Figure 67 Configuration BPDU

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Confi guration B PDU s Plow from the root bridge to others. A BPDU is an IEEE 802.3 frame with 802.2 LLC SAP header. The Type is 0 because it is a configuration BPDU rather than a topology change BPDU. The root bridge at the time of this capture was the CatOS sw2900 that has a MAC that ends in 1400° The defau 1 t priority is 8000 hex. The root path cost is IL0. The sending BID is from the sw1 9900 tlcat Li as a MAC that ends in 7680. The max age is 20 seconds, the hellos are every 2 seconds, and the forward delay is 15 seconds as discussed earlier. Compare Figure 6-7 to the following headers:

- Source MAC is toe Catalyso port that senP the BPDU o

- Destination MAC i s rhe IEEE 802.1D STP mult ¡cast address u 180c2000000. • LLC header:

- DSAP a p0 SSAP are both 0x42 fot STP. • The configuration BPDU:

rProtocol ID Always 0. -Vemion A Iways 0.

-Tyge 0 for configunation PP DU; 0x80 foa topo logy chang e n otiPcatiot (TCN BPDU). -Flags Topology c hange or acknowledgement.

-Root IBID Concatenation of priority an d MAC to popm tPe nridge ID foo t he root bridge.

-Root Path Cost Cumulative cost of links toward the root bridge. -Sender BID Bridge ID of the sender of the BPDU. -Port ID Unique value for port that sent the BPDU.

-Message Age Time since the root bridge first created this BPDU. This is not max age. Think of it like a reverse TTL where 1 is added at every bridge hop.

-Max Age Time period to save BPDU information.

-Rello Time Time interval between BPDUs.

-Forwand Delay For listening and learning states.

STP convergence time is 20 max age + 30 for the two 15-second forward delays, for a total of 50 seconds. Tools such as Sniffer Pro and the CatOS or IOS spanning-tree commands in Table 6-7 or even more automated tools such as the CiscoWorks Campus Manager can lend a hand in drawing a very informative Layer 2 STP diagram to assist you in supporting STP issues.

TCN BPDUs are one way to speed up this convergence. They flow from the spoke bridges to the center on the wheel ro ot b ridge to let the root bri dge know t hat the topolo gy has ch anged. You acklyze TCNs in more detail in the Trooble Tickets.

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