Hi all,
I have some confusion of the spanning tree terminologies I came across while studying. Can someone explain me clearly on what they and how different they are from others?
Sorry for posting so many questions in one thread but I just do not what to run multiple threads on this.
1. PVST - I know that this is the legacy STP protocol (cisco proprietary) and will run on ISL trunk links. But in my router I see only 3 modes for STP namely PVST, MST and RAPID-PVST. So if I choose to run pvst mode over dot1q trunk links do I essentially run PVST+ or still I run only PVST?
2. PVST+ - Continuation to above question how to run PVST+ mode on my routers as I see only the above mentioned 3 modes available.
3. Spanning tree enabled protocol ieee - When I see this when I execute "show spanning-tree vlan x" which mode do I assume PVST or PVST+?
4. CST - What is a common spanning tree and why it is needed? I read in some document mentioning that is will run on STP instance for all VLANs to reduce switch load but does it essentially mean a flavor of MST mode?
5. IST - Why do we need an IST instance inside an MST regions? It says that it will only generate the BPDUs inside a region but how will it differentiate different instances inside MST? Any useful document link or video link here would be very useful to me.
6. rapid-pvst - When I choose to run this mode do I essentially run r-pvst or r-pvst+ mode?
7. Root bridge selection - I know the process of a root bridge selection but my doubt is why when a switch with a lowest bridge is added to the network again an election happens? I am comparing this to DR/BDR election in OSPF network where the re-election will happen only when the already elected routers fail.
8. BPDUGUARD Vs BPDUFILTER - What is the difference between these two features? Both of them are anyways filtering the bpdu then why two separate features?
9. Aging Time 300 - I see this under "Bridge ID" section of "show spanning-tree vlan x" output. What does this signify and how this is helpful? The mac-address table aging time?
10. show spanning-tree bridge id - The output of this command is as follows.
nPE1#show spanning-tree bridge id
VLAN0001 8001.0021.d8ca.c340
VLAN0002 8002.0021.d8ca.c340
What does the 8001 and 8002 signify? I was thinking of bridge priority (with vlan number) and mac-address as BID of a switch. But this command yields some different output.
11. RLQ request/response - I know that these messages are used in backbonefast feature but are they a different type of BPDUs or it is embedded in conf BPDUs? To my knowledge I know that only 2 types of BPDUs exists. Configuration and TCN BPDUs. Are these are different kind? I tried to captures after enabling the backbonefast feature but I could not find in my wireshark. I turned on the debugs where it says that it received rlq messages but I couldnt see in packet capture
12. STP/R-STP Timers - In legacy stp, all switches send the BPDUs to the root and the root inturn propagates to all other switches. So why the timer configured on root bridge will take effect on all swithces. But in RSTP all switches all the capability to send BPDUs then if there is a mismatch in STP timers between itself and the root, which one will be agreed upon?
Hopefully I have listed down all the doubts I had in my mind. I will be glad enough if someone clarifies all these points.
Thanks,
Madhu
