AT-GS950/16PS Switch Web Interface User’s Guide
363
Associating VLANs to MSTIs
When you are using Multiple Spanning Tree, Allied Telesis recommends that
you assign each VLANs to one of the existing MSTIs on a switch. You should
not leave any VLAN unassigned including the Default VLAN. This is to prevent
the blocking of a port that should be in the forwarding state. The reason for
this guideline is explained below.
An MSTP BPDU contains information identifying the Multiple Spanning Tree
instance that is associated with the port transmitting the BPDU packet. By
default, all ports of the AT-GS950/16PS switch belong to the CIST instance.
So the CIST identification is always included in the BPDU. If the port is also a
member of a VLAN that has been assigned to a MSTI, that information is
included in the BPDU too.
This is illustrated in Figure 142. Port 1 in switch A is a member of the Default
VLAN and has been assigned to MSTI ID 10 and port 8 is a member of VLAN
3 assigned to MSTI ID 10. The BPDUs transmitted by port 8 to switch B
indicate that the port is a member of both CIST 0 and MSTI 15, while the
BPDUs from port 1 indicate the port is a member of the CIST 0 and MSTI 10.
Figure 142. CIST and VLAN Guideline - Example 1
At first glance, it might appear that because both ports belong to CIST, a loop
would exist between the switches and that MSTP would block a port to stop
the loop. However, within a region, MSTI takes precedence over CIST. When
switch B receives a packet from switch A, it uses MSTI, not CIST, to determine
whether a loop exists. And because both ports on switch A belong to different
MSTIs, switch B determines that no loop exists.
A problem can arise, however, if you assign some VLANs to MSTIs while
leaving others assigned only to CIST. Figure 143 on page 364 illustrates the
issue. The network is the similar as the previous example. The primary
difference is that the VLAN 2 containing port 1 on Switch A has not been
assigned to a MSTI, and only belongs to CIST (MSTI ID 0).
Содержание AT-GS950/16PS
Страница 12: ...Figures 12...
Страница 14: ...List of Tables 14...
Страница 18: ...Preface 18...
Страница 20: ...20...
Страница 52: ...Chapter 2 System Configuration 52...
Страница 54: ...54...
Страница 92: ...Chapter 5 Multiple Spanning Tree Protocol 92...
Страница 102: ...Chapter 6 Static Port Trunking 102...
Страница 118: ...Chapter 8 Port Mirroring 118...
Страница 140: ...Chapter 11 IGMP Snooping 140...
Страница 174: ...Chapter 14 GVRP 174...
Страница 186: ...Chapter 15 Quality of Service and Cost of Service 186...
Страница 188: ...188...
Страница 216: ...Chapter 17 SNMPv3 216...
Страница 244: ...Chapter 18 Access Control Configuration 244...
Страница 282: ...Chapter 21 Security 282...
Страница 288: ...Chapter 22 Power Over Ethernet PoE 288...
Страница 289: ...289 Chapter 23...
Страница 310: ...Chapter 24 LLDP 310...
Страница 322: ...322...
Страница 334: ...Chapter 26 Software Configuration Updates 334...
Страница 346: ...Chapter 28 Rebooting the AT GS950 16PS 346...
Страница 368: ...Appendix A MSTP Overview 368...
Страница 386: ...Appendix B AT GS950 16PS Default Parameters 386...