Avaya 580 - Manual (Page 203)

Document No. 10-300077, Issue 2 7-5
Configuring Rapid Spanning Tree
Per-VLAN
Spanning Tree Per-VLAN mode is the Default Spanning Tree setting on all multiservice
switches. In this mode, the switch runs a separate spanning process for each
VLAN. Each logical Spanning Tree has its own BPDUs which are tagged
with the appropriate VLAN Identifier. In this configuration, the switch can
participate in as many Spanning Tree domains as there are VLANs defined
on the switch. This conforms to a “virtual” bridging model where the switch
runs as if each VLAN is a separate logical bridge (separate Address
Forwarding Tables, separate spanning trees, etc.). If spanning tree is not
required on all VLANs, you can disable it for individual VLANs on a
VLAN by VLAN basis. Also, different root bridges can be configured with
different bridge priorities based upon VLAN. This will allow load sharing
to occur based upon VLAN. Similarly link costs and priorities can be
adjusted on a per-VLAN basis allowing further load sharing per VLAN.
Per VLAN Spanning Tree is the recommended method whenever many
VLANs are passed over interswitch links, but not every VLAN resides on
every interswitch link. If IEEE 802.1D Spanning Tree mode is used without
care under these constraints, VLANs can be split due to blocking on
inappropriate links. The only time that Per-VLAN Spanning Tree may
become undesirable is when you have more VLANs to manage than the
number of Spanning Tree Domains. The only other constraint is to limit the
number of outgoing BPDUs.
It is required that the number of outgoing BPDUs be less than 500/second
on all multiservice switches. This means that if you have 50 VLANs you
cannot have more than 20 possible paths back to the root bridge (50
VLANS X 20 LINKS X 0.5 BPDU/sec. = 500), or if you have 100 VLANs
you cannot have more that 10 possible paths back to the root bridge. These
constraints are not generally exceeded in real networks. If these limits are
exceeded, you must use 802.1D Spanning Tree mode.
This method can easily interoperate with legacy IEEE802.1D devices. The
legacy devices become part of the VLAN associated with the VLAN port
binding with which they are attached. In other words, if three P580 switches
are in a network running VLANA and VLANB, and three legacy devices
connect up through a port configured in VLAN A, the legacy devices will
become part of the VLAN A Spanning Tree. The P580 switches will
participate in two Spanning Tree domains, one for VLAN A containing
three bridges and one for VLAN B containing 6 bridges. The legacy devices
need not be VLAN aware.
Dual-Layer
Spanning Dual-Layer spanning tree mode is a variation of per-VLAN spanning tree
mode with many of the same features. However, instead of using normal
BPDUs, which are clear (free of VLAN tags) on clear links and tagged on
tagged links, as is the case with Per-VLAN Spanning Tree mode, dual-layer
uses a proprietary BPDU. This proprietary BPDU is sent to a special
multicast address and contains information about which VLAN the BPDU
is associated with. This has an advantage over per-VLAN spanning trees in
that this method can support multiple VLANs over a non-tagging link, or
when connecting to a bridge/router.