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SWITCHING AND STP?

kollagouthami
Level 1
Level 1

                   hi ,

please help me in solving these questions?

1) what will switch first check when a frame enters its port, is it VLAN or D MAC?

2)in STP when a switch with higher BID(>root BID) is added to the network  and which wont form any loop in the topology, will the STP recalculation occurs  or not? will the new switch sends TCN BPDU to root switch or not?

3)if TCN reaches root bridge then what the root bridge generates TCN ACK or TC ?

4)what is the difference between TCN ACK and TC in root bridge perspective? how these effect the all switches in the topology?

please explain every thing with reason!!!!!

5 Replies 5

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

1) what will switch first check when a frame enters its port, is it VLAN or D MAC? 

Principialy, it needs to evaluate both at the same time because the DMAC has to be matched only among those rows of MAC address table whose VLAN is identical to the VLAN ID in the frame.

To be very precise, when a VLAN-aware switch receives a frame, it must look into the EtherType field of the frame to see whether it carries the value of 0x8100 indicating that this is a tagged frame. If the EtherType is set to 0x8100, the switch must extract both the DMAC and the VLAN ID from the frame and use them in the MAC address table lookup. If the EtherType is different from 0x8100, there is no tag in the frame, and the lookup in the MAC address table lookup is performed using the DMAC and the access/native VLAN of the ingress port.

2)in STP when a switch with higher BID(>root BID) is added to the  network  and which wont form any loop in the topology, will the STP  recalculation occurs  or not? will the new switch sends TCN BPDU to root  switch or not?

If a new switch is added to the network, at least one new link must have come up - the one that connects the added switch to the rest of the network. Both ports on this link have become forwarding - the existing switch will have a Designated Forwarding port on that link, the new switch will have Root Forwarding port on such link. This is the "STP recalculation" in the strict sense of the word. For both STP and RSTP, whenever a port becomes Forwarding, a TCN is generated. So in fact, both switches will generate a TCN message because on both of them, at least one port has become Forwarding. Note that the TCN is not a trigger for "STP recalculation" - the opposite is true: a TCN is generated after a topology change has taken place. The TCN does not serve the purpose of finding a new active topology - it only serves the purpose of flushing MAC address table entries expressly.

3)if TCN reaches root bridge then what the root bridge generates TCN ACK or TC ?

It should generate both - first the TCA, and then the TC.

4)what is the difference between TCN ACK and TC in root bridge perspective? how these effect the all switches in the topology?

The TCA confirms the arrival of a TCN BPDU to its sender. If a switch forwards a TCN out its root port, it expects that the upstream switch confirms this by sending back a TCA. If it does not, the TCN is retransmitted via the root port again and again, each 2 seconds by default, until the TCA arrives back.

Note that a root bridge never receives a TCA - because it does not send TCNs to anyone else so there is nothing to acknowledge. A root bridge only sends a TCA if a TCN arrived through a designated port.

The arrival of a TCN triggers the sending of TC from the root switch. Other switches relay received BPDUs with the TC bit set so that the information about the topology change is spread throughout the entire network, and the switches shorten the MAC address table aging time as a result.

Feel welcome to ask further!

Best regards,

Peter

kollagouthami
Level 1
Level 1

Hi Peter,

Thank you so much for you help.

please explain me the following

1)if two ports of a switch are having the roles as root port and blocking port if root port goes down then how does a TCN BPDU is generated by the swicth to notify the change to the root bridge as port in blocking state can only receive BPDU but cannot process and send?

2)if blocking port is changing to forwarding then how much time does it takes for STP to recalculate ?

is it like 50sec where port will be in blocking for MAX age time ie 20s plus forward delay- in this case what about the ports which are not in blocking (is it just forward delay ie 30s)?

so the bottom line  is   :  STP calculation is port based or switch based in STP?

Regards,

Bharad.

Hi bharad vege,

1)if two ports of a switch are having the roles as root port and blocking port if root port goes down then how does a TCN BPDU is generated by the swicth to notify the change to the root bridge as port in blocking state can only receive BPDU but cannot process and send?

suppose the 0/3 of sw3 fails then when sw3 will not get the superior  bpdu from the root then it will start making its block port 0/1 to  forwarding so it will then generate the TCN.

2)if blocking port is changing to forwarding then how much time does it takes for STP to recalculate ?

From blocking to forwarding it takes 50 sec.---->20sec extra just to age out the previous superior bpdu of some root port

From disable to forwarding it takes 30 sec.

Hi,

what are the RFCs for STP ?

There are no RFCs for STP. They are IEEE standards. The latest one was 802.1D-2004 I think. That one includes RSTP. If you want to see legacy STP then look at 802.1D-1998.

Daniel Dib
CCIE #37149

Daniel Dib
CCIE #37149
CCDE #20160011

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