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NETW206 Ch 05
Spanning Tree Protocol (STP)
| Question | Answer |
|---|---|
| Alternate Port | Port in an RSTP topology that offers an alt path towards the root bridge. Assumes a discarding state in a stable active topology. Will be present on nondesignated bridges and will make a transition to a designated port if the current path fails. |
| Backup Port | In RSTP topology, this is a port on a design'd bridge w/a redundant link to the segment for which the switch is designated. Has a higher port ID than the design'd port on the designated bridge.Assumes discarding state in a stable active topology |
| Blocking State | Port is blocking state if it is a nondesignated port & does not participate in frame fwding. Conts. to process rec'd BPDU frames to determine the location and root ID of the root bridge and what port role it should assume in the final active STP topology. |
| Bridge ID (BID) | is composed of a priority value, an extended system ID, and the reserved MAC address from the switch. Is used by the spanning-tree algorithm; in particular, the BID is used for root bridge election. |
| Bridge Priority | The first parameter used in building a spanning-tree topology. The switch with the lowest bridge ID will be elected as the root bridge. |
| Bridge Protocol Data Unit (BPDU) | A spanning-tree Ethernet frame that is sent out at regular intervals to exchange information among bridges in the network relating to spanning-tree topology formation. |
| Configuration BPDU | Used by STP to build a loop-free topology. In normal STP operation, a switch receives configuration BPDU frames from the root bridge on its root port; never sends a config BPDU toward the root bridge, in contrast to topology change notification BPDUs. |
| Designated Bridge | The bridge that incurs the lowest path cost when forwarding a frame from a segment to the root bridge. Exactly one end of each trunk link in a spanning-tree topology is the designated bridge for that link. |
| Designated Port | In spanning tree, a nonroot switch port that is permitted to forward traffic on the network. For a trunk link, one end connects to the designated bridge via the designated port. Only one end of a trunk link connects to a designated port. |
| Disable Port | A port that is administratively shut down. |
| Disabled State | A switch port is in the spanning-tree disabled state if it is administratively shut down. A disable port does not function in the spanning-tree process. |
| Discarding State | An RSTP port state seen in both a stable active topology and during topology synchronization changes. The discarding state prevents the forwarding of data frames, thus breaking the continuity of a Layer 2 loop. |
| Edge Port | An RSTP edge port is a switch port that is never intended to be connected to another switch device. Transitions to forwarding state when enabled. Conceptually similar to PortFast enabled ports in the Cisco implementation of IEEE 802.1D |
| Extended System ID | Constitutes 12 bits of the 8-byte BID and contains the ID of the VLAN with which an STP BPDU is associated. The presence of the extended system ID results in bridge priority values incrementing in multiples of 4096. |
| Forward Delay | The time a switch port spends in the STP listening state after the port is activated for bridging and before forwarding begins. |
| Forwarding State | An STP port in forwarding state is considered part of the active topology and forwards data frames as well as sending and receiving BPDU frames. |
| Hello Time | The value of a field in a BPDU frame that specifies how frequently BPDUs are transmitted. The default is 2 seconds. |
| Learning State | The IEEE 802.1D learning state is seen in both stable active topology and during topology synch changes. During the learning state a port accepts data frames to populate the MAC address table in an effort to limit flooding of unknown unicast frames. |
| Listening State | The IEEE 802.1D listening state is seen in both stable active topology and during topology synch changes. In listening state, the port can't send or receive data frames; however the port is allowed to receive and send BPDUs. |
| Load Balance | The capability of a networking device to distribute traffic over some of its ports on a path to the destination. Load balancing increases the utilization of network segments, this increasing the effective network bandwidth. |
| Maximum Age | An STP timer that controls the maximum length of time a switch port saves configuration BPDU information. The default is 20 seconds, but can be tuned between 6 & 40 seconds. |
| Message Age | A field in the BPDU that specifies the metric for distance from the root bridge. The root bridge sends all its BPDUs with a message age value of 0, and all subsequent switches add 1 to this value. |
| Multiple Instance Spanning Tree Protocol (MISTP) | A prestandard version of MSTP used on Catalyst 6000 family switches running CatOS. |
| Multiple Spanning Tree Protocol (MSTP) | Enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of instances needed to support a large number of VLANs. |
| Non-Designated Port | An STP port dynamically configured to be in a blocking state to prevent loops. |
| Out-of-Band | Refers to any type of access of a networking device operating system by means other than the network itself; with Cisco devices, out-of-band almost always refers to console access. |
| Path Cost | The cumulative STP cost from a device to the root bridge; it is a function of the bandwidths of the individual links connecting the device to the root bridge. |
| Per-VLAN Spanning Tree (PVST) | A Cisco proprietary STP implementation that maintains a spanning-tree instance for each VLAN configured in the network. PVST relies on ISL for VLAN trunk encapsulation. |
| Per-VLAN Spanning Tree Plus (PVST+) | provides the same functionality as PVST, including PortFast, and adds support for IEEE 802.1Q. PVST+ is not supported on non-Cisco devices. |
| Point-to-Point Link Type | In RSTP topology nonedge ports are catag'd into 2 link types; PTP & Shared. The link type is auto determined, but can be overwritten with an explicit port conf. PTP link types are used except on links connected to a shared multiaccess half-duplex environ. |
| Port Cost | The spanning-tree port cost is a measure assigned on a per-link basis in a switched LAN; it is determined by the link bandwidth, with a higher bandwidth giving a lower port cost. |
| Port ID | The spanning-tree port priority coupled with the port number of the switch port. |
| Port Number | A numerical value associated with switch ports and used to break ties in spanning-tree calculations.F0/1 equals 1 and so on. |
| Port Priority | A configurable spanning-tree parameter assigned to a switch port. The default setting is 128. |
| PortFast | When a switch port config'd as an access port is config'd with PortFast, the port transitions from blocking to forwarding state immidiately, bypassing the usual STP listening and learning states. Used on ports with end devices like workstations, servers |
| Proposal and Agreement Process | RSTP speeds up convergence relative to IEEE 802.1D STP implementations by functioning on a link-by-link basis w/o relying on timers expiring before ports can transition between states. The link-by-link process consists of a proposal & agreement process. |
| Rapid Per-VLAN Spanning Tree Plus (Rapid PVST+) | A Cisco implementation of RSTP. It supports one instance of RSTP for each VLAN. |
| Rapid Spanning Tree Protocol (RSTP) | Specified by IEEE 802.1W, is a dramatic improvement to IEEE 802.1D, providing very fast spanning-tree convergence on a link-by-link basis using a proposal and agreement process independent of timers. |
| Root Bridge | Root bridge exchanges topology info with the other bridges in a spanning-tree topology to notify all other bridges in the network when topology changes are required; this prevents loops and provides a measure of defense against link failure. |
| Root ID | The bridge ID of the root bridge in a spanning-tree topology. |
| Root Port | The unique port on a nonroot bridge that has the lowest path cost to the root bridge. Every nonroot bridge in an STP topology must elect a root port. The root port on a switch is used for communication between the switch and the root bridge. |
| Shared Link Type | Nonedge ports are categorized into 2 link types; PTP & Shared. The link type is auto determined, but can be overwritten with an explicit port conf. The shared link type is associated with ports connecting to a shared multi-access half-duplex environment. |
| Spanning Tree Protocol (STP) | Bridge protocol that utilizes the spanning-tree algorithm, enabling a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. |
| Spanning-Tree Algorithm (STA) | Used by STP to create a spanning tree. |
| Switch Diameter | The number of intermediary switches between two endpoints. |
| Time-to-Live (TTL) | The field in an IP header that indicates how long a packet is considered valid; each routing device that an IP packet passes through decrements the TTL by 1. |
| Topology Change (TC) Bit | A bit in a configuration BPDU that is used only when a switch is acknowledging the receipt of a TCN BPDU. The TC bit is set by the root bridge for a period of 35 seconds, by default. |
| Topology Change Acknowledgment (TCA) | A BPDU with the TC bit set to 1. When the TC bit is set to 1, it indicates that a switch has received a TCN BPDU. This process is repeated until the root bridge receives and responds to the TCN BPDU. |
| Topology Change Notification (TCN) | A special BPDU used when a switch needs to signal a topology change. A switch sends TCNs out of its root port, toward the root bridge. The TCN is a very simple BPDU that contains no information and is sent out at the hello time interval. |
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