CNIC Information System with Pakdata Cf In Pakistan
Packet Tracer: Cisco, Spanning tree protocol
1. Rafat Khandaker
Packet Tracer
Spanning Tree Protocol/PVST / RSTP
05/05/16
ABSTRACT
In this Lab simulation, four switches have been set up to separate multiple hosts on a network.
The purpose of this simulation is to understand the concept of the spanning tree protocol. The
hosts on the network have been separated into different vlans in order to enable PVST
configuration.
INTRODUCTION:
The Spanning tree protocol was designed in order to prevent redundant paths within a network.
When 3 or more switches are connected together with more than one possible path to designated
target host; it is possible to create an endless loop, creating a broadcast storm. This problem is
due to the switches' lacking ability to separate broadcast domains; for example, when a packet
with an unknown destination mac address enters a switch, the switch will send a broadcast arp
request until it learns the destination to the host. At this time, the switch to switch recieving the
broadcast of FF:FF:FF:FF:FF:FF will be forwarded out of all ports and the cycle will continue
through other switches, creating denial of service or bandwith overload. To fix this problem,
often times a network in the below example will have separate VLANS along with RSTP +
PVST configured.
The RSTP (802.1W) protocol is an enhancement of the original STP 802.1D protocol. Rapid
spanning tree combines the operations of Learning, Blocking and disabled port state into one
operation, called discarding state, It has a much faster convergance time. When configured with
Rapid PVST, RSTP can converge for each virtual lan interface. According to cisco, the default
spanning tree protocol is set to Rapid PVST+. STP also use BPDU ( Bridged Protocol Data Units
) , which are stp frames, to construct a loop free path. BPDUs are 16 bit added information,
attached to the frame of the packet.
Spanning tree topology is calculated by using the root bridge as the primary sender for all
BPDUs. The Root bridge will have all it's interfaces as designated ports. The other switches
should have atleast one root bridge port, which is the shortest path to the bridge. One interace on
the all the switches should be in the discarding state.
9. RESOURCE : http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-
switches/72836-rapidpvst-mig-config.html
According to the cisco documentation, R-PVST configuration are used with two or more core
distribution switches. The distribution switches are manually assigned to carry vlan trunk
information on vlan groups. This is most likely done to reduce bandwidtch congestion and
security. I realized that even though this network is configured with R-PVST , it is configured
with RSTP for each vlan. We can actually see from the Show span configuration that the
spanning tree is autoconfigured with a priority for each Vlan, separately.
The configuration I found in the cisco documentation clearly shows the difference, in an actual
R-PVST network.
In here trunk links between the distribution switches are manually assigned between 2
disribution devices. In this configuration, Distribution 1 will be the root bridge, manually
assigned for vlan 10,30,100. Distribution will be the root bridge, manually assigned for vlan
20,40,200.
the configuration:
10. We can see that the individual trunk links for each switch are manually set to carry certain vlan
information.
Another Source from Youtube
11. https://www.youtube.com/watch?v=6AxDqaG9M7I
From this example, I realized that my previous configuration can be set to PVST or R-PVST
simply by manually configuring the root bridge for each vlan. I can also manually configure the
trunk links to only send vlan traffic in certain links.
12. CONCLUSION
I have successfully configured STP and RSTP, P-VST on cisco switches.
The concepts and actual implementation of PVST had to be analyzed in depth.
From this simulation, I was able to reassess and correct my knowledge of the STP Protocol.
