802.1s MSTP

MSTP(Multiple Spanning Tree Protocol)
2002 年 IEEE 802.1s mstp標準被提出

Multiple instances of STP can be used, with each instance supporting a different group of VLANs(mapping one or more VLANs to a single STP instance)

The MSTP was developed to address the lack of and surplus of STP instances.
the network administrator can configure exactly the number of STP instances that makes sense for the enterprise network, no matter how many VLANs are in use.
提供multi-forward path及load balance的機制,而且也與STP及RSTP相容

將需要進行相同STP path計算的VLAN mapping到同1個STP instance中
無論有多少vlan,只要實際產生n個path,就維護n個stp instance,可節省系統資源
ex:switch上有20個VLAN,這20個VLAN的流量只需要3條不同path,則只需维護3個STP instance即可
ex:switch上有20個VLAN,而CST只維護1個STP instance
PVST+:每個VLAN運作一個STP instance,將不同VLAN的流量放在不同path上傳輸,但每個VLAN都運作一個STP instance,所以會很消耗系統資源
ex:switch上有20個VLAN,而PVST+維護20个STP instance

1. The number of STP instances needed to support the desired topologies
2. Whether to map a set of VLANs to each instance

MST is different from 802.1Q and PVST+, although it can interoperate with them
if an MST region connects with a switch running traditional PVST+
MST can detect this situation by listening to the received BPDUs.
If BPDUs are heard from more than one VLAN (the CST), PVST+ must be in use.
When the MST region sends a BPDU toward the PVST+ switch, the IST BPDUs are replicated into all the VLANs on the PVST+ switch trunk.

MST region
In most networks, a single MST region is sufficient, although you can configure more than one region
If a switch is configured to use MST, it somehow must figure out which of its neighbors are using which type of STP.This is done by configuring switches into common MST regions,where every switch in a region runs MST with compatible parameters.

MST region attribution
all switches must run the instance of MST that is defined by the following attributes:
MST configuration name (32 characters)
MST configuration revision number (0 to 65535)
MST instance-to-VLAN mapping table (4096 entries)

MST configuration revision number
1.for tracking changes to the MST region configuration
2.Each time you make changes to the configuration, you should increase the number by one
3.the region configuration (including the revision number) must match on all switches in the region. Therefore, you also need to update the revision numbers on the other switches to match.

MST region attribution relation
if two switches region 3 attributes
 they belong to the same MST region.
 the STP instances within MST can be shared as part of the same region.
not same:
  they belong to two independent regions
 a switch is seen to be at the MST region boundary,
MST region boundary where one region meets another or one region meets traditional 802.1D STP.
MST BPDUs contain configuration attributes so that switches receiving BPDUs can compare them against their local MST configurations.

entire MST instance-to-VLAN mapping table
1 The table contents:
 is not sent in the BPDUs ,
 because the instance mappings must be configured on each switch.
2 a digest/hash code computed from the table contents:
 is sent in the BPDUs
 As the contents of the table change, the digest value will be different.
a switch quickly can compare a received digest to its own to see if the advertised table is the same

MSTI(MST Instances)
1.different MSTIs can exist within a single MST region
2.Each of the MSTIs is significant only within a region, even if an adjacent region has the same MSTIs in use.
MST region “compnayregion” include MSTI1=vlan2-3 stp ,MSTI2=vlan4-5 stp
In other words, the MSTIs combine with the IST only at the region boundary to form a subtree of the CST. That means only IST BPDUs are sent into and out of a region.
MSTI number為bridge id中的extend system id

only the MSTI is allowed to send and receive MST BPDUs.
Information about each of the other MSTIs is appended to the MST BPDU as an M-record.
By default, all VLANs will belong to MSTI 0
if a region has all 16 instances active, only 1 BPDU is needed to convey STP information about them all.

IST(Internal Spanning Tree) Instances
1. Something other than CST must work out a loop-free topology inside each MST region.
2. Within a single MST region,
an IST instance runs to work out a loop-free topology
between “the links where CST meets the region boundary” and “all switches inside the region”.
the IST instance as a locally significant CST, bounded by the edges of the region.
The IST presents the entire region as a single virtual bridge to the CST outside.
BPDUs are exchanged at the region boundary only over the native VLAN of trunks, as if a single CST were in operation

the IST instance is active on every port on a switch. Even if a port does not carry VLANs that have been mapped to the IST, IST must be running on the port.