Supermicro L2 User manual

L2 / L3 Switches
Spanning Tree
Configuration Guide
Revision 1.1

Spanning TreeConfiguration Guide
Supermicro L2/L3 Switches Configuration Guide
2
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Manual Revision 1.0
Release Date: February 24, 2013
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Contents
1 Spanning Tree Configuration Guide ...................................................................................................... 4
1.1 Spanning Tree Basics ..................................................................................................................... 4
1.1.1 Root Switch Election Procedure ............................................................................................ 6
1.2 Spanning Tree Support.................................................................................................................. 7
1.3 Spanning Tree Defaults ................................................................................................................. 7
1.4 Enabling / Disabling Spanning Tree ............................................................................................... 8
1.4.1 Enable / Disable Spanning Tree Globally .............................................................................. 8
1.4.2 Enable / Disable Spanning Tree on Ports .............................................................................. 9
1.5 Configuring MST .......................................................................................................................... 10
1.5.1 Configuring MST region and instances ............................................................................... 11
1.6 Configuring RSTP ......................................................................................................................... 13
1.7 Spanning Tree Compatibility ....................................................................................................... 13
1.8 Configuring Root Switch (or) Priority .......................................................................................... 15
1.9 Port Priority ................................................................................................................................. 16
1.10 Path Cost ..................................................................................................................................... 18
1.11 Hello Time ................................................................................................................................... 20
1.12 Max ge ...................................................................................................................................... 22
1.13 Forwarding Time ......................................................................................................................... 23
1.14 Max Hops .................................................................................................................................... 24
1.15 Path Cost Long / Short ................................................................................................................ 25
1.16 Transmit Hold Count ................................................................................................................... 26
1.17 Root Guard .................................................................................................................................. 27
1.18 Topology Change Guard .............................................................................................................. 28
1.19 Port Fast ...................................................................................................................................... 30
1.20 uto Edge .................................................................................................................................... 31
1.21 Link Type ..................................................................................................................................... 32
1.22 Spanning Tree Configuration Examples ...................................................................................... 34

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1Spanning Tree Configuration Guide
This document describes the Spanning Tree feature supported in Supermicro Layer 2 / Layer 3 switch
products.
This document covers the Spanning Tree configurations for the below listed Supermicro switch products.
The majority of this document applies to all the above listed Supermicro switch products. In any
particular sub section however, the contents might vary across these switch product models. In those
sections the differences are clearly identified with reference to particular switch product models. If any
particular switch product model is not referenced, the reader can safely assume that the content is
applicable to all the above listed models.
Throughout this document, the common term “switch” refers to any of the above listed
Supermicro switch product models unless a particular switch product model is noted.
1.1 Spanning Tree Basics
Switches are interconnected to provide network access to a large number of end stations. In complex
networks it is possible to have multiple network paths between any two end devices. These multiple
paths form network loops that lead to packet flooding by forwarding broadcast and multicast packets
repeatedly over the looped connections. Flooding makes the network unusable until the looped
connections are disconnected and flooding stopped.
Spanning tree protocols help to prevent the flooding on network loops. Spanning tree protocols form a
loop-free tree, logically structured network topology over physical network connections.
Top of Rack Switches
• SSE-G24-TG4
• SSE-G48-TG4
• SSE-X24S
• SSE-X3348S
• SSE-X3348T
Blade Switches
• SBM-GEM-X2C
• SBM-GEM-X2C+
• SBM-GEM-X3S+
• SBM-XEM-X10SM

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Spanning tree enabled switches exchange spanning tree protocol messages (BPDU) to form a loop-free
topology. Based on the exchanged BPDU information, the spanning tree algorithm selects one of the
switches on the network as the root switch for the tree topology. ll other switches on the network
choose a best loop free path to reach the root switch. The redundant paths to root switch are then
blocked to form a loop-free topology.
The spanning tree algorithm assigns one of the following roles to every port on the switches.
When a network connection status changes, spanning tree recalculates the paths to form a loop-free
topology. Spanning tree calculations are based on the following three key factors:
Bridge Identifier: Combination of a switch’s M C address and switch’s spanning tree priority
Path Cost: Spanning tree path cost to the root switch
Port Identifier: Combination of port number and port priority
When a switch boots up, it assumes its role as the root switch. It sends out spanning tree BPDUs with its
bridge id as the root bridge id. When a switch receives spanning tree BPDUs, it compares the received
BPDU information. If the received BPDU information is superior, the switch uses the received BPDU
information to determine the root bridge and recalculates the spanning tree. If the received BPDU
information is inferior, the switch ignores the received BPDU.
Spanning tree operates the switch ports in different states while calculating the loop-free topology.
BPDU exchanges between switches take a few seconds in a large L N. To avoid any temporary loops
while forming spanning tree topology, the switch ports are moved through different states to reach a
forwarding state. Switch ports stay in one of the following spanning tree states:
•Port to reach the root switch with lowest path cost
•Root ports forward the traffic
Root Port
•Loop-free connection to the other switch on the L N
• Designated ports forward the traffic
Designated Port
• Redundant path to the root switch
• lternate ports do not forward the traffic
lternate Port
•Redundant path to other switches on the L N
•Blocked ports do not forward the traffic
Blocked Port

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Since spanning tree forms a logical loop-free topology, it helps to have physical loop connections on the
network for redundancy purposes. When an active connection fails, spanning tree enables the blocked
redundant connection automatically.
Rapid spanning tree protocol (RSTP) provides faster topology convergence. While spanning tree (STP)
takes more than 30 seconds to move a port to a forwarding state, RSTP can move a port to the
forwarding state within 3 times of the hello interval (the default hello interval is 2 seconds). RSTP is
compatible with STP.
Multiple spanning tree protocol (MSTP) extends RSTP to provide separate spanning trees for different
VL Ns or VL N groups. This helps use alternate paths efficiently by only blocking the ports for the
required VL Ns. MSTP is compatible with RSTP.
1.1.1 Root Switch Election Procedure
Spanning tree protocol selects one switch as the root switch for every switched L N. This root switch is
used as the reference point to decide the spanning tree topology. Based on the connections to this root
switch, the redundant links on the L N are identified and blocked. Spanning tree runs an election
process to determine the root switch.
Spanning tree selects the switch with the lowest bridge ID as the root switch. Every switch on the L N
has a bridge ID. The bridge ID has two components – its priority and the M C address of the switch. The
spanning tree priority occupies the most significant two bytes of the bridge ID. The default spanning tree
priority is 32768.
Blocking
•Receives BPDUs
•Discards packets
•Does not learn M C
Listening
•Receives BPDUs
•Discards packets
•Does not learn M C
Learning
•Receives BPDUs
•Discards packets
•Learns M C addresses
Forwarding
•Receives BPDUs
•Forwards packets
•Learns M C addresses
Disabled
•Does not receive BPDUs
•Discards packets
•Does not learn M C

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When a switch starts spanning tree it sends out BPDUs with its bridge ID as the root bridge ID. When a
switch receives the BDPUs, it compares the received root bridge ID with its own bridge ID. If the
received root bridge ID is lower than its own bridge ID, the received switch accepts the other switch as
the root switch. If the received root bridge ID is higher than its own bridge ID, the received switch
ignores the received BPDU and continue to act as the root switch.
If the priorities of all switches are the same, the switch M C addresses decide the lowest bridge ID and
hence the switch with the lowest M C address will be elected as the root switch.
1.2 Spanning Tree Support
Supermicro switches support STP, RSTP and MSTP protocols based on standards IEEE 802.1D 2004 and
802.1s.
1.3 Spanning Tree Defaults
Parameter Default Value
Spanning tree global status
Enabled
Spanning tree port status
Enabled
Spanning tree mode
MST
Switch priority
32768
Port priority
128
Port cost
Port Speed Default Path
Cost
10 Mbps
2000000
100 Mbps
200000
1 bps
20000
10 bps
2000
40 bps
500
Hello time
2 seconds
Forwarding
time
15 seconds
Maximum aging time
20 seconds
Transmit hold count
3
Max hops
20
Path cost method
long
Bridge ID
STP Priority - 2 bytes
(Default 32
7
6
8)
Switch MAC – 6 bytes

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MST region name
Switch M C address
MST region revision
0
Spanning tree compatibility
In MSTP mode, the default
compatibility is MSTP and in RSTP
mode the default compatibility is
RSTP
Root guard
Disabled
Topology change guard
Disabled
Port fast
Disabled
uto edge
Enabled
Link type
Full duplex ports
–
point to point links
Half duplex ports – shared L N links
1. Enabling / Disabling Spanning Tree
1. .1 Enable / Disable Spanning Tree Globally
Spanning tree is enabled by default in Supermicro switches globally.
Follow the steps below to disable the spanning tree globally.
Step Command Description
Step 1
configure terminal
Enters the configuration mode
Step 2
no spanning
-
tree
Disables
the
spanning tree
globally
Step 3
end
Exits the configuration mode.
Step 4
show
spanning
-
tree
Displays the
spanning tree
information.
Step 5
write startup
-
config
Optional step
–
saves this
spanning tree
configuration to be part of startup
configuration.
The
“spanning
-
tree”
command
enables the spanning tree globally.
The examples below show ways to disable / enable the spanning tree function on Supermicro switches.
Disable the spanning tree.
SMIS# configure terminal
SMIS(config)# no spanning-tree

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SMIS(config)# end
Enable the spanning tree.
SMIS# configure terminal
SMIS(config)# spanning-tree
SMIS(config)# end
1. .2 Enable / Disable Spanning Tree on Ports
Spanning tree is enabled by default on all the ports and port channels in Supermicro switches.
Follow the steps below to disable the spanning tree on ports.
Step Command Description
Step 1
configure terminal
Enters the configuration mode
Step 2
interface
<interface-type> <interface-id>
or
interface range <interface-type> <interface-id>
….
Enters the port interface mode.
interface-type – may be any of the
following:
gigabit-ethernet – gi
extreme-ethernet – ex
qx-ethernet – qx
port-channel – po
interface-id is in slot/port format for all
physical interfaces. It may be the port
channel identifier for port channel
interfaces.
To configure multiple interfaces, use
the “interface range …” command. To
provide a range use a hypen (-)
between the start and end interface
numbers.
E.g.: int range gi 0/1-10
To provide multiple interfaces or
ranges, separate with a comma (,).
E.g.: int range gi 0/1-10, gi 0/20
Step 3
To
disable the spanning tree in RST mode:
spanning-tree disable
To disable the default MST instance spanning tree:
spanning-tree disable
To disable the particular MST instance spanning
Disables the spanning tree
on the port.
instance-id – The MST instance
identifier may be from 1 to 16.

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tree.
spanning-tree mst <instance-id> disable
Step
4
end
Exits the configuration mode.
Step
5
show spanning
-
tree
interface
<interface-type
>
<interface-id>
show running-config interface <interface-type
>
<interface-id>
Displays the spanning tree
port
information.
Step
6
write startup
-
config
Optional step
–
saves this spanning tree
configuration to be part of startup
configuration.
“
no
spanning
-
tree disable”
command
enables the spanning tree on ports.
The examples below show various ways to disable / enable the spanning tree on ports.
Disable the spanning tree on ports ex 0/1 and ex 0/2.
SMIS# configure terminal
SMIS(config)# interface range ex 0/1-2
SMIS(config-if)# spanning-tree disable
SMIS(config)# end
Enable the spanning tree on port ex 0/1.
SMIS# configure terminal
SMIS(config)# interface ex 0/1
SMIS(config-if)# no spanning-tree disable
SMIS(config)# end
1.5 Configuring MST
Spanning tree is enabled by default in MST mode in Supermicro switches.
In case the switch was earlier configured in RST mode, follow the steps below to change to MST mode.
Step Command Description
Step 1
configure terminal
Enters the configuration mode
Step 2
spanning
-
tree mode mst
Configures the switch to operate in
MST mode.