RSTP and PC reboots

[mikehjorten]

I have a network of 4 switches in a ring config, 3 2955 and 1 2950. They have a few different versions of IOS, 12.1(19)EA1c to 12.1(22)EA1. I have RSTP enabled on all switches with default RSTP settings.

The backbone ports are set:

interface FastEthernet0/25

switchport mode trunk

no ip address

spanning-tree link-type point-to-point

or

interface FastEthernet0/13

switchport mode trunk

switchport nonegotiate

auto qos voip trust

macro description cisco-switch

spanning-tree link-type point-to-point

depending on whether i had the CNA macro availible or not.

The issue is that anytime that i plug in a device or reboot a device that is connected to an edge port the network reconverges. I have configured all the edge ports on the net with portfast to minimize traffic disruption, but i view that as a band-aid.

I thought that RSTP only reconverges when it gets a BPDU that contains new information about a path to the root. It seems that the act of plugging any device in causes a TCN to propagate through the network. I thought that only plugging in another switch would cause a TCN to be generated. Any ideas or clarification?

Thanks,

Mike

[smothuku]

Hi Mike ,

Rapid Convergence of RSTP :

The RSTP provides for rapid recovery of connectivity following the failure of switch, a switch port, or a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through point-to-point links as follows:

Edge ports?If you configure a port as an edge port on an RSTP switch by using the spanning-tree portfast interface configuration command, the edge port immediately transitions to the forwarding state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports that connect to a single end station.

Root ports?If the RSTP selects a new root port, it blocks the old root port and immediately transitions the new root port to the forwarding state.

Point-to-point links?If you connect a port to another port through a point-to-point link and the local port becomes a designated port, it negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology.

And

The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch on that LAN. The port role in the proposal message is always set to the designated port.

The sending switch sets the agreement flag in the RSTP BPDU to accept the previous proposal. The port role in the agreement message is always set to the root port.

The RSTP does not have a separate topology change notification (TCN) BPDU. It uses the topology change (TC) flag to show the topology changes. However, for interoperability with 802.1D switches, the RSTP switch processes and generates TCN BPDUs.

The learning and forwarding flags are set according to the state of the sending port.

check the below link for RSTP.

http://www.cisco.com/en/US/customer/products/hw/switches/ps628/products_configuration_guide_chapter09186a00800c6ef8.html

Thanks,

satish

[tdrais]

Part of your issue is that the way cisco defines a "EDGE" port in RSTP is with the portfast option. If you do not use this option it treats the port as a normal port and will generate topology change notifications. How big a issue if any this will cause will depend where the port is in relation to the root bridge.

[mikehjorten]

So if i plug anything into anyport i'll get a topology change? I understand that portfast will basically take the port out of the RSTP reconverge and forward traffic right away. The main question that I have is, why do I get a topology change when i plug in a PC? I thought that only plugging a swtich in would do that.

[Francois Tallet]

As soon as a port comes forwarding, RSTP generates a TC. As you said, this behavior is not desirable on edge ports, where you connect your pc. STP has no way of guessing whether the port is connected to a PC or a switch (a switch might be silent and may never send out a BPDU). So you have to configure this information using the portfast command. This is not a band-aid solution, this is the way the protocol is intended to be configured. Note that configuring portfast does not remove the port from the spanning-tree. As it was said earlier it:

-transition the port to forwarding immediately at link up

-prevents topology changes from being generated when the link goes forwarding.

If you receive a BDPU later on an edge port, it loses its status of edge port. STP is still running on edge ports, and BPDU are still continuously being sent on them.

Regards,

Francois

PS: a TC is not really considered as a reconvergence of the STP. It just updates (in fact flush) the cam entries of the switches in the network. The handling of the cam entries is done independently of STP (even though STP has an influence on it).