04-05-2012 02:34 AM - edited 03-07-2019 05:58 AM
does pvst+ BPDU inculde VLAN tag(IN TLV) on access interface or native vlan
thank you!
04-05-2012 06:15 AM
Hey Fly
:
Depends on which trunking protocol you are using.
:
ISL or 802.1Q
:
ISL attaches a VLAN ID header on all frames
802.1q inserts a VLAN ID inside all non-native frames
:
HTH
Frank
04-10-2012 07:57 PM
Hi Frank,
Thank you for quick answer.
but my question is not only about 802.1q tag on trunk interface.
I mean vlan tag inside BPDU frame.
I know for PVST+, non native vlan. cisco will inject a vlan tag inside BPDU TLV. when BPDU passthrough 802.1Q trunk interface. this BPDU will be encapsulated with a vlan tag. if Received switch find 802.1Q trunk vlan tag is not same as vlan tag in BPDU TLV, receive swtich will report inconsistent status and block related VLAN.
my questions are:
switchA(native vlan10)---(native vlan 10)SwitchB,
If BPDU create on native vlan 10 on switchB, does PVST+ inject VLAN tag in BPDU TLV filed?
switchA(native vlan 1)------(native vlan20)SwtichB----802.1Qtrunk-----switchC(vlan20),
IF vlan 20 BPDU create on switchC, switchC PVST+ will inject vlan tag20 in BPDU TLV, when this BPDU passthrough switchB, go out switchB to switchA, switchA config native vlan 20 on port to switchA, does switchB remove BPDU vlan tag from TLV field. if doesn't switchA receive this BPDU, will orignal a inconsistent warning .because native vlan on switchA port is VLAN 1.
i don't have time to test this. SwitchB may be a third vendor switch.
Am I right?
thank you!
Fly
04-11-2012 01:52 AM
Hi Fly,
About you first question and from the information in
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml#topic1
If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:
VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.
VLAN 1 STP BPDUs are also sent to the IEEE STP MAC address on the Native VLAN of the IEEE 802.1Q trunk, untagged.
Non-VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.
Note: Native VLAN STP BPDUs are sent untagged.
And regarding the second one, from the same link
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml#topic2
If native vlans are not the same on each end of the trunks, a PVID-inconsistent state will be created.
Note that third party switches do not understand the Cisco PVST+ mac address (0100.0ccc.cccd) as spanning-tree realted, thus forwarding them just like normal multicast traffic
Regards.
Karim
04-11-2012 02:54 AM
Hello Fly,
To answer your original question, on access ports, only standard STP or RSTP BPDUs are sent. There is no PVST+ or RPVST+ running on access ports, and therefore, the BPDUs do not contain the originating VLAN TLV. This TLV is placed only into PVST+ or RPVST+ BPDUs sent and received across trunk ports.
Best regards,
Peter
04-11-2012 05:18 AM
Hello, Peter.
Maybe you (or smb else) can aswell clarify for me - which vlan MSTP uses to send its BPDU? Have no free switches to make lab and check
But as i suppose:
And if regarding third paragraph i am rather sure - this behaviour well known and described in many articles, but for first two i have doubts, cos i think it may be that MSTP use vlan 1 and send untagged frames if vlan 1 native and tagged with vlan 1 if other vlan choosed as native.
Both theory have rights to life (at least its seems to me so) but i want to know exactly
04-11-2012 03:39 PM
Hello Alexey,
You are correct all three accounts. MSTP BPDUs are sent untagged, regardless of the native VLAN, on internal ports and on boundary ports towards other regions or towards pure IEEE switches.
On boundary ports towards a (R)PVST+ region, the switch takes the VLAN1 BPDU and replicates it into (R)PVST+ BPDUs for each existing VLAN, tagged with the appropriate VLAN tag except the VLAN that is defined as native on that boundary port - that (R)PVST+ BPDU is sent untagged.
Best regards,
Peter
04-11-2012 11:31 PM
Tnx for answer Peter! I highly appreciate your responces
Maybe you can help me with few following assumptions too (confirm or make corrections):
1) When MSTP Region1 conected with CST to MSTP Region2 them exchaneges IEEE BPDU (version = 3) with no MSTI information attached - only IST information included.
2) When MSTP Region1 conected with CST to PVST+ switch them exchaneges IEEE (version = 0) BPDU replicated on each vlan with no MSTI information and no IST information - whole region presented as 1 switch
3) When MSTP Region1 conected with CST to RPVST+ switch them exchaneges IEEE BPDU (version = 2) replicated on each vlan with no MSTI information and no IST information - whole region presented as 1 switch
And I must confess that Im not sure about BPDU types used by STP, MSTP, PVST+ - it seems to me logical that all of them use same BPDU format (PVST+ use also SSTP BPDU but here i dont take it in to consideration) but with different version numbers and differently treat flags fields, but from other point - MSTP bpdu have so much additional fields that it seems to be other BPDU format. I tried to read http://www.ieee802.org/1/files/public/docs2009/aq-seaman-merged-bpdu-encoding-0509.pdf quote from there:
The format of MST BPDUs is compatible with that specified
for RST BPDUs, with the addition of fields to convey
information for the IST and each MSTI
From here i can make assumption that in 1st my question - MSTP BPDU used not IEEE, and in 3rd question MSTP BPDU (version=3) and maybe even contain IST information included.
So things a bit messed up in my head
Few switches will help me a lot in finding answers, but i will get them from warehouse only in two - three weeks, so this post will at least serve as mention
04-12-2012 03:39 PM
Hello Alexey,
1) When MSTP Region1 conected with CST to MSTP Region2 them exchaneges IEEE BPDU (version = 3) with no MSTI information attached - only IST information included.
I am not 100% sure here, but if I remember correctly, the switches will still send fully populated MST BPDUs including MSTI records to each other, however, only the CST information will be processed from these BPDUs, and the data in the MSTI records will be ignored.
2) When MSTP Region1 conected with CST to PVST+ switch them exchaneges IEEE (version = 0) BPDU replicated on each vlan with no MSTI information and no IST information - whole region presented as 1 switch
Basically correct, although I would not say that the MSTP Region1 is connected to PVST+ switch with CST, rather, it is running PVST+ on the boundary port. The MSTP switch takes the IST ( = MSTI0 ) data and replicates it into PVST+ BPDUs sent out for each existent VLAN allowed on that trunk. This way, the entire MST region behaves as a single PVST+ switch identically configured for all VLANs. Certainly, PVST+ BPDUs have no MSTI records.
3) When MSTP Region1 conected with CST to RPVST+ switch them exchaneges IEEE BPDU (version = 2) replicated on each vlan with no MSTI information and no IST information - whole region presented as 1 switch
The situation here is absolutely identical to the previous point. In addition, it has been my observation that in case of interconnecting the MST and RPVST+ region using trunk links, the MST reverts to PVST+ and not to RPVST+ operation. I was once told that Cisco engineers at the time allegedly considered running RPVST+ Simulation to be too complex (because of the added state related to Proposal/Agreement to keep on per-VLAN basis) and not worthy of implementation. So while in this case the MST region should correctly revert to RPVST+ on its boundary port, in reality, it reverts to PVST+.
And I must confess that Im not sure about BPDU types used by STP, MSTP, PVST+ - it seems to me logical that all of them use same BPDU format (PVST+ use also SSTP BPDU but here i dont take it in to consideration) but with different version numbers and differently treat flags fields
STP and PVST+ use exactly the same BPDU format, with these exceptions:
RSTP and RPVST+ BPDUs differ in exactly the same ways, i.e. their format is absolutely identical, just the encapsulation is different and the RPVST+ BPDU contains the originating VLAN TLV.
MST BPDUs are different because their format is an extension of the RSTP BPDU format - it has the RSTP-compatible part describing the CST data, and MST extension part containing info about MSTIs. However, ordinary RSTP switches should be able to process the CST part from the MST BPDU and ignore the MSTI record.
Does this help a little?
Best regards,
Peter
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