Chapter 23: Multiple Spanning Tree Protocol
260
Section V: Spanning Tree Protocols
Common and
Internal
Spanning Tree
(CIST)
MSTP has a default spanning tree instance called the Common and
Internal Spanning Tree (CIST). This instance has an MSTI ID of 0.
This instance has unique features and functions that make it different from
the MSTIs that you create yourself. First, you cannot delete this instance
and you cannot change its MSTI ID. Second, when you create a new port-
based or tagged VLAN, it is by default associated with the CIST and is
automatically given an MSTI ID of 0. The Default_VLAN is also associated
by default with CIST.
Another critical difference is that when you assign a VLAN to another
MSTI, it still partially remains a member of CIST. This is because CIST is
used by MSTP to communicate with other MSTP regions and with any
RSTP and STP single-instance spanning trees in the network. MSTP uses
CIST to participate in the creation of a spanning tree between different
regions and between regions and single-instance spanning tree, to form
one spanning tree for the entire bridged network.
MSTP uses CIST to form the spanning tree of an entire bridged network
because CIST can cross regional boundaries, while a MSTI cannot. If a
port is a boundary port, that is, if it is connected to another region, that port
automatically belongs solely to CIST, even if it was assigned to an MSTI,
because only CIST is active outside of a region.
As mentioned earlier, every MSTI must have a root bridge, referred to as a
regional root, in order to locate loops that might exist within the instance.
CIST must also have a regional root. However, the CIST regional root
communicates with the other MSTP regions and single-instance spanning
trees in the bridged network.
The CIST regional root is set with the
CIST Priority
parameter. This
parameter, which functions similar to the RSTP bridge priority value,
selects the root bridge for the entire bridged network. If the AT-9400
Switch has the lowest CIST Priority value among all the spanning tree
bridges, it functions as the root bridge for all the MSTP regions and STP
and RSTP single-instance spanning trees in the network.
MSTP with STP
and RSTP
MSTP is fully compatible with STP and RSTP. If a port on the AT-9400
Switch running MSTP receives STP BPDUs, the port sends only STP
BPDU packets. If a port receives RSTP BPDUs, the port sends MSTP
BPDUs because RSTP can process MSTP BPDUs.
A port connected to a bridge running STP or RSTP is considered to be a
boundary port of the MSTP region and the bridge as belonging to a
different region.
An MSTP region can be considered as a virtual bridge. The implication is
that other MSTP regions and STP and RSTP single-instance spanning
trees cannot discern the topology or constitution of a MSTP region. The
only bridge they are aware of is the regional root of the CIST instance.
Содержание AT-S63
Страница 14: ...Figures 14 ...
Страница 18: ...Tables 18 ...
Страница 28: ...28 Section I Basic Operations ...
Страница 58: ...Chapter 1 Overview 58 ...
Страница 76: ...Chapter 2 AT 9400Ts Stacks 76 Section I Basic Operations ...
Страница 96: ...Chapter 5 MAC Address Table 96 Section I Basic Operations ...
Страница 114: ...Chapter 8 Port Mirror 114 Section I Basic Operations ...
Страница 116: ...116 Section II Advanced Operations ...
Страница 146: ...Chapter 12 Access Control Lists 146 Section II Advanced Operations ...
Страница 176: ...Chapter 14 Quality of Service 176 Section II Advanced Operations ...
Страница 196: ...196 Section III Snooping Protocols ...
Страница 204: ...Chapter 18 Multicast Listener Discovery Snooping 204 Section III Snooping Protocols ...
Страница 216: ...Chapter 20 Ethernet Protection Switching Ring Snooping 216 Section III Snooping Protocols ...
Страница 218: ...218 Section IV SNMPv3 ...
Страница 234: ...234 Section V Spanning Tree Protocols ...
Страница 268: ...268 Section VI Virtual LANs ...
Страница 306: ...Chapter 27 Protected Ports VLANs 306 Section VI Virtual LANs ...
Страница 320: ...320 Section VII Internet Protocol Routing ...
Страница 360: ...Chapter 30 BOOTP Relay Agent 360 Section VII Routing ...
Страница 370: ...Chapter 31 Virtual Router Redundancy Protocol 370 Section VII Routing ...
Страница 372: ...372 Section VIII Port Security ...
Страница 402: ...Chapter 33 802 1x Port based Network Access Control 402 Section VIII Port Security ...
Страница 404: ...404 Section IX Management Security ...
Страница 436: ...Chapter 36 PKI Certificates and SSL 436 Section IX Management Security ...
Страница 454: ...Chapter 38 TACACS and RADIUS Protocols 454 Section IX Management Security ...
Страница 462: ...Chapter 39 Management Access Control List 462 Section IX Management Security ...
Страница 504: ...Appendix B SNMPv3 Configuration Examples 504 Security Model Security Level Storage Type SNMPv3 Parameters Continued ...
Страница 532: ...Appendix D MIB Objects 532 ...