it is marked as a root

node. Since the algorithm is statistical, it may not find any root nodes.

The list of the root

nodes found by this auto-detect stage is used by the ranking process stage.

The user can override the node list manually

If this stage cannot find any root nodes, and the user did not specify a guid list file,

OpenSM defaults back to the Min Hop routing algorithm.

2. Ranking process - All root switch nodes (found in stage 1) are assigned a rank of 0. Using the

BFS algorithm, the rest of the switch nodes in the subnet are ranked incrementally. This rank-

ing aids in the process of enforcing rules that ensure loop-free paths.

3. Min Hop Table setting - after ranking is done, a BFS algorithm is run from each (CA or

switch) node in the subnet. During the BFS process, the FDB table of each switch node tra-

versed by BFS is updated, in reference to the starting node, based on the ranking rules and

guid values.

At the end of the process, the updated FDB tables ensure loop-free paths through the subnet.

Up/Down routing does not allow LID routing communication between switches that

are located inside spine “switch systems”. The reason is that there is no way to allow a

LID route between them that does not break the Up/Down rule. One ramification of

this is that you cannot run SM on switches other than the leaf switches of the fabric.

UPDN Algorithm Usage

Activation through OpenSM

• Use '-R updn' option (instead of old '-u') to activate the UPDN algorithm.

• Use '-a <root_guid_file>' for adding an UPDN guid file that contains the root nodes for

ranking. If the `-a' option is not used, OpenSM uses its auto-detect root nodes algorithm.

Notes on the guid list file:

• A valid guid file specifies one guid in each line. Lines with an invalid format will be dis-

carded

• The user should specify the root switch guids

3.2.2.5.3 Fat-tree Routing Algorithm

The fat-tree algorithm optimizes routing for "shift" communication pattern. It should be chosen if

a subnet is a symmetrical or almost symmetrical fat-tree of various types. It supports not just K-