Example 10-3
Examining the 16-bit Priority as Interpreted in Cisco show Commands
SW1# show spanning-tree vlan 9
VLAN0009
Spanning tree enabled protocol rstp
Root ID Priority 24585
Address 1833.9d7b.0e80
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32777 (priority 32768 sys-id-ext 9)
Address f47f.35cb.d780
! Output omitted for brevity
RSTP Methods to Support Multiple Spanning Trees
Although the history and configuration might make the BID priority idea seem a bit convo-
luted, having an extra 12-bit field in the BID works well in practice because it can be used to
identify the VLAN ID. VLAN IDs range from 1 to 4094, requiring 12 bits.
For the purposes of discussion, focus on the standard RSTP and its Cisco-proprietary cousin
RPVST+. Both use the RSTP mechanisms as discussed in Chapter 9, “Spanning Tree Protocol
Concepts,” but RPVST+ uses the mechanisms for every VLAN, while standard RSTP does
not. So how do their methods differ?
■
RSTP creates one tree—the Common Spanning Tree (CST)—while RPVST+ creates one
tree for each and every VLAN.
■
RSTP sends one set of RSTP messages (BPDUs) in the network, no matter the number of
VLANs, while RPVST+ sends one set of messages per VLAN.
■
RSTP and RPVST+ use different destination MAC addresses: RSTP with multicast address
0180.C200.0000 (an address defined in the IEEE standard), and RPVST+ with multicast
address 0100.0CCC.CCCD (an address chosen by Cisco).
■
When transmitting messages on VLAN trunks, RSTP sends the messages in the native
VLAN with no VLAN header/tag. RPVST+ sends each VLAN’s messages inside that
VLAN—for instance, BPDUs about VLAN 9 have an 802.1Q header that lists VLAN 9.
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Chapter 10: RSTP and EtherChannel Configuration 247
■
RPVST+ adds an extra type-length value (TLV) to the BPDU that identifies the VLAN ID,
while RSTP does not (because it does not need to, as RSTP ignores VLANs.)
■
Both view the 16-bit priority as having a 12-bit System ID Extension, with RSTP setting
the value to 0000.0000.0000, meaning “no VLAN,” while RPVST+ uses the VLAN ID.
In other words, standard RSTP behaves as if VLANs do not exist, while Cisco’s RPVST+ inte-
grates VLAN information into the entire process.
NOTE
Some documents refer to the feature of sending BPDUs over trunks with VLAN
tags matching the same VLAN as BPDU tunneling.
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