Purpose of Spanning Tree Redundancy at OSI Layers 1 and 2 Multiple cabled paths between switches: • Provide physical redundancy in a switched network. • Improves the reliability and availability of the network. • Enables users to access network resources, despite path disruption.
Purpose of Spanning Tree Issues with Layer 1 Redundancy: MAC Database Instability • Ethernet frames do not have a time to live (TTL) attribute. • Frames continue to propagate between switches endlessly, or until a link is disrupted and breaks the loop. • Results in MAC database instability. • Can occur due to broadcast frames forwarding. • If there is more than one path for the frame to be forwarded out, an endless loop can result. • When a loop occurs, it is possible for the MAC address table on a switch to constantly change with the updates from the broadcast frames, resulting in MAC database instability.
Purpose of Spanning Tree Issues with Layer 1 Redundancy: Broadcast Storms • A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. It is also known as denial of service • A broadcast storm is inevitable on a looped network. • As more devices send broadcasts over the network, more traffic is caught within the loop; thus consuming more resources. • This eventually creates a broadcast storm that causes the network to fail.
Purpose of Spanning Tree Issues with Layer 1 Redundancy: Duplicate Unicast Frames • Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device. • Most upper layer protocols are not designed to recognize, or cope with, duplicate transmissions. • Layer 2 LAN protocols, such as Ethernet, lack a mechanism to recognize and eliminate endlessly looping frames.
STP Operation Spanning Tree Algorithm: Introduction • STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop. • A port is considered blocked when user data is prevented from entering or leaving that port. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops. • The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. • If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active.
Minimum Spanning Trees (MST) • To find the minimum number of edges necessary to connect all the edges in a graph. • It can be implemented using DFS or BFS. A A B C D E Extra Edges B C D E Minimum Number of Edges
Searches • Depth-first search • The algorithm acts as through it wants to get as far away from the starting point as quickly as possible. • Can use a stack • Breadth-first search • The algorithm likes to stay as close as possible to the starting points. • Can use a queue
Depth-First Search B A F H C D E G I • Stack Contents Event Stack Visit A A Visit B AB Visit F ABF Visit H ABFH Pop H ABF Pop F AB Pop B A Visit C AC Pop C A Visit D AD Visit G ADG Visit I ADGI Pop I ADG Pop G AD Pop D A Visit E AE Pop E A Pop A Empty
Breadth-First Search B F H • Queue Contents Event Queue (Front to Rear) Visit A A C D E G I Visit B B Visit C BC Visit D BCD Visit E BCDE Remove B CDE Visit F CDEF Remove C DEF Remove D EF Visit G EFG Remove E FG Remove F G Visit H GH Remove G H Visit I HI Remove H I Remove I Empty Done
STP Operation Extended System ID In the example, the priority of all the switches is 32769. The value is based on the 32768 default priority and the VLAN 1 assignment associated with each switch (32768+1).
PVST+ Overview of PVST+ Networks running PVST+ have these characteristics: • A network can run an independent IEEE 802.1D STP instance for each VLAN in the network. • Optimum load balancing can result. • One spanning-tree instance for each VLAN maintained can mean a considerable waste of CPU cycles for all the switches in the network. In addition to the bandwidth that is used for each instance to send its own BPDU.
PVST+ Configuration PortFast and BPDU Guard When a switch port is configured with PortFast that port transitions from blocking to forwarding state immediately. BPDU guard puts the port in an error-disabled state on receipt of a BPDU.
PVST+ Configuration PVST+ Load Balancing • Another method to specify the root bridge is to set the spanning tree priority on each switch to the lowest value so that the switch is selected as the primary bridge for its associated VLAN.
Rapid PVST+ Overview of Rapid PVST+ • RSTP is the preferred protocol for preventing Layer 2 loops in a switched network environment. • With Rapid PVST+, an independent instance of RSTP runs for each VLAN. • RSTP supports a new port type: an alternate port in discarding state. • There are no blocking ports. RSTP defines port states as discarding, learning, or forwarding. • RSTP (802.1w) supersedes STP (802.1D) while retaining backward compatibility • RSTP keeps the same BPDU format as IEEE 802.1D, except that the version field is set to 2 to indicate RSTP, and the flags field uses all 8 bits.
Rapid PVST+ Link Types The link type can determine whether the port can immediately transition to forwarding state. Edge port connections and point-to-point connections are candidates for rapid transition to forwarding state.
Summary • IEEE 802.1D is implemented on Cisco switches on a per-VLAN basis in the form of PVST+. This is the default configuration on Cisco switches. • RSTP, can be implemented on Cisco switches on a per-VLAN basis in the form of Rapid PVST+. • With PVST+ and Rapid PVST+, root bridges can be configured proactively to enable spanning tree load balancing.