CCNP Switching Exam Notes: Building Cisco Multilayer Switched Networks : Spanning Tree Protocol

BCMSN - Building Cisco Multilayer Switched networksexam is a requirement towards obtaining CCNP certification. Skills measured are: Designing and implementing complex switched networks.Valid CCNA certification is a pre-requisite for obtaining CCNP certification.

To be CCNP certified, the following exams need to be successfully completed:

Exam Exam Code Study material covering exam objectives
BSCI 642-801 Building Scalable Cisco Internetworks. Note that BSCI replaces Routing Exam (640-603).
Switching Exam 642-811 Building Cisco Multi-layer Switched Network or BCMSN
Remote Access Exam 642-821 Building Cisco Remote Access Networks
Support Exam 642-831 Cisco Internetwork Troubleshooting

Alternatively, one can take the following exams to obtain CCNP certification:

Exam Exam Code Study material covering exam objectives
Foundations Exam 642-891 Composite Building Scalable Cisco Networks,
Building Cisco Multi-layer Switched Network.
Remote Access 642-821 Building Cisco Remote Access Networks
Support Exam 642-831 Cisco Internetwork Troubleshooting.

Exam cram

1.0 Hardware based routing:

Layer 2 switching is based on hardware based bridging, whereas Layer 3 switching is based on hardware based routing. Layer 2 switching is done based on physical (MAC) addresses, whereas Layer 3 switching is based on logical address.

2.0 Spanning Tree Protocol (STP):

The function of Spanning Tree Protocol (STP) is to avoid loops in a switched network. It is achieved by determining the redundant links, and blocking the same. Spanning Tree Algorithm (STA) is used by STP to find redundant links. If the bridging loops are not prevented, a packet may get forwarded again and again, indefinitely.

During the process of Spanning-Tree Algorithm execution, some redundant ports need to be blocked. This is required to avoid bridging loops. To choose which port to use for forwarding frames, and which port to block, the following three components are used by the Spanning-Tree Protocol:
1. Path Cost: The port with lowest path cost is placed in the forwarding mode. Other ports are placed in blocking mode.
2. Bridge ID: If the path costs are equal, then the bridge ID is used to determine which port should forward. The port with the lowest Bridge ID is elected to forward, and all other ports are blocked.
3. Port ID: If the path cost and bridge ID are equal, the Port ID is used to elect the forwarding port. The lowest port ID is chosen to forward. This type of situation may araise when there are parallel links, used for redundancy.

3.0 A switch, participating in Spanning-Tree protocol, passes through the following states:
1. Blocked state: This is the initial state. All ports are put in a blocked state to prevent bridging loops. 
2. Listen state: This is the second state of switch ports. Here all the ports are put in listen mode. The port can listen to frames but can't send. The period of time that a switch takes to listen is set by "fwd delay" . 
3. Learn state: Learn state comes after Listen state. The only difference is that the port can add information that it has learned to its address table. The period of time that a switch takes to learn is set by "fwd delay". 
4. Forward state: A port can send and receive data in this state. Before placing a port in forwarding state, Spanning-Tree Protocol ensures that there are no redundant paths or loops.
5. Disabled state: This is the state when the switch port is disabled. A switch port may be disabled due to administrative reasons or due to switch specific problems.

4.0 When a bridge starts up, the bridge ID is set as root ID. That is, it considers itself as the root bridge. However, while exchanging of BDPUs, if it comes across a BDPU that has a bridge ID lower than its own, then the bridge corresponding to the BDPU is considered as root bridge, and the information is propagated. The bridge ID consists of the following:
1. 2-byte priority: The default value on Cisco switches is 0X8000 (32,768), lower the priority, higher the chances of becoming a root bridge.
2. MAC address: The 6 byte MAC address of the bridge. Lower the MAC address, higher the chances of becoming a root bridge. 
Note that, the bridge (or switch) with lowest value of 2-byte priority will become the root bridge. If the priority value is same, then the bridge with lowest value of 6-byte MAC address will become the root bridge.

5.0 The following methods are used for implementing Spanning-Tree in a VLAN environment:
1. PVST (Per VLAN Spanning Tree): This is a Cisco proprietary method. Requires Cisco ISL encapsulation. Separate instances of Spanning-Tree are for every VLAN.
2. CST (Common Spanning Tree): This is supported by IEEE802.1Q. Here, A single instance of Spanning Tree runs for all VLANs. BPDU information is exchanged on VLAN1
3. PVST+ (Per VLAN Spanning Tree Plus: This is also a Cisco proprietary method for implementing STP in VLAN environment.

PVST+ is available with Catalyst 4.1 release or above. Switches before release 4.1 are compatible with PVST implementation of Spanning-Tree. Note that PVST+ is backward compatible. PVST+ is also compatible with 802.1Q implementation of CST (Common Spanning Tree) protocol. PVST+ is in fact requires no configuration to make it compatible with PVST (Plug and play compatible).

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