Exam 350-401 topic 1 question 598 discussion

This seems to be a frequent topic in cisco forums. DSW2 is not going to switch to PVST+ mode, only the affected interfaces are going to adjust their behavior to the old style PVST+ device. The output shows that DSW2 runs "protocol rstp" i.e. cisco rapid-pvst+, whereas the new switch (connected to DSW2) runs PVST+. Hence we see "P2p Peer(STP)" under "Type". Source: https://ccie2012.wordpress.com/2011/12/27/what-p2p-peerstp-means-in-the-show-spanning-tree-output/ "The RPVST+ switch should detect that the other switch is running PVST+ by the incoming BPDUs. It would then revert to sending 802.1D BPDUs on that interface and rely on timers instead of synchronization." Source: https://community.cisco.com/t5/switching/rstp-rapid-pvst-compatibility/td-p/2400863 "An RSTP bridge that receives an STP BPDU know that it's connected to a legacy device and start sending STP BPDUs itself. This mechanism will happen on a per-vlan basis with Cisco PVST/Rapid-PVST." Source: https://community.cisco.com/t5/switching/mix-pvst-and-rstp/td-p/1345045

I think it is A = the RSTP one downgrades to PVST+ so both run PVST+ now. https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/17-3/configuration_guide/lyr2/b_173_lyr2_9500_cg/configuring_spanning_tree_protocol.html#spanning-tree-interoperability-backward-compatibility

D is correct

stupid question. both A and D can be correct, depending on how you want to understand the question. the DSW2 interface connected to the new switch will run PVST+ and other interfaces will be RPVST+ anyway I will stay with D

I believe it is D. It is true that DSW2's interfaces connected to the PVST+ peer will operate in PVST+ mode (Peer(STP)), but the mode of operation remains Rapid PVST+, as highlighted by the line "Spanning tree enabled protocol rstp"

D is the correct answer. Rapid PVST+ is backwards compatible with PVST.

From the output we see DSW2 is running in RSTP mode (in fact Rapid PVST+ mode as Cisco does not support RSTP alone). When a new switch running PVST+ mode is added to the topology, they keep running the old STP instances as RSTP (in fact Rapid PVST+) is compatible with PVST+.

Tested in LAB. Just on that shared interface both switches run PVST+. Switch runs RSTP on other interfaces.

sw1#show spanning-tree vlan 10 detail | i compat VLAN0010 is executing the rstp compatible Spanning Tree protocol sw1# !!!!!!!!!!! sw1 [Rapid PVST + ] <---> sw2 [PVST + ] sw2#show spanning-tree vlan 10 detail | i compat VLAN0010 is executing the ieee compatible Spanning Tree protocol sw2#

D. looks true, but not sure ... sw1# sw1#sh runn | include spann spanning-tree mode pvst spanning-tree extend system-id sw1# sw1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address aabb.cc00.4000 Cost 100 Port 66 (Port-channel2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.cc00.1000 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/3 Desg FWD 100 128.4 Shr Po2 Root FWD 100 128.66 Shr

A looks OK to me.