STP mode or default, whats the difference?

[Anonymous]

Hi there,

Working on a site that has adopted an odd configuration for spanning-tree that I'm trying to make sense of. The below is an edited example taken from an edge switch. The network consists of all extreme devices.

Its seems as though PVST+ has been adopted as the spanning tree protocol between switchs and RSTP else where.

One of the things I can't make complete sense of is the 'mode', and then the 'default-encapsulation', then configuration of the spanning-tree protocol on the uplink ports.

My question is, if you configure the mode, say dot1w, then does configuring the default-encapsulation for that same STP domain (say pvst-plus) override it?

Also, as in the case of the STP domain of s1, is there any need to specify pvst-plus on the port 1:48 if you have already configured the 'default-encapsulation' as pvst-plus?

create stpd s1
configure stpd s1 mode dot1w
enable stpd s1 rapid-root-failover
configure stpd s1 default-encapsulation pvst-plus
create stpd s2
configure stpd s2 mode dot1w
enable stpd s2 rapid-root-failover
create stpd s3
configure stpd voice mode dot1w
enable stpd voice rapid-root-failover
configure stpd voice default-encapsulation dot1d

configure stpd mangament tag 1000
configure stpd data tag data 500
configure stpd data tag data 300

enable stpd management
enable stpd data
enable stpd voice

enable stpd s0 auto-bind vlan Default
enable stpd s1 auto-bind vlan management
enable stpd s2 auto-bind vlan data
enable stpd s3 auto-bind voice

configure stpd s1 add vlan managment ports 1:48 pvst-plus
configure stpd s2 add vlan data ports 1:48 pvst-plus
configure stpd s3 add vlan voice ports 1:48 pvst-plus

I believe the answer is that the default encapsulation will override it, and with auto-bind on each of the stp domains of each of the ports will automatically bind to the default encapsulation type of the Vlans configured. So there is no need to add the addition of pvst-plus on port 1:48 for s1?

Should the default encapsulation type not be compatible it will step back to that configured for the mode?

I base that on the output below that shows

STACK.7 # show stpd "voice" ports 1:2 detail
Stpd: DATA Port: 1:2 PortId: 8002 Stp: ENABLED Path Cost: 200000
Port Mode: 802.1D
Port State: FORWARDING Topology Change Ack: FALSE
Port Priority: 128
Designated Root: 10:00:02:04:96:27:89:1c Designated Cost: 18000
Designated Bridge: 80:00:02:04:96:83:78:bf Designated Port Id: 8002
Partner STP version: Dot1w
Restricted Role: Disabled
Active Role: Disabled
Edge Port Safe Guard: Disabled
Bpdu Restrict: Disabled
Participating Vlans: voice

STACK.8 # show stpd "management" ports 1:2 detail
Stpd: managment Port: 1:2 PortId: 8002 Stp: ENABLED Path Cost: 200000
Port Mode: PVST+
Port State: FORWARDING Topology Change Ack: FALSE
Port Priority: 128
Designated Root: 10:00:02:04:96:27:89:1c Designated Cost: 18000
Designated Bridge: 80:00:02:04:96:83:78:bf Designated Port Id: 8002
Partner STP version: Dot1w
Restricted Role: Disabled
Active Role: Disabled
Edge Port Safe Guard: Disabled
Bpdu Restrict: Disabled
Participating Vlans: management

Many thanks in advance

[Anonymous]

Hi Brandon,

Thanks for posting back..... do have another question to assert my understanding:

So in the examples given the following would apply:

configure stpd s1 mode dot1w
enable stpd s1 rapid-root-failover
configure stpd s1 default-encapsulation pvst-plus
enable stpd s1 auto-bind vlan management
configure stpd s1 add vlan managment ports 1:48 pvst-plus

The stpd domain of s1 is configured for RSTP, but due to the 'default-encapsulation pvst-plus' then all vlans in that domain (s1) will transmit PVST+ (one-to-one) BPDU's (In this case only the
management vlan is configured).

Is there therefore any need to configure 'configure stpd s1 add vlan managment ports 1:48 pvst-plus' if the the 'default-encapsulation' is already sending out pvst-plus BPDU's as its part of the management Vlan belonging to the s1 spanning tree domain?

Many thanks,

Martin.

[BrandonC]

If the default encapsulation is already pvst-plus, there is no need to specify the encapsulation for each port. Specifying it per port allows you to override the default encapsulation.

Also, note that in order to do PVST+, you must create a separate STP domain for each VLAN participating in PVST+.

[BrandonC]

Hi Martin,

The STP mode is the operational mode of the STP domain. For example, dot1d means that the the domain will operate according to 802.1d, and dot1w means that it will operate according to RSTP (802.1w).

MSTP will set it to 802.1s, and then you will also need to specify whether this STP domain is for the CIST, or if it is an MST instance.

The encapsulation defines how the BPDUs will be transmitted. If the default-encapsulation is set to dot1d, the BPDUs will be sent untagged. EMISTP is an Extreme proprietary encapsulation mode, that send the BPDUs encapsulated in an 802.1Q tag with an STPD ID in the VLAN ID field. The pvst-plus encapsulation mode is for compatibility for third-party switches running PVST+. In this mode, STP domains must have a one-to-one relationship with VLANs.

Let me know if you have any questions.

-Brandon

[Mike_D]

Hi Martin,

We're working on picking these up a bit faster than this. I'll post something for you tomorrow; requires some study on my side.

Best regards,
Mike