Michael McNamara

technology, networking, virtualization and IP telephony

Cisco 3702e Access Point & 5Ghz Performance Problems

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It seems that everyone I talk to has wireless challenges in their environment and network. I recently deployed ~ 60 Cisco 3702e Access Points paired with a set of Cisco 5508 Wireless LAN Controllers running software release 7.6.100 in a newly renovated building. I wasn’t anticipating any issues or problems but you can guess what I ended up stumbling upon. While we thought everything was working fine we quickly discovered that 5Ghz clients were having severe performance and connectivity issues. I could run continuous ICMP pings but the moment I put the client under load I would either loose connectivity or experience severe packet loss upwards of 80-90%. We had the same results from a Lenovo T420 and T430 laptop and a MacBook Air.

We opened a case with Cisco TAC and discovered that we were experiencing a known issue;

  • CSCuj17283 – Cisco AP3700 used 8 replay counters with clients that support only one (ARP failed).

The bug above is resolved in software release 7.6.110, here are the release notes. We’re currently testing software release 7.6.110 although Cisco has just released 7.6.120 – release notes.

If your going to be deploying the Cisco 3700 Access Points you’ll want to make sure that your WLC is running a minimum software release of 7.6.110.

Cheers!

Short Story – switchport trunk allowed vlan

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I like sharing these stories because they help me document some really simple problems that can sometimes take a few minutes to troubleshoot and ultimately resolve. The moral of this story resolves around the much used command “switchport trunk allowed vlan x,y,z” and the often overlooked commands “switchport trunk allowed vlan add|remove x,y,z“.

I decided to write about this topic since I recently encountered operational “difficulties” with both my prior employer and my current employer that involved the same near identical mistake of a network engineer accidentally overwriting the list of allowed VLANs. In the most recent case it was a simple oversight on the engineer’s part and the problem was quickly corrected. On the prior case the engineer had issued the command “no switchport trunk allowed vlan x” which seems to have given NXOS a bit of a fit. The ports that were in that vPC needed to be shutdown and then enabled to clear what appeared to be a software bug. While the running-config indicated that the VLANs were being trunked on the ports, the MAC/FDB table had no entries of those VLANs on the affected ports.

I strongly recommend that folks prune VLANs that aren’t being used from their trunks, however, you need to be very careful with how you add and/or remove VLANs from the list once the trunk is up and running.

In the past I’ve seen folks accidentally overwrite the VLAN allowed add list by using the “switchport trunk allowed vlan” command. Look at this sample configuration;

interface port-channel2
  description VPC_CISCO_NEXUS_5010
  switchport
  switchport mode trunk
  switchport trunk allowed vlan 150-154
  switchport trunk allowed vlan add 155
  mtu 9216
  vpc 2

Now let’s say we wanted to add VLAN 156 and we using the following command, taking the allowed VLAN list and adding VLAN 156;

switchport trunk allowed vlan 152-154,156

The problem with this is that we just missed the fact that VLAN 155 was also on that trunk and we just removed it from the trunk with the that previous command.

The morale of the story – be careful when you add/remove VLANs from trunk ports and make sure you use the switchport trunk allow vlan add|remove command.

Cheers!

Image Credit: Roger Kirby

BGP Soft Reset – Cisco IOS

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I just recently learned that the BGP Soft Reset feature in Cisco IOS is automatically implemented in software release 12.0(2)S and later. Earlier software releases required the neighbor soft-reconfiguration in the BGP configuration to dynamically update BGP route-maps, local preference, etc. Without the neighbor soft-configuration enabled any configuration changes required a hard reset of the BGP peer which would interrupt network traffic. There was a memory penalty paid to having the neighbor soft-reconfiguration enabled since the router would keep a duplicate copy of the BGP route table in memory;

Before the BGP Soft Reset Enhancement feature, a soft reset for inbound routing table updates was performed by entering the neighbor soft-reconfiguration router configuration command. This command was used to configure the local BGP router to store all received (inbound) routing policy updates. However, this method uses too much memory because inbound updates are not modified and is not recommended.

I’m guessing this new feature had a significant impact for anyone taking a full Internet BGP table?

Cheers!

Image Credit: Earth 3D by Jan

LACP Configuration Examples (Part 6)

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While we’re at it let’s add two Cisco Catalyst 2950 switches to our topology and detail how to configure those additional switches. This has already been documented hundreds of times across the Internet so I’m doing this more for myself than for anyone else. The Cisco 2950 supports EtherChannel in one of these modes: Port Aggregation Protocol (PAgP) or Link Aggregation Control Protocol (LACP). There always seems to be some confusion regarding the configuration between PAgP and LACP so let me quote straight from the Cisco documentation:

Switch interfaces exchange PAgP packets only with partner interfaces configured in the auto or desirable modes. Switch interfaces exchange LACP packets only with partner interfaces configured in the active or passive modes. Interfaces configured in the on mode do not exchange PAgP or LACP packets.
…
Both the auto and desirable PAgP modes allow interfaces to negotiate with partner interfaces to determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.
…
Both the active and passive LACP modes allow interfaces to negotiate with partner interfaces to determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels, trunking state, and VLAN numbers.

We’ll be configuring our EtherChannel for LACP so we’ll use channel-group x mode active on both the Cisco 3750 and 2950 switches.

Sample Topology

AvayaJuniperCisco-MSTP3

Cisco Catalyst 3750E Switch

enable
config t
interface gig1/0/37
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 3 mode active
interface gig1/0/38
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 3 mode active

interface gig1/0/47
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 4 mode active
interface gig1/0/48
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 4 mode active
exit
exit

Cisco Catalyst 2950 Switch 1 & 2

enable
config t
vlan 100
name "192-168-100-0/24"
exit
vlan 200
name "192-168-200-0/24"
exit

interface vlan 100
ip address 192.168.100.40 255.255.255.0
no shut
exit

spanning-tree mode mst

spanning-tree mst configuration
 name AcmeNetworks
 revision 1
 instance 1 vlan 100
 instance 2 vlan 200

interface fas0/1
switchport mode trunk
channel-protocol lacp
channel-group 1 mode active

interface fas0/2
switchport mode trunk
channel-protocol lacp
channel-group 1 mode active

interface port-channel 1
switchport mode trunk

interface fas0/31
switchport mode trunk
channel-protocol lacp
channel-group 2 mode active

interface fas0/32
switchport mode trunk
channel-protocol lacp
channel-group 2 mode active

interface port-channel 2
switchport mode trunk
exit
exit

That’s all well and good but I’m sure you want to see the output… is it working as expected?

Cisco Catalyst 2950 Switch #1

C2950-SW1#show lacp neighbor
Flags:  S - Device is requesting Slow LACPDUs
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode

Channel group 1 neighbors

Partner's information:

                  LACP port                        Oper    Port     Port
Port      Flags   Priority  Dev ID         Age     Key     Number   State
Fa0/1     SA      32768     0064.xxxx.4d80   7s    0x3     0x126    0x3D
Fa0/2     SA      32768     0064.xxxx.4d80  21s    0x3     0x127    0x3D

Channel group 2 neighbors

Partner's information:

                  LACP port                        Oper    Port     Port
Port      Flags   Priority  Dev ID         Age     Key     Number   State
Fa0/31    SA      32768     0019.xxxx.49c0  23s    0x2     0x1F     0x3D
Fa0/32    SA      32768     0019.xxxx.49c0  21s    0x2     0x20     0x3D

C2950-SW1#show spanning-tree

MST00
  Spanning tree enabled protocol mstp
  Root ID    Priority    16384
             Address     3475.xxxx.a400
             Cost        0
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768  (priority 32768 sys-id-ext 0)
             Address     0018.xxxx.4a40
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Desg FWD 100000    128.66   P2p

MST01
  Spanning tree enabled protocol mstp
  Root ID    Priority    16385
             Address     54e0.xxxx.d441
             Cost        110000
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0018.xxxx.4a40
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Desg FWD 100000    128.66   P2p

MST02
  Spanning tree enabled protocol mstp
  Root ID    Priority    16386
             Address     0064.xxxx.4d80
             Cost        100000
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0018.xxxx.4a40
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Desg FWD 100000    128.66   P2p

Let’s have a look at the other Cisco 2950 switch;

Cisco Catalyst 2950 Switch #2

C2950-SW2#show lacp neighbor
Flags:  S - Device is requesting Slow LACPDUs
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode

Channel group 1 neighbors

Partner's information:

                  LACP port                        Oper    Port     Port
Port      Flags   Priority  Dev ID         Age     Key     Number   State
Fa0/1     SA      32768     0064.xxxx.4d80   9s    0x4     0x130    0x3D
Fa0/2     SA      32768     0064.xxxx.4d80  10s    0x4     0x131    0x3D

Channel group 2 neighbors

Partner's information:

                  LACP port                        Oper    Port     Port
Port      Flags   Priority  Dev ID         Age     Key     Number   State
Fa0/31    SA      32768     0018.xxxx.4a40   4s    0x2     0x1F     0x3D
Fa0/32    SA      32768     0018.xxxx.4a40  25s    0x2     0x20     0x3D

C2950-SW2#show spanning-tree

MST00
  Spanning tree enabled protocol mstp
  Root ID    Priority    16384
             Address     3475.xxxx.a400
             Cost        0
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768  (priority 32768 sys-id-ext 0)
             Address     0019.xxxx.49c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Altn BLK 100000    128.66   P2p

MST01
  Spanning tree enabled protocol mstp
  Root ID    Priority    16385
             Address     54e0.xxxx.d441
             Cost        110000
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0019.xxxx.49c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Altn BLK 100000    128.66   P2p

MST02
  Spanning tree enabled protocol mstp
  Root ID    Priority    16386
             Address     0064.xxxx.4d80
             Cost        100000
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0019.xxxx.49c0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p
Po2              Altn BLK 100000    128.66   P2p

We can see that ports Fas0/31 and Fas0/32 that make up Port-Channel 2 are in Alternate Blocking mode on SW2. This is expected since the bridge address of SW1 (0018.xxxx.4a40) is lower than SW2 (0019.xxxx.49c0) and the links are equal cost.

Cheers!
Image credit: Ravenel Bridge Charleston by Roger Kirby

LACP Configuration Examples (Part 5)

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Let’s keep going… let’s bring a Cisco 3750E into the topology and let’s talk about utilizing Spanning Tree. Let’s get this out the way, Avaya does NOT recommend that you disable Spanning Tree. Avaya’s Split MultiLink Trunking (SMLT) is not compatible with the Spanning Tree Protocol so you can’t run STP over SMLT links. You can still run STP on edge ports and even ports utilizing MultiLink Trunking (MLT) or LACP/802.3ad. This is in contrast to Cisco’s Virtual Port Channel (vPC) which is interoperable with Spanning Tree.

Let’s look at expanding the topology from our last post adding a Cisco 3750E;

AvayaJuniperCiscoAgain, that’s pretty straight forward and isn’t too exciting. Although if we leave every uplink/downlink as a member of VLAN 100 and VLAN 200 we’ll end up with a loop in our topology – not a Spanning Tree Loop. What if we add Multiple Spanning Tree Protocol (MSTP) to our configuration just to make it interesting? Our topology might look like this with 2 instances of MSTP running, one for each VLAN.

AvayaJuniperCisco-MSTP2

We’ll make the Avaya switch the root bridge for CIST. We’ll make the Juniper switch the root bridge for MST 1, and we’ll make the Cisco switch the root bridge for MST 2.

That’s interesting… let’s see what we need to-do in order to configure everything up. I’m going to pickup the configuration as I had it setup in the previous post, LACP Configuration  Examples (Part 4). We’ll need to add another LACP group/pair to our Avaya and Juniper switches as well as configure the Cisco switch. We’ll also need to enable MSTP on each switch, add the VLANs to the correct MSTP instances and set the correct bridge priority for each.

Juniper EX2200-C Switch

configure
set chassis aggregated-devices ethernet device-count 2

delete interfaces ge-0/0/4 unit 0
delete interfaces ge-0/0/5 unit 0

set interfaces ge-0/0/4 ether-options 802.3ad ae1
set interfaces ge-0/0/5 ether-options 802.3ad ae1
set interfaces ae1 aggregated-ether-options lacp active
set interfaces ae1 aggregated-ether-options lacp periodic fast

set interfaces ae1 unit 0 family ethernet-switching
set interfaces ae1 unit 0 family ethernet-switching port-mode trunk
set interfaces ae1 unit 0 family ethernet-switching port-mode trunk vlan members VLAN-100 members VLAN-200

delete protocols rstp

set protocols mstp configuration-name AcmeNetworks
set protocols mstp revision-level 1
set protocols mstp msti 1 vlan 100
set protocols mstp msti 2 vlan 200

set protocols mstp msti 1 bridge-priority 16384
commit and-quit

Avaya Ethernet Routing Switch 5520

config t
spanning-tree mode mst
exit
boot

You’ll need to reboot the switch in order to enable MSTP, so go ahead and reboot before continuing the steps;

config t
vlan ports 25,26 tagging tagAll

interface fastEthernet 25,26
lacp key 25
lacp mode active
lacp timeout-time short
lacp aggregation enable
exit

spanning-tree mstp msti 1
spanning-tree mstp msti 1 add-vlan 100
spanning-tree mstp msti 2
spanning-tree mstp msti 2 add-vlan 200
spanning-tree mstp priority 4000

You’ll notice that the Avaya switch accepts a hexadecimal value for the priority, so 4000 in hex = 16384 in decimal.

spanning-tree mstp region region-name AcmeNetworks
spanning-tree mstp region region-version 1
exit

Cisco Catalyst 3750E Switch

config t
vlan 100
name "192-168-100-0/24"
exit
vlan 200
name "192-168-200-0/24"
exit

interface vlan 100
ip address 192.168.100.30 255.255.255.0
no shut
exit

interface vlan 200
ip address 192.168.200.30 255.255.255.0
no shut
exit

interface gig1/0/13
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 1 mode active

interface gig1/0/14
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 1 mode active

interface gig1/0/25
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 2 mode active

interface gig1/0/26
switchport trunk encapsulation dot1q
switchport mode trunk
channel-protocol lacp
channel-group 2 mode active

spanning-tree mode mst

spanning-tree mst configuration
name AcmeNetworks
revision 1
instance 1 vlan 100
instance 2 vlan 200
exit
spanning-tree mst 2 priority 16384
exit

Let’s have a look at our work and see what everything looks like from both a LACP and Spanning Tree perspective.

Cisco Catalyst 3750E Switch

Switch#show lacp neighbor
Flags: S - Device is requesting Slow LACPDUs
F - Device is requesting Fast LACPDUs
A - Device is in Active mode P - Device is in Passive mode

Channel group 1 neighbors

Partner's information:

LACP port Admin Oper Port Port
Port Flags Priority Dev ID Age key Key Number State
Gi1/0/13 FA 127 54e0.xxxx.d440 5s 0x0 0x2 0x3 0x3F
Gi1/0/14 FA 127 54e0.xxxx.d440 5s 0x0 0x2 0x4 0x3F

Channel group 2 neighbors

Partner's information:

LACP port Admin Oper Port Port
Port Flags Priority Dev ID Age key Key Number State
Gi1/0/25 FA 32768 3475.xxxx.a400 14s 0x0 0x3019 0x19 0x3F
Gi1/0/26 FA 32768 3475.xxxx.a400 16s 0x0 0x3019 0x1A 0x3F

Switch#show spanning-tree

MST0
Spanning tree enabled protocol mstp
Root ID Priority 16384
Address 3475.xxxx.a400
Cost 0
Port 496 (Port-channel2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 0064.xxxx.4d80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Po1 Desg FWD 10000 128.488 P2p
Po2 Root FWD 10000 128.496 P2p

MST1
Spanning tree enabled protocol mstp
Root ID Priority 16385
Address 54e0.322a.d441
Cost 10000
Port 488 (Port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0064.xxxx.4d80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Po1 Root FWD 10000 128.488 P2p
Po2 Desg FWD 10000 128.496 P2p

MST2
Spanning tree enabled protocol mstp
Root ID Priority 16386
Address 0064.xxxx.4d80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 16386 (priority 16384 sys-id-ext 2)
Address 0064.xxxx.4d80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Po1 Desg FWD 10000 128.488 P2p
Po2 Desg FWD 10000 128.496 P2p

We can see that LACP is up and running to both the Avaya and Juniper switches. We can also see that the Cisco switch is the root bridge for MSTI 2 and the root port for MSTI 1 is Port-channel 1 (link to Juniper EX2200-C) while the root port for the CIST is Port-channel2 (link to Avaya ERS 5520). All ports are designated and forwarding traffic.

 Juniper EX2200-C Switch

root> show lacp interfaces
Aggregated interface: ae0
LACP state: Role Exp Def Dist Col Syn Aggr Timeout Activity
ge-0/0/0 Actor No No Yes Yes Yes Yes Fast Active
ge-0/0/0 Partner No No Yes Yes Yes Yes Fast Active
ge-0/0/1 Actor No No Yes Yes Yes Yes Fast Active
ge-0/0/1 Partner No No Yes Yes Yes Yes Fast Active
LACP protocol: Receive State Transmit State Mux State
ge-0/0/0 Current Fast periodic Collecting distributing
ge-0/0/1 Current Fast periodic Collecting distributing

Aggregated interface: ae1
LACP state: Role Exp Def Dist Col Syn Aggr Timeout Activity
ge-0/0/4 Actor No No Yes Yes Yes Yes Fast Active
ge-0/0/4 Partner No No Yes Yes Yes Yes Slow Active
ge-0/0/5 Actor No No Yes Yes Yes Yes Fast Active
ge-0/0/5 Partner No No Yes Yes Yes Yes Slow Active
LACP protocol: Receive State Transmit State Mux State
ge-0/0/4 Current Slow periodic Collecting distributing
ge-0/0/5 Current Slow periodic Collecting distributing

root> show spanning-tree bridge

STP bridge parameters
Context ID : 0
Enabled protocol : MSTP

STP bridge parameters for CIST
Root ID : 16384.34:75:xx:xx:a4:00
Root cost : 0
Root port : ae0.0
CIST regional root : 16384.34:75:xx:xx:a4:00
CIST internal root cost : 10000
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Hop count : 19
Message age : 0
Number of topology changes : 2
Time since last topology change : 14690 seconds
Topology change initiator : ae0.0
Topology change last recvd. from : 34:75:xx:xx:a4:01
Local parameters
Bridge ID : 32768.54:e0:xx:xx:d4:41
Extended system ID : 0
Internal instance ID : 0

STP bridge parameters for MSTI 1
MSTI regional root : 16385.54:e0:xx:xx:d4:41
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Number of topology changes : 5
Topology change initiator : ae1.0
Topology change last recvd. from : 00:64:xx:xx:4d:8d
Local parameters
Bridge ID : 16385.54:e0:xx:xx:d4:41
Extended system ID : 0
Internal instance ID : 1

STP bridge parameters for MSTI 2
MSTI regional root : 16386.00:64:xx:xx:4d:80
Root cost : 10000
Root port : ae1.0
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Hop count : 19
Number of topology changes : 6
Topology change initiator : ae1.0
Topology change last recvd. from : 00:64:xx:xx:4d:8d
Local parameters
Bridge ID : 32770.54:e0:xx:xx:d4:41
Extended system ID : 0
Internal instance ID : 2

Avaya Ethernet Routing Switch 5520

5520-48T-PWR#show lacp port 13-14,25-26
Admin Oper Trunk Partner
Port Priority Lacp A/I Timeout Key Key AggrId Id Port Status
---- -------- ------- --- ------- ----- ----- ------ ----- ------- ------
13 32768 Active A Short 1 12289 8224 32 1 Active
14 32768 Active A Short 1 12289 8224 32 2 Active
25 32768 Active A Short 25 12313 8223 31 282 Active
26 32768 Active A Short 25 12313 8223 31 283 Active

5520-48T-PWR#show spanning-tree mstp config
Maximum Mst Instance Number: 8
Number of Msti Supported: 2
Cist Bridge Priority (hex): 4000
Stp Version: Mstp Mode
Cist Bridge Max Age: 20 seconds
Cist Bridge Forward Delay: 15 seconds
Tx Hold Count: 3
Path Cost Default Type: 32-bit
Max Hop Count: 2000

VLAN members
------ ------ ------ ------ ------ ------ ------ ------ ------ ------
1

Msti Config Id Selector: 0
Msti Region Name: AcmeNetworks
Msti Region Version: 1
Msti Config Digest: 6D:A4:B5:0C:4F:D5:87:75:7E:EF:03:56:75:36:05:E1

5520-48T-PWR#show spanning-tree mstp msti config 1
Msti Bridge Regional Root:  40:00:54:E0:xx:xx:D4:41
Msti Bridge Priority (hex): F000
Msti Root Cost:             10000
Msti Root Port:             MLT 32
Msti State:                 Enabled

VLAN members
------ ------ ------ ------ ------ ------ ------ ------ ------ ------
100

5520-48T-PWR#show spanning-tree mstp msti config 2
Msti Bridge Regional Root:  40:00:00:64:xx:xx:4D:80
Msti Bridge Priority (hex): F000
Msti Root Cost:             10000
Msti Root Port:             MLT 31
Msti State:                 Enabled

VLAN members
------ ------ ------ ------ ------ ------ ------ ------ ------ ------
200

5520-48T-PWR#show spanning-tree mstp msti port role 1
Port Role State STP Status Oper Status
---- ---------- ---------- ---------- -----------
13 Root Forwarding Enabled Enabled
14 Root Forwarding Enabled Enabled
25 Alternate Discarding  Enabled Enabled
26 Alternate Discarding  Enabled Enabled

5520-48T-PWR#show spanning-tree mstp msti port role 2
Port Role State STP Status Oper Status
---- ---------- ---------- ---------- -----------
13 Alternate Discarding  Enabled Enabled
14 Alternate Discarding  Enabled Enabled
25 Root Forwarding Enabled Enabled
26 Root Forwarding Enabled Enabled

We can see from the output above that ports 13,14 are Alternate Discarding for MSTI 1 while ports 25,26 are Alternate Discarding for MSTI 2.

In the output we can see which port is the root bridge port for each switch, we can also see the MSTP config digest which should match on every switch in the topology. In order for the configuration to be valid the MST region name, version and config selector need to match along with correct VLAN IDs matched to the correct MST instance.

Cheers!
Image Credit: New York City Brooklyn Bridge by Diogo Ferrari

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