[c-nsp] "The TCN only has an impact on the aging time"

[Kamlesh Sharma]

Hi Kim,
Hi Phil,

I just got this information just check it out.
<cisco-nsp at puck.nether.net>TCN from Root will set the aging time = (max age
+ forwarding delay)
but switches will set aging time 15 sec.

And there are some rules like
Ingress and Egress Rules

•The CST BPDU (of VLAN 1, by default) is sent to the IEEE address.

•All the other BPDUs are sent to Shared Spanning Tree Protocol
(SSTP)-Address and encapsulated with Logical Link Control-Subnetwork Access
Protocol (LLC-SNAP) header.

•The BPDU of the CST and BPDU of the VLAN equal to the PVID of the
802.1Qtrunk are sent untagged.

•All other BPDUs are sent tagged with the VLAN ID.

•The CST BPDU is also sent to the SSTP address.

•Each SSTP-addressed BPDU has a Tag-Length-Value (TLV) appended to it. This
TLV contains the VLAN ID of the spanning tree to which the BPDU belongs and
is used to check the PVID.

The BPDU reception on the 802.1Q port of a PVST+ router will follow these
rules:

•All untagged SSTP-addressed BPDUs must be received on the PVID of the
802.1Q port.

•IEEE-addressed BPDUs will be processed by the CST.

•If the SSTP-addressed BPDU does not have an 802.1Q tag (that is, if it
originated from the PVID of the sending 802.1Q port), it will still contain
a TLV. The TLV contains the PVID of the sending port, which the receiver
will compare with the PVID of the receiving port. If these two values do not
match, the port is put into Blocking state due to PVID inconsistency.

•All SSTP-addressed BPDUs whose VLAN ID is not equal to the CST are
processed by the spanning tree of that particular VLAN ID.

•The SSTP-addressed BPDUs whose VLAN ID is equal to the CST are dropped.
These BPDUs are used for consistency checking only.

Simply it does mean that if we recieve the SSTP TCN from the root switch for
the tagged VLAN than that VLAN mac address only will get age out.

Now need to lab it up
thanks
kamlesh sharma

On 7/14/07, Phil Bedard <philxor at gmail.com> wrote:
>
> With 802.1d, when the switch receives a TCN from the root it will
> drop the mac-address aging time to 15 seconds.
> The root switch will send PDUs with the TCN flag set for max age
> +forwarding delay= 35 seconds.
>
> I'm fairly certain it only ages out the mac table entries associated
> with the STP instance that received
> the TCN.  Using per-vlan spanning tree.
>
> If you are using Portfast on the access port, it doesn't generate a TCN.
>
> All of this only applies to 802.1d, if you are using 802.1w (RSTP) it
> uses a different mechanism for topology notification
> and just flushes the mac table instead of lowering the aging time.
>
> Phil
>
> On Jul 14, 2007, at 9:41 AM, Kamlesh Sharma wrote:
>
> >  As stated in the sentence
> > "The TCN only has an impact on the aging time"
> >
> > When there is a TC-N received by the switch. It will change it's
> > aging time
> > to 15 sec or max_age + forwarding delay = 35.
> >
> > My confusion is :
> > it will change it's aging time for whole mac forwarding table
> > or
> >
> > Case 1 - TCN recieved on access port for VLAN 20
> > age out all mac address learned on that access VLAN 20
> >
> > Case 2 - TCN received on trunk port for VLAN 2 and VLAN 3
> >
> > age out for all mac address learned on the trunk for VLAN 2 and VLAN 3
> >
> > Case 3 - TCN received on trunk port for all allowed VLAN
> >
> > age out for whole MAC forwarding table.
> >
> > Thanks
> >
> > --
> > Thanks
> > Kamlesh Sharma
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>
>
>
>
>


-- 
Thanks
Kamlesh Sharma