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000-958 Questions & Answers

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Exam Code: 000-958

Exam Name: Enterprise Storage Technical Support V3

Vendor: IBM

Certs Covered: Certified Specialist

No of Questions: 143

Last Updated: June 11, 2014

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Transparent Bridging Fundamentals


Bridging Overview

Bridging takes place at Data-link layer of the OSI model. At Layer 2, bridging performs the following functions:

  • Receives the incoming frame.

  • Once received, the frame is either forwarded out a port or out all ports of the bridge, or the frame is dropped.


Bridges existed before hubs and routers did. A bridge is device that connects two network segments that utilize a common communications protocol. The bridge passes data between the two network segments. Bridges and switches essentially perform the same function, that is, they break up collision domains on a LAN.


The features provided by bridges include:

  • Control data flow.

  • Handle transmission errors.

  • Provide physical addressing

  • Manage access to the physical medium for technologies such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI).


To forward frames, bridges build a Bridging table which includes all the MAC addresses and port associations which it has learned. When a bridge receives a frame, it checks the content of the Bridging table to determine how the frame should be forwarded.


The main benefits of bridging are:

  • Bridges can forward protocols which are not routable to remote segments. This includes protocols such as SNA and LAT.

  • Bridges isolate traffic as it flows between network segments. Traffic which should be forwarded to a segment is the only traffic sent. In this way, bridging assists in reducing traffic on the segments, which in turn leads to fewer collisions.

  • Bridges overcome the physical segment size limitations of Ethernet.



Bridges versus Routers

The similarities between bridges and routers are listed here:

  • Bridges and routers each examine tables to decide what should happen when an incoming frame or data is received.


The main differences between bridges and routers are listed here

  • Bridges make decisions using Data-link layer information of the OSI Reference Model. Routers use Network layer information of the OSI Reference Model

  • While routers check the Routing table for the longest match to the destination, a bridge checks for an exact match.

  • Bridging is much faster and less processor intensive than routing.

  • While configuring a bridge is a simple process, configuring a router is much more intricate. There are loads of standards for the routing protocols. Bridging on the other hand uses standardized inter-bridge protocols.



Bridges versus Switches

The similarities between bridges and switches are listed here:

  • Bridges and switches both discover MAC addresses by examining the source address of frames which they receive.

  • Bridges and switches examine the layer 2 addresses, and make forwarding decisions based on the destination MAC address of the frame.

  • Bridges and switches both forward layer 2 broadcasts.


The main differences between bridges and switches are listed here:

  • Bridges use software to create and manage a filter table. This makes bridges software based. Switches use application-specific integrated circuits (ASICs) to create and maintain filter tables, thereby making switches hardware based.

  • Switches are regarded as being multiport bridges.

  • While a bridge can only have one spanning tree instance, a switch can have many spanning tree instances.

  • Bridges can have a maximum of 16 ports, while switches can have hundreds of ports.



Transparent Bridging Overview

Transparent bridging is the term used to refer to the method used in Ethernet and IEEE 802.3 networks wherein frames are passed on or forwarded one hop at a time. Frames are forwarded based on the tables that associate network end nodes with bridge ports. Transparent bridging forwards frames without actually modifying the frames. With transparent bridging, the presence of bridges is transparent to network end nodes. It combines multiple physical segments into a single data-link LAN, as perceived by network end nodes.


A transparent bridge connects two or more Ethernet networks, and then performs the following main functions:

  • Learning

  • Forwarding

  • Filtering

  • Loop Avoidance


The MAC address types defined by the IEEE for Ethernet are:

  • Unicast addresses: Also called Ethernet addresses, LAN address, or MAC addresses. The format of LAN addresses and assignment of LAN addresses are defined and controlled by the Institute of Electrical and Electronics Engineers (IEEE). One requirement defined by the IEEE is the existence of unique MAC addresses on all LAN interface cards. A MAC address is a 6 byte long (48-bit) hardware address, which is expressed in the hexadecimal format. The MAC address is encoded on the Ethernet card by the Ethernet card manufacturers to guarantee uniqueness.

  • Broadcast addresses: Broadcast addresses are 32-bit numbers used to specify all hosts in a specific network. The broadcast address is used by both hosts and applications to send data to all hosts residing on the particular network. Broadcast addresses mean that the frame should be processed by all devices on the LAN.

  • Multicast addresses: Multicast addresses support IP multicasting. IP multicasting allows a single address to be utilized to transmit information to multiple destinations. Multicast addresses are used to enable a group of devices residing on the LAN to communicate. Multicast addresses can be used on Ethernet and FDDI. Multicast addresses are not burned into the LAN card. The software configures the NIC to listen for a specific multicast address.


Because transparent bridges only forward frames when it has to, transparent bridges have to be able to perform the following functions to achieve this:

  • Check the source MAC address of each frame to learn the MAC addresses.

  • Determine when to forward a frame and when to filter a frame, based on the destination MAC address.

  • Use the Spanning Tree Protocol (SPT) to create a loop-free environment.



How transparent bridges learn MAC addresses

For transparent bridges to forward or filter frames, it has to build and main an accurate bridging table. Transparent bridges essentially have to listen to incoming frames, and then check the source MAC address within the frame. After monitoring the source MAC address, and a bridge determines that the specific source MAC address is not in its bridging table, the bridge updates its bridging table to include the MAC address.


When a transparent bridge updates its bridging table with source MAC address information, it adds both the MAC address and the interface in which the frame arrived. Updating of the bridging table is a continuous process.


Entries in the bridging table expire when a station is not heard from within a predefined time period. Expired entries are removed from the bridging table.



How transparent bridges determine whether to forward incoming frames

To determine whether and where incoming frames should be forwarded, a transparent bridge checks the destination address of the frame, and then uses its bridging table. The bridging table contains MAC address and interface associations.


Bridges can forward frames out either one of its ports, or out all its ports:

  • A single port of the bridge: When a MAC address entry exists for the frame in the bridging table, the bridge forwards the frame from a single port.

  • All ports of the bridge: When the bridging table does not contain a MAC address entry, the bridge forwards the frame out all ports of the bridge. In this case, it is said that the MAC address is an unknown station. The term used to refer to the process of bridges forwarding frames out all ports, is flooding.


When an unknown station replies, the bridge adds the MAC address of the unknown station to its bridging table.


The term filtering is used to describe the occurrence of a bridge dropping an incoming frame.



Why bridges need Spanning Tree Protocol (SPT)

All bridging devices, which include transparent bridges and switches, use Spanning Tree Protocol (STP) to avoid loops.


STP is a bridge-to-bridge protocol which was designed to find and remove bridging loops. Digital Equipment Corporation (DEC) developed the initial STP. The DEC STP protocol formed the basis of the IEEE 802.1D version of STP. The IEEE 802.1D version of STP and the DEC STP protocol are not compatible.


Without STP, frames would continue to loop endlessly in networks that have physically redundant links. STP monitors the layer 2 network to locate all links, and to determine whether loops exist. STP then blocks or shuts down some ports so that one active path exists between any pair of LAN segments. While this prevents frames from looping infinitely, redundant links are blocked to prevent frames from looping. Bear in mind though that should frames loop infinitely, the LAN would become unusable. STP operates the same for a transparent bridge and a switch. STP makes use of the spanning-tree algorithm (STA) to create its topology database.


The STP creates a spanning tree of interfaces that places the interface in either of these modes/states:

  • Forwarding state

  • Blocking state


To determine which interfaces should be placed in the forwarding state, STP uses the following criteria:

  • STP chooses the root bridge. Interfaces that exist on the root bridge are placed in the forwarding state.

  • STP places the root port in the forwarding state.

  • The designated port is also placed in the forwarding state.


All interfaces, other than these interfaces are placed in the blocking state



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