CCNP3 v3 Module 3: Spanning Tree
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Fundamental topics continued from the previous curriculum Spanning Tree Protocol concepts •STP Processes •Steps to STP convergence, root and designated port elections and states •BPDU format •Topology changes •Enhancements such as portfast, uplink fast and backbone fast •Etherchannel groups •Configuration of STP fundamentals
© 2002, Cisco Systems, Inc. All rights reserved.
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New Topics in this curriculum Rapid Spanning Tree Protocol 802.1w •States •port roles •timers •BPDU format Evolution of Spanning Tree •PVST+ 802.1q and Enhanced PVST + •MST 802.1s •Load Balancing •BPDU guard •Root guard Configuration of RSTP, 802.1q, 802.1s, load balancing, BPDU guard and Root Guard © 2002, Cisco Systems, Inc. All rights reserved.
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New Configurations covered in this curriculum Implementation of : •Rapid Spanning Tree Protocol 802.1w (RSTP); •Per VLAN Spanning Tree 802.1q (PVST +); •Multiple Spanning Tree 802.1s (MST); •Load balancing across links; •BPDU guard; •Root Guard; and •Uni-Directional Link Detection (UDLD)
New Topic Areas will be are covered in detail in the remainder of this presentation © 2002, Cisco Systems, Inc. All rights reserved.
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802.1w Rapid Spanning Tree Protocol The IEEE 802.1w specification, Rapid Spanning Tree Protocol, provides for subsecond reconvergence of STP after failure of one of the uplinks in a bridged environment. 802.1w provides the structure on which the 802.1s features such as multiple spanning tree operates. There are only three port states left in RSTP corresponding to the three possible operational states Learning ,Forwarding and Discarding. Rapid Transition to Forwarding State is the most important feature introduced by 802.1w: • RSTP actively confirms safe port transition to forwarding without relying on timers; • There is now a real feedback mechanism that takes place between RSTPcompliant bridges. •In order to achieve fast convergence on a port, the protocol relies upon two new variables: edge ports and link type. © 2002, Cisco Systems, Inc. All rights reserved.
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RSTP Port Roles The RSTP role is now a variable assigned to a given port: •Root Port •Designated Ports •Alternate Ports •Backup Ports RSTP calculates final topology using exactly the same criteria as 802.1d. There is now a difference between the role the protocol has determined for a port and its current state.
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RSTP Timers When STA selects a port to become a designated port, under 802.1d it waits twice seconds (2x15 by default) before transitioning it to the forwarding state. In RSTP a root bridge aware of a change in the topology, sets the TC flag on the BPDUs it sends out, which are then relayed to all the bridges in the network.
© 2002, Cisco Systems, Inc. All rights reserved.
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RSTP BPDU Format A few changes have been introduced by RSTP to the BPDU format. Only two flags, Topology Change (TC) and TC Acknowledgment (TCA), were defined in 802.1d, however RSTP now uses all six remaining bits of the flag byte
BPDU are sent every hello-time (not simply relayed anymore). BPDUs are now used as a keep-alive mechanism between bridges. A bridge considers that it has lost connectivity to its direct neighboring root or designated bridge if it misses three BPDUs in a row.
© 2002, Cisco Systems, Inc. All rights reserved.
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Evolution of Spanning Tree The following developments in Spanning Tree Protocol are examined: Per-VLAN Spanning Tree (PVST) is a Cisco-proprietary implementation requiring ISL trunk encapsulation. PVST+ provides Layer 2 load balancing for the VLAN on which it runs. MST (IEEE 802.1s) extends the IEEE 802.1w Rapid Spanning Tree (RST) algorithm to multiple spanning-trees. Enhanced PVST + or Multiple Instance of Spanning Tree Protocol (MISTP), a compromise between PVST+ and MST.
© 2002, Cisco Systems, Inc. All rights reserved.
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Evolution of Spanning Tree Load sharing divides the bandwidth supplied by parallel trunks connecting switches. BPDU guard •The STP PortFast BPDU guard enhancement is designed to allow network designers to enforce the STP domain borders and keep the active topology predictable. Root Guard •The Root Guard ensures that the port on which it is enabled is the designated port
© 2002, Cisco Systems, Inc. All rights reserved.
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Implementing New Spanning Tree features RST and MSTP configuration Configuring MST : •on a root switch; •Port Priority; •Path cost; •Switch priority; •MTP timers; and •Max Hop count. •BPDU guard and Root Guard Uni-Directional Link Detection (UDLD)
© 2002, Cisco Systems, Inc. All rights reserved.
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© 2002, Cisco Systems, Inc. All rights reserved.
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