Cisco
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Rapid Spanning Tree Protocol (802.1W)
IEEE 802.1W or Rapid Spanning Tree Protocol (RSTP) takes the improvements and concepts from propriety protocols such as PVST and PVST+, which were spanning tree instances that ran on a per-VLAN basis rather than across the entire topology. A per-VLAN spanning tree instance can introduce advantages such as allowing simple load balancing and traffic steering.…
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STP Topology Changes
When a change in the topology occurs on a spanning tree protocol activated network, it has an affect on all switches in that topology. The switch that detects a link status change on one of its links will send out a topology change notification (TCN) BPDU towards the root bridge out of its root port.…
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STP Topology Generation
Election of a Root Bridge The first step in any Spanning Tree Protocol topology generation is to identify the root bridge. When a switch initialises it will assume that it is the root bridge and uses its local bridge identifier as a root bridge identifier. The ports on the local switch will then transition to…
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Spanning Tree Protocol IEEE 802.1D
The original version of the spanning tree protocol comes from the IEEE 802.1D standard. Port States It provides support to ensure a loop free topology on a single VLAN by transitioning network ports through various states (some are timed delayed by a STP Forwarding timer that by default is 15 seconds): State Description Disabled The…
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Spanning Tree Protocol
Spanning Tree Protocol, or STP, allows switches to be aware of other switches through the sending and receiving of BPDUs (Bridge Protocol Data Units). Spanning Tree Protocol can build a Layer 2 loop free topology by blocking traffic on redundant ports. Spanning Tree Protocol operates by selecting a master switch and running an algorithm to…
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Switching Database Manager (SDM) Templates
The number of MAC addresses that a switch may need to store in comparison to the number of routes it needs to hold depends on where it is deployed in the network. The memory used for TCAM tables is limited and allocated during the boot-up process on the switch. When a section of hardware resources…
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Stateful Switchover (SSO)
Routers designed for high availability (HA) include hardware redundancy such as dual power supplies (PSU) and router processors (RP). The RP is responsible for for learning the network topology and building the route table (RIB). If the RP fails it can trigger routing protocol adjacencies to reset resulting in network instability. During a RP failure…
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Hardware Cisco Express Forwarding (CEF)
ASICs in hardware platforms are expensive to design, produce and troubleshoot and limited in their functionality yet they allow for very high packet rates due to only being programmed for specific tasks. Routers can also be equipped with network processing units (NPUs). NPUs have an advantage over ASICs as they are programmable meaning their programming…
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Software Cisco Express Forwarding (CEF)
Software CEF, more commonly known as software Forwarding Information Base (FIB) consists of two parts: Forwarding Information Base The forwarding information base is built from the routing table on the local device and contains the next hop IP for each destination that the device knows on the network. When a routing or topology change occurs…
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Distributed Forwarding
Distributed forwarding is when there are line cards that can make decisions without the input of a route processor. When a packet is received on the ingress line card, it is transmitted to the local forwarding engine. The forwarding engine performs a packet lookup and determines if the outbound interface is local. If the outbound…