1. Purpose and Functionality

• RSTP (Rapid Spanning Tree Protocol):

  • RSTP is an evolution of the original IEEE 802.1D STP (Spanning Tree Protocol). Its primary goal is to improve the convergence time of STP.
  • Faster Convergence: RSTP significantly reduces the time required for the network to converge (after a topology change, such as a link failure). RSTP achieves convergence within seconds (typically 1–2 seconds), compared to STP which can take 30–50 seconds.
  • Backwards Compatibility: RSTP is backward compatible with legacy 802.1D STP devices. Devices that support RSTP can still operate in mixed networks with devices running the older STP.

• MSTP (Multiple Spanning Tree Protocol):

  • MSTP is an extension of RSTP that allows for the creation of multiple spanning tree instances (MSTIs) across the network. It is defined in IEEE 802.1s, which is an extension to the original 802.1Q (VLAN Tagging) standard.
  • Multiple VLANs in a Single STP Instance: MSTP allows multiple VLANs to be mapped to a single spanning tree instance, enabling more efficient use of network resources.
  • Optimized for VLAN-Based Networks: MSTP is particularly useful in large VLAN-based networks, as it allows for more granular control over which VLANs are assigned to different spanning tree instances.

2. Convergence Time

• RSTP:

  • Rapid Convergence: RSTP offers much faster convergence than the original STP by improving how switches transition between states.
  • Edge Port and Proposal/Agreement Mechanism: RSTP uses a proposal/agreement mechanism and edge port concept to speed up convergence. When a topology change happens, the network is able to quickly converge to a loop-free state.
  • Convergence Time: Typically within 1–3 seconds after a link failure.

• MSTP:

  • Convergence Based on RSTP: MSTP inherits the rapid convergence characteristics of RSTP. However, since MSTP deals with multiple spanning tree instances (each with its own topology), it may involve more complexity in determining the optimal path across those instances.
  • Convergence Time: The convergence time in MSTP is generally faster than legacy STP, but may vary depending on the number of MSTI regions and the network's overall design.

3. Spanning Tree Instances

• RSTP:
  • RSTP only uses a single spanning tree instance for the entire network, meaning one topology for all VLANs.
  • Every VLAN shares the same STP instance, which can lead to inefficiencies in networks with many VLANs.
• MSTP:
  • MSTP allows the creation of multiple spanning tree instances (MSTIs). Each MSTI can have its own topology and root bridge, which is particularly beneficial in multi-VLAN networks.
  • This means that different VLANs can have different spanning tree paths, which improves network resource utilization and load balancing by allowing more efficient use of redundant links across VLANs.

4. Compatibility with VLANs

• RSTP:
  • Single STP Instance: In RSTP, all VLANs share a single spanning tree, so the same topology is used for every VLAN. This can create inefficiencies in large VLAN environments because all VLANs will use the same set of paths, even if certain paths are more efficient for some VLANs than others.
• MSTP:
  • Multiple STP Instances: MSTP supports multiple spanning tree instances and allows administrators to assign specific VLANs to particular instances. This means each VLAN or group of VLANs can have its own spanning tree topology, optimizing the network’s performance and load balancing across different VLANs.
  • MSTP maps multiple VLANs to MSTI instances, allowing better use of redundant links and reducing the number of blocked ports for VLAN traffic.

5. Protocol Design

• RSTP:

  • RSTP enhances the original STP by introducing new port roles and states to enable quicker convergence.
    • Port Roles in RSTP:
      • Root Port (RP): The port on a non-root bridge that is closest to the root bridge.
      • Designated Port (DP): The port on a bridge that has the best path to a particular segment of the network.
      • Alternate Port: A backup port that can quickly take over if the active port fails.
    • Port States in RSTP: The states include Discarding, Learning, and Forwarding, but the transition times are significantly faster than in standard STP.

• MSTP:

  • MSTP uses the same rapid convergence mechanisms as RSTP but extends this functionality to work with multiple spanning tree instances (MSTIs).
  • MSTP creates MST regions, where multiple switches agree on the configuration of the multiple spanning tree instances (MSTI). A unique MSTP configuration is shared across switches in the same region.

6. Network Complexity

• RSTP:
  • Simpler Configuration: RSTP is simpler to configure because it uses a single spanning tree for the entire network. It doesn't require complex mappings between VLANs and spanning tree instances.
• MSTP:
  • More Complex: MSTP is more complex because it allows for multiple spanning tree instances and requires careful configuration of which VLANs map to which MSTI.
  • MSTI Regions: MSTP requires the network to be divided into MST regions, and the configuration must be consistent across all switches in the region to avoid misconfigurations.

7. Use Cases

• RSTP:

  • Best for smaller to mid-sized networks that don't have a high number of VLANs or complex topologies.
  • Suitable for networks where rapid convergence is needed but there is no significant need to separate VLANs into different spanning tree instances.

• MSTP:

  • Ideal for large VLAN-based networks that require more efficient use of redundant links and better load balancing across VLANs.
  • Large enterprise networks or data centers where multiple VLANs need to be handled with distinct STP topologies for improved efficiency.

Summary of Key Differences

Purpose of the Root Bridge in STP

1. Central Point for Path Selection:

   • The root bridge is the reference point for all path calculations in the STP topology. All other switches in the network calculate the shortest path to the root bridge using a tree-like structure called the spanning tree.
   • It ensures that the entire network converges to a loop-free topology by making all switches aware of a single point of reference for forwarding traffic.

2. Determines the Best Paths:

   • The root bridge determines the shortest path from itself to all other switches in the network. Each switch in the network determines its root path cost to the root bridge, which is based on the port and link costs.
   • The switch with the lowest path cost to the root bridge is selected as the root port on each non-root bridge (switch).

3. Prevents Broadcast Storms and Network Loops:

   • STP ensures there are no bridging loops in the network, which can occur when redundant paths exist between switches. If loops form, broadcast frames will circulate endlessly, overwhelming the network and causing instability.
   • The root bridge helps avoid such loops by providing a consistent topology where only one active path is used to reach any given destination. Redundant paths are put into a blocked state to prevent loops, though they remain available in case the active path fails.

4. BPDU Election Process:

   • The root bridge is elected through a process in which all switches in the network exchange Bridge Protocol Data Units (BPDUs). Each BPDU contains information about the bridge priority, MAC address, and path cost of the sending switch.
   • Initially, all switches believe themselves to be the root bridge. As BPDUs are exchanged, each switch evaluates the BPDUs it receives and selects the switch with the lowest bridge ID (which consists of the bridge priority and MAC address) as the root bridge.
   • The root bridge has the lowest bridge ID across the entire network, and it is the switch that sends out the "best" (lowest-cost) BPDUs.

5. Root Bridge Role in Forwarding Traffic:

   • Root Bridge: The root bridge has no root port since it is the root of the network. It can forward traffic to other switches, but it does not have to choose a path to reach other switches because it is already at the top of the tree.
   • Non-Root Bridges: These switches have a root port (the port with the lowest cost path to the root bridge). Each non-root bridge also has designated ports, which forward traffic towards the root bridge or out to other networks.

6. STP Stability and Network Convergence:

   • The root bridge plays a key role in network stability by establishing a consistent point of reference for network-wide path selection and ensuring that all switches in the topology converge to the same loop-free topology.
   • Once the root bridge is elected, all switches converge on the root bridge and calculate the best forwarding paths, which minimizes network instability and ensures convergence after topology changes.

How the Root Bridge is Selected

• When STP is first enabled, all switches start by assuming they are the root bridge. Each switch sends out BPDUs containing its Bridge ID (a combination of its MAC address and priority value).
• The root bridge is chosen based on the lowest bridge ID. The priority value can be adjusted to influence the election process, but by default, it is 32768. The switch with the lowest priority value (and, in case of a tie, the lowest MAC address) becomes the root bridge.
• Once the root bridge is selected, other switches in the network adjust their roles and assign root ports (ports that provide the lowest cost path to the root bridge) and designated ports (ports responsible for forwarding traffic).

Key Roles of the Root Bridge in STP:

• Root Bridge Election: Determines the central point of the network topology.
• Path Calculation: All other switches in the network calculate their best paths to the root bridge.
• Loop Prevention: Ensures the network topology remains loop-free by blocking redundant paths and maintaining a tree-like structure.
• Traffic Forwarding Reference: All non-root bridges forward traffic towards the root bridge through their root ports and designated ports.

BGP Path Selection Criteria

1. Highest Weight:

   • The path with the highest weight is preferred. Weight is a Cisco-specific parameter and is local to the router on which it is configured. It is not propagated to other routers.

2. Highest Local Preference:

   • The path with the highest local preference is preferred. Local preference is a well-known discretionary BGP attribute and is used within an autonomous system (AS) to prefer one path over others.

3. Locally Originated:

   • Paths that are originated by the local router (indicated by the network or aggregate BGP subcommands) are preferred over paths received from other routers.

4. Shortest AS Path:

   • The path with the shortest AS path is preferred. The AS path length is the number of ASes the route has traversed. This helps avoid routing loops and generally indicates a shorter path.

5. Lowest Origin Type:

   • The origin attribute indicates how BGP learned about the route. The preference order is: IGP (i), EGP (e), and Incomplete (?), where IGP is preferred over EGP, and EGP is preferred over Incomplete.

6. Lowest Multi-Exit Discriminator (MED):

   • The path with the lowest MED is preferred. MED is used to influence inbound traffic from neighboring ASes and is propagated between different ASes.

7. eBGP over iBGP Paths:

   • Paths learned from eBGP peers are preferred over paths learned from iBGP peers. This preference helps ensure that the path with the shortest external route is used.

8. Lowest IGP Metric to the BGP Next-Hop:

   • The path with the lowest IGP metric to the BGP next-hop is preferred. This means that the path which is closest (in terms of IGP cost) to the next-hop router is chosen.

9. Oldest Path:

   • If multiple paths are still equal after considering all the above criteria, the oldest path (the one that has been in the BGP table the longest) is preferred. This helps ensure stability by reducing route flapping.

10. Lowest BGP Router ID:

    • If paths are still equal, the path with the lowest BGP router ID is preferred. The router ID is a unique identifier for the BGP router, usually the highest IP address on the router or a manually configured value.

11. Lowest Neighbor Address:

    • If all else is equal, the path with the lowest neighbor IP address (the IP address of the BGP peer) is preferred.

Example Scenario

Imagine a BGP router has received multiple routes to the same destination from different BGP peers. Here's how it might select the best path:

1. Check Weight: If one path has a higher weight than the others, it is chosen.
2. Check Local Preference: If weights are equal, the router checks local preference. The path with the highest local preference is chosen.
3. Check Origin: If local preferences are equal, the router checks if any of the routes were originated locally.
4. Check AS Path Length: If local origination is equal, the router checks the AS path length and prefers the shortest one.
5. Check Origin Type: If AS paths are equal, the router checks the origin type and prefers IGP over EGP, and EGP over Incomplete.
6. Check MED: If origin types are equal, the router checks the MED and prefers the lowest value.
7. Check eBGP over iBGP: If MEDs are equal, the router prefers eBGP paths over iBGP paths.
8. Check IGP Metric to Next-Hop: If the eBGP/iBGP status is equal, the router checks the IGP metric to the BGP next-hop and prefers the lowest metric.
9. Check Oldest Path: If IGP metrics are equal, the router checks which path has been in the BGP table the longest and prefers it.
10. Check Router ID: If paths are still equal, the router checks the BGP router ID and prefers the lowest ID.
11. Check Neighbor Address: If router IDs are equal, the router checks the neighbor IP address and prefers the lowest one.

• AS Path
• Next HOP
• ORIGIN
• Local Preference