GATE IT 2005 | Question: 28

Question

Which of the following statements is FALSE regarding a bridge?

• A. Bridge is a layer $2$ device
• B. Bridge reduces collision domain
• C. Bridge is used to connect two or more LAN segments
• D. Bridge reduces broadcast domain

Comments

Please explain in simple words relationship of bridges with broadcast & collision domain.

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A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

A collision domain is a network segment connected by a shared medium or through repeaters where data packets may collide with one another while being sent. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet.

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is bridge in syllabus? I think this has been removed.

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Ethernet bridge is in syllabus.

Answer 1

Answer: D

Bridges are Data Link layer devices used to connect  LANs. Bridges are collision domain separator but unable to separate Broadcast domain.

Comments

Bridge can increase collision domains but how can it reduce?

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Any device which is having Network Layer can reduce the Broadcast domain.

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We can have L2 VLANs inside a switch, which will also limit, to what all ports packet is flooded. Flooding is broadcast to unknown ports. in that sense, the switch can also split broadcast domain using L2 VLANs, isn't it? I understand that bridge is a two ported switch used to interconnect 2 networks. Also, understand that L3 VLANs will split broadcast domain and that is what discussed in the above comments. 

I found these discussion threads as well which discusses my point... 

https://networkengineering.stackexchange.com/questions/9770/how-do-vlans-affect-the-broadcast-domain

https://www.reddit.com/r/ccna/comments/44j4hk/only_routers_breakup_broadcast_domains_but_also/

Answer 2

Terms you need to understand.

Broadcast: In this case, a Data-Link layer (Layer 2) message sent to ALL devices attached to the same network segment.

Collision: When more then one device transmit data at the same time on shared media, the packets collide, data becomes corrupted and after a random period of time, devices have to re-transmit the data.

Broadcast Domain: All devices that a Broadcast message reaches at the Data -Link layer within a network is called a Broadcsat domain. A network can have more then one Broadcast domain and a Broadcast domain is delimeted by devices that stop Broadcast messages.

Collision Domain: Anywhere within a network where a Collision can occur. The limits of a Collision domain are marked by those devices that break up Collision Domains.

Network Devices and their role regarding Broadcast and Collision domains:

 

• Switch: Expands Broadcast domains and breaks up Collision domains.
  • Every device attached to a Switch belongs to the same Broadcast domain and this is because every port on a Switch belongs to the same Brodcast domain. If you were to attach another Switch to the original Switch, every single device attached to that second Switch will also belong to the same original Broadcast domain. In other words, every time you add a switch to a network, you expand the Broadcast domain.
  • It is said that a Switch breaks up Collision domains because each port on it is its own collision domain, as apposed to a Hub, for example, where all of its ports belong to the same Collision domain.

 

• Bridge: Expands Broadcast domains and breaks up Collision Domains.
  • The Bridge is just like a Switch; it expands Broadcast domains and breaks up Collision Domain. Each port on a Bridge is its own Collision domain.

 

• Hub: Expands both Broadcast and Collision domains.
  • A Hub will expand a Broadcast domain just like a Switch does, but, as opposed to a Switch, it does not break up a Collision domain because every single port on a Hub belongs to the same Collision domain. In other words, a frame that came in or out on one port can collide with any other frame from any other of the Hub's ports.
    Imaging yourself adding Hubs to a network, every time you add a Hub, both the Broadcast and the Collision domains get expanded.

 

• Router: Breaks up both Broadcast and Collision domains.
  • Each interface on a Router is its own Broadcast and Collision domain. This means that Routers stop Broadcast messages, they will not route a Broadcast message from one Broadcast domain to another Broadcast domain.

 

Broadcast Domains.

Look at the picture bellow, you can identify 4 different Broadcast domains, let's see why:

 

 

Broadcast domain #1:

• Each interface on a Router belongs to its own Broadcast domain: TRUE.
• Each port on a Switch belongs to the same Broadcast domain: TRUE.
• Each port on a Hub belongs to the same Broadcast domain: TRUE.

Broadcast domains #2 & #3:

• Each interface on a Router belongs to its own Broadcast domain: TRUE.

 

Brocast domain #4:

• Each interface on a Router belongs to its own Broadcast domain: TRUE.
• Each port on a Bridge belongs to the same Broadcast domain: TRUE.

 

Collision Domains.

Looking at the picture below you can identify 10 different Collision domains, let's see why:

 

Collision domain #1:

• All the ports on a Hub belong to the same Collision domain: TRUE.
• Each port on a Switch is its own Collision domain: TRUE.

Collision domains #2, #3 & #4:

• Each port on a Switch is its own Collision domain: TRUE.
• Each interface on a Router belongs to one Broadcast domain: TRUE.

Collision domain #5, #6 & #7:

• Each interface on a Router belongs to one Broadcast domain: TRUE.
• Each port on a Switch is its own Collision domain: TRUE.

Collision domain #8 & #9:

• Each port on a Bridge is its own Collision domain: TRUE.
• Each port on a Switch is its own Collision domain: TRUE.

Collision domain #10:

• Each port on a Switch is its own Collision domain: TRUE

Reference:https://learningnetwork.cisco.com/docs/DOC-30227

 

Comments

Best answer!

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Such a nice explanation

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Thank you for the explanation!
In case anyone prefers a video answer, this one explains the same in a very clear and concise format.

Answer 3

Answer->D

Bridge does not reduce broadcast domain.It remains same.

Answer 4

A

Since bridge is layer two device clearly bcz it uses MAC address to built static or dynamic or learning table so option A is right option for it.

B

Bridge reduces the collision domain since there is no noise or collision inside bridege is zero but may be collision due to other connected devices with bridge as HUB or repeater so it reduces not zero.

C

Since there is no capability of fragmentation therefore it is used to connect two lan segments of same type.

Option D will be False be router and gateway has capability to prevent broadcost domain not bridge or switch or HUB,