USE OF MAC LAYER IN OSI MODEL

The MAC sub-layer has two primary responsibilities:

1)Data encapsulation includes frame assembly before transmission and frame parsing /error detection during and after reception.

2) Media access control, including initiation of frame transmission and recovery from transmission failure.

 

DIFFERENCE BETWEEN TCP & UDP (TCP v/s UDP)

	TCP	UDP
Acronym for	Transmission Control Protocol	User Datagram Protocol or Universal Datagram Protocol
Connection	TCP is a connection-oriented protocol.	UDP is a connectionless protocol.
Function	As a message makes its way across the internet from one computer to another. This is connection based.	UDP is also a protocol used in message transport or transfer. This is not connection based which means that one program can send a load of packets to another and that would be the end of the relationship.
Usage	TCP is suited for applications that require high reliability, and transmission time is relatively less critical.	UDP is suitable for applications that need fast, efficient transmission, such as games. UDP's stateless nature is also useful for servers that answer small queries from huge numbers of clients.
Use by other protocols	HTTP, HTTPs, FTP, SMTP, Telnet	DNS, DHCP, TFTP, SNMP, RIP, VOIP.
Ordering of data packets	TCP rearranges data packets in the order specified.	UDP has no inherent order as all packets are independent of each other. If ordering is required, it has to be managed by the application layer.
Speed of transfer	The speed for TCP is slower than UDP.	UDP is faster because there is no error-checking for packets.
Reliability	There is absolute guarantee that the data transferred remains intact and arrives in the same order in which it was sent.	There is no guarantee that the messages or packets sent would reach at all.
Header Size	TCP header size is 20 bytes	UDP Header size is 8 bytes.
Common Header Fields	Source port, Destination port, Check Sum	Source port, Destination port, Check Sum
Streaming of data	Data is read as a byte stream, no distinguishing indications are transmitted to signal message (segment) boundaries.	Packets are sent individually and are checked for integrity only if they arrive. Packets have definite boundaries which are honored upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.
Weight	TCP is heavy-weight. TCP requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control.	UDP is lightweight. There is no ordering of messages, no tracking connections, etc. It is a small transport layer designed on top of IP.
Data Flow Control	TCP does Flow Control. TCP requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control.	UDP does not have an option for flow control
Error Checking	TCP does error checking	UDP does error checking, but no recovery options.
Fields	1. Sequence Number, 2. AcK number, 3. Data offset, 4. Reserved, 5. Control bit, 6. Window, 7. Urgent Pointer 8. Options, 9. Padding, 10. Check Sum, 11. Source port, 12. Destination port	1. Length, 2. Source port, 3. Destination port, 4. Check Sum
Acknowledgement	Acknowledgement segments	No Acknowledgment
Handshake	SYN, SYN-ACK, ACK	No handshake (connectionless protocol)

% INSPIRATIONAL QUOTES %

If you don’t like something, change it. If you can’t change it, change your attitude.
- Maya Angelou


Yesterday I got a mail from one of my friend asking about common misconfiguration of MSTP

and i decided to post a blog on this topic so many of us get help from it......:)

Common Misconfigurations

The independence between instance and VLAN is a new concept that implies you must carefully plan your configuration. The IST Instance is Active on All Ports, Whether Trunk or Access section illustrates some common pitfalls and how to avoid them.

IST Instance is Active on All Ports, Whether Trunk or Access

This diagram shows Switches A and B connected with access ports each located in different VLANs. VLAN 10 and VLAN 20 are mapped to different instances. VLAN 10 is mapped to instance 0, while VLAN 20 is mapped to instance 1.

This configuration results in pcA 's inability to send frames to pcB. The show command reveals that Switch B is blocking the link to Switch A in VLAN 10, as shown in the this diagram:

How is that possible in such a simple topology, with no apparent loop?

This issue is explained by the fact that MST information is conveyed with only one BPDU (IST BPDU), regardless of the number of internal instances. Individual instances do not send individual BPDUs. When Switch A and Switch B exchange STP information for VLAN 20, the switches send an IST BPDU with an MRecord for instance 1 because that is where VLAN 20 is mapped. However, because it is an IST BPDU, this BPDU also contains information for instance 0. This means that the IST instance is active on all ports inside an MST region, whether these ports carry VLANs mapped to the IST instance or not.

This diagram shows the logical topology of the IST instance:

Switch B receives two BPDUs for instance 0 from Switch A (one on each port). It is clear that Switch B has to block one of its ports in order to avoid a loop.

The preferred solution is to use one instance for VLAN 10 and another instance for VLAN 20 to avoid mapping VLANs to the IST instance.

An alternative is to carry those VLANs mapped to the IST on all links (allow VLAN 10 on both ports, as in this diagram).

Two VLANs Mapped to the Same Instance Block the Same Ports

Remember that VLAN no longer means spanning tree instance. The topology is determined by the instance, regardless of the VLANs mapped to it. This diagram shows a problem that is a variant of the one discussed in the IST Instance is Active on All Ports, Whether Trunk or Access section:

Suppose that VLANs 10 and 20 are both mapped to the same instance (instance 1). The network administrator wants to manually prune VLAN 10 on one Uplink and VLAN 20 on the other in order to restrict traffic on the Uplink trunks from Switch A to distribution Switches D1 and D2 (an attempt to achieve a topology as described in the previous diagram). Shortly after this is completed, the network administrator notices that users in VLAN 20 have lost connectivity to the network.

This is a typical misconfiguration problem. VLANs 10 and 20 are both mapped to instance 1, which means there is only one logical topology for both VLANs. Load-sharing cannot be achieved, as shown here:

Because of the manual pruning, VLAN 20 is only allowed on the blocked port, which explains the loss of connectivity. In order to achieve load balancing, the network administrator must map VLAN 10 and 20 to two different instances.

A simple rule to follow to steer clear of this problem is to never manually prune VLANs off a trunk. If you decide to remove some VLANs off a trunk, remove all the VLANs mapped to a given instance together. Never remove an individual VLAN from a trunk and not remove all the VLANs that are mapped to the same instance.