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Spanning-tree protocol is a protocol USED in switching network to create a loop-free topology. STP is enabled by default on all VLANs on Catalyst switches. STP switches send BPDU’s (Bridge Protocol Data UNITS) to each other to form their topology databases. BPDU’s are SENT out all ports every two seconds, are forwarded to a specific MAC MULTICAST address: 0180.c200.0000. 

Spanning-tree protocol is a protocol used in switching network to create a loop-free topology. STP is enabled by default on all VLANs on Catalyst switches. STP switches send BPDU’s (Bridge Protocol Data Units) to each other to form their topology databases. BPDU’s are sent out all ports every two seconds, are forwarded to a specific MAC multicast address: 0180.c200.0000. 

Spanning TREE Protocol (STP) is a protocol which prevents layer 2 loops. STP enables switches to BECOME AWARE of each other so that they can NEGOTIATE a Loop-Free path through network.

In practical Scenario, Redundant links are created to avoid COMPLETE network failure in an event of failure of one link.

Spanning Tree Protocol (STP) is a protocol which prevents layer 2 loops. STP enables switches to become aware of each other so that they can negotiate a Loop-Free path through network.

In practical Scenario, Redundant links are created to avoid complete network failure in an event of failure of one link.

STP chooses a REFERENCE point (Root BRIDGE) in the network and calculates all the redundant PATHS to that reference point. Than it picks ONE path which to forward frames and blocks other redundant paths. When blocking hapeens, Loops are prevented.

STP chooses a Reference point (Root Bridge) in the network and calculates all the redundant paths to that reference point. Than it picks one path which to forward frames and blocks other redundant paths. When blocking hapeens, Loops are prevented.

1. Disabled - A port in the disabled state does not participate in the STP.
2. Blocking - A blocked port does not forward FRAMES. It only listens to BPDUs. The purpose of the blocking state is to prevent the use of LOOPED paths.
3. Listening - A port in listening state prepares to forward data frames without populating the MAC address table. The port also sends and listens to BPDUs to make SURE no LOOPS occur on the network.
4. Learning - A port in learning state populates the MAC address table but doesn’t forward data frames. The port still sends and receives BPDUs as before.
5. Forwarding - The port now can send and receive data frames, collect MAC addresses in its address table, send and receive BPDUs. The port is now a fully functioning switch port within the spanning-tree topology.

STP uses three timers to make sure that a network converges properly before a bridging loop can form.

Hello timer - The time INTERVAL between Configuration BPDUs sent by the root bridge. It is 2 seconds by default.

Forward Delay timer - The time interval that a switch port spends in both the Listening and LEARNING states.

The default VALUE is 15 seconds.

MAX (Maximum) Age timer - Maximum length of time a BPDU can be stored without receiving an update. It can also be define as a time interval that a switch stores a BPDU before discarding it. It is 20 seconds by default.

STP uses three timers to make sure that a network converges properly before a bridging loop can form.

Hello timer - The time interval between Configuration BPDUs sent by the root bridge. It is 2 seconds by default.

Forward Delay timer - The time interval that a switch port spends in both the Listening and Learning states.

The default value is 15 seconds.

Max (Maximum) Age timer - Maximum length of time a BPDU can be stored without receiving an update. It can also be define as a time interval that a switch stores a BPDU before discarding it. It is 20 seconds by default.

1. Root port - The root port is always the LINK directly connected to the root bridge, or the SHORTEST path to the root bridge. It is always on Non-Root Bridge.
2. Designated port - A designated port is ONE that has been determined as having the best (lowest) cost. A designated port will be marked as a forwarding port. It can be on both Root Bridge & Non Root Bridge. All PORTS of Root Bridge are Designated Port.
3. Forwarding port - A forwarding port forwards frames.
4. Blocked port - A blocked port is the port that is used to prevent loops. It only listens to BPDUs. Any port other than Root port & Designated port is a Block Port.

All the switches EXCHANGE information to SELECT Root BRIDGE as well as for configuration of the network.

This is done through Bridge Protocol Data Unit (BPDU). Each switch compares the parameters in the BPDU that it SENDS to one NEIGHBOR with the one that it receives from another neighbor.

All the switches exchange information to select Root Bridge as well as for configuration of the network.

This is done through Bridge Protocol Data Unit (BPDU). Each switch compares the parameters in the BPDU that it sends to one neighbor with the one that it receives from another neighbor.

Bridge Protocol DATA UNITS (BPDUS) FRAMES are sent out as at multicast destination MAC address

01:80:c2:00:00:00.

Bridge Protocol Data Units (BPDUs) frames are sent out as at multicast destination MAC address

01:80:c2:00:00:00.

Two types of BPDU exist:-

1. CONFIGURATION BPDU - USED for Spanning-Tree Computation.
2. TOPOLOGY Change Notification (TCN) BPDU - Used to announce changes in the Network Topology.

Two types of BPDU exist:-

The bridge ID is used to elect the root bridge in the STP domain. This ID is 8 bytes long and includes both the priority and the MAC address of the device.

Switch with the lowest Bridge ID is elected as the Root bridge which MEANS Switch with the lowest priority will become Root Bridge if TWO or more switches have same priority than switch with lowest mac address will become Root Bridge.

The bridge ID is used to elect the root bridge in the STP domain. This ID is 8 bytes long and includes both the priority and the MAC address of the device.

Switch with the lowest Bridge ID is elected as the Root bridge which means Switch with the lowest priority will become Root Bridge if two or more switches have same priority than switch with lowest mac address will become Root Bridge.

The SPANNING TREE Cost Value is INVERSELY proportional to the BANDWIDTH of the link and therefore a path with a low cost value is more preferable than a path with high cost value.
Link Bandwidth Cost Value

• 10 Gbps 2
• 1 Gbps 4
• 100 Mbps 19
• 10 Mbps 100

The Spanning Tree Cost Value is inversely proportional to the bandwidth of the link and therefore a path with a low cost value is more preferable than a path with high cost value.
Link Bandwidth Cost Value

Once the Root Switch is elected, every other Switch in the network MUST select a SINGLE port on itself to reach the Root Switch. The port with the LOWEST root path cost (lowest cumulative cost to reach root switch) is elected as the root port and is placed in the forwarding state. Root Bridge will never have a Root Port.

Once the Root Switch is elected, every other Switch in the network must select a single port on itself to reach the Root Switch. The port with the lowest root path cost (lowest cumulative cost to reach root switch) is elected as the root port and is placed in the forwarding state. Root Bridge will never have a Root Port.

The EXTENDED SYSTEM ID is utilized by spanning-tree to include the VLAN ID information inside 16-bit STP

The Extended System ID is utilized by spanning-tree to include the VLAN ID information inside 16-bit STP

STP can be configured only on SWITCHES and not on ROUTERS. STP is used to PREVENT frame looping

STP can be configured only on switches and not on routers. STP is used to prevent frame looping

It is not required. STP is USED to SHUTDOWN redundant LINKS between switches to prevent LOOPS.

It is not required. STP is used to shutdown redundant links between switches to prevent loops.

STP is a protocol. It has it’s own FRAME when configured. So it WOULD not affect a GENERIC ethernet frame on the network.

STP is a protocol. It has it’s own frame when configured. So it would not affect a generic ethernet frame on the network.

All PORTS on a ROOT BRIDGE is DESIGNATED ports.

All ports on a root bridge is designated ports.

If the priority VALUE of the two SWITCHES are same, which switch would be ELECTED as the ROOT BRIDGE

The switch with the lowest mac-address value would be elected as the root bridge

If the priority value of the two switches are same, which switch would be elected as the root bridge

The switch with the lowest mac-address value would be elected as the root bridge

Spanning tree protocol is used to prevent looping of frames. Unlike IP, which has a TTL value field in the packet which is decremented every time the packet PASSES through a router and is dropped once the value reaches 0, the frame does not have any SIMILAR field. So if a scenario arises where a frame is looping in a switched environment, the frame would not AUTOMATICALLY time out. The spanning tree protocol is used for preventing the looping.

Spanning tree protocol is used to prevent looping of frames. Unlike IP, which has a TTL value field in the packet which is decremented every time the packet passes through a router and is dropped once the value reaches 0, the frame does not have any similar field. So if a scenario arises where a frame is looping in a switched environment, the frame would not automatically time out. The spanning tree protocol is used for preventing the looping.

Yes. The IP DATAGRAM IDENTIFIER FIELD is different for each IP datagram transmitted. The fragments of an IP datagram will have the same identifier VALUE.

Yes. The IP datagram identifier field is different for each IP datagram transmitted. The fragments of an IP datagram will have the same identifier value.

The TTL field is used to limit the lifetime of a IP datagram and to prevent INDEFINITE looping of IP datagrams.

TIME To Live is used to limit the period of time of TRANSMISSION of network technology that ba UNIT of data can experience before it should be discarded.

The TTL field is used to limit the lifetime of a IP datagram and to prevent indefinite looping of IP datagrams.

Time To Live is used to limit the period of time of transmission of network technology that ba unit of data can experience before it should be discarded.

RFC 1349 DISCUSSES the TYPE Of SERVICE (TOS) FIELD.

RFC 1349 discusses the Type Of Service (TOS) field.

The typical VALUE for a TTL field is 32 or 64.

The typical value for a TTL field is 32 or 64.

If a datagram cannot be DELIVERED to the DESTINATION HOST due to some REASON, it is considered an undeliverable datagram.

If a datagram cannot be delivered to the destination host due to some reason, it is considered an undeliverable datagram.

The size of the IP header is not fixed. Depending on the IP options PRESENT, the size of the IP header will VARY. A separate field for the IP header LENGTH is added, so that the DESTINATION system can separate the IP DATAGRAM header from the payload.

The size of the IP header is not fixed. Depending on the IP options present, the size of the IP header will vary. A separate field for the IP header length is added, so that the destination system can separate the IP datagram header from the payload.

All the DATAGRAM headers in the TCP/IP protocol suite are TRANSMITTED in the "BIG endian" BYTE order. i.e. The most significant byte is transmitted FIRST. This is also called as "network byte order".

All the datagram headers in the TCP/IP protocol suite are transmitted in the "big endian" byte order. i.e. The most significant byte is transmitted first. This is also called as "network byte order".

The TTL field CONTAINS a counter value SET by the source HOST. Each GATEWAY that processes this datagram, decreases the TTL value by one. When the TTL value reaches zero, the datagram is DISCARDED.

The TTL field contains a counter value set by the source host. Each gateway that processes this datagram, decreases the TTL value by one. When the TTL value reaches zero, the datagram is discarded.