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Understanding STP for EX Series Switches
Ethernet networks are susceptible to broadcast storms
if loops are introduced. However, an Ethernet network should always
include loops because they provide redundant paths in case of a link
failure. Spanning-tree protocols address both of these issues because
they provide link redundancy while simultaneously preventing undesirable
loops.
Juniper Networks EX Series Ethernet Switches provide Layer 2
loop prevention through Spanning–Tree Protocol (STP), Rapid
Spanning–Tree Protocol (RSTP), Multiple Spanning-Tree Protocol
(MSTP), and VLAN Spanning-Tree Protocol (VSTP). Configure STP when
you need to support older 802.1D 1998 bridges. However, note that
EX Series switches configured to use STP actually run RSTP force version
0, which is compatible with STP. For an explanation of RSTP, see Understanding RSTP for EX Series Switches
Selecting a Spanning-Tree Protocol
The default factory configuration for EX Series switches is
RSTP, a faster version of STP. To determine which spanning-tree protocol
is best for your situation, see Table 1 below.
Table 1: Selecting a Spanning-Tree
Protocol
| Protocol | Advantages | Disadvantages |
|---|
RSTP | - Rapid Spanning-Tree Protocol is the default switch configuration
and is recommended for most network configurations because it converges
more quickly than STP after a failure.
- Voice and video work better with RSTP than they do with
STP.
- RSTP is backward compatible with STP so switches do not
all have to run RSTP.
- RSTP supports more ports than MSTP or VSTP
| - RSTP does not work with 802.1D 1998 bridges.
- RSTP is not recommended for multiple VLAN networks because
it is not VLAN-aware—as a result, all VLANs within a LAN share
the same spanning-tree. This limits the number of forwarding paths
for data traffic.
|
STP | - Spanning-Tree Protocol works with 802.1D 1998 bridges.
- RSTP is backward compatible with STP so switches do not
all have to run STP.
| |
MSTP | - Multiple Spanning-Tree Protocol works with most VLANs.
- RSTP and STP are recognized as distinct spanning-tree
regions by MSTP.
| - Some protocols require compatibility that is not provided
by MSTP. In this case, use VSTP.
- MSTP supports a limited number of ports.
- MSTP uses more CPU than RSTP and does not converge as
fast as RSTP.
|
VSTP | - VLAN Spanning-Tree Protocol works with VLANs that require
device compatibility.
- VSTP and RSTP are the only spanning-tree protocols that
can be configured concurrently on a switch.
| - With VSTP, there can be only one STP instance per VLAN,
whereas MSTP lets you combine multiple VLANs in one instance.
- VSTP supports a limited number of ports compared to RSTP.
- VSTP uses more CPU than RSTP and does not converge as
fast as RSTP.
- Having a large number of VSTP and RSTP instances can cause
continuous changes in the topology. As a workaround, reduce the number
of VSTP instances to fewer than 190.
|
Modified: 2016-01-12
