Chapter #5 - Internetworking

What is a Repeater and at which layer does it operate?

Layer 1 (Physical). Copies/amplifies bits between cable segments to extend range. No frame modification.

What is a Bridge and at which layer?

Layer 2 (Data Link). Relays frames between LANs at MAC level; minor frame modifications; increases number of stations.

How does a L2 Switch differ from a Bridge?

Switch: hardware based + fast, Supports cut-through switching

A Bridge is CPU based/Software + slow, always store-and-forward.

Both do the same task: Relay Frames between LANs at MAC level

What is a Router and at which layer?

Layer 3 (Network). Relays packets between different networks; converts addressing; operates end-to-end across heterogeneous subnetworks.

What is a Gateway and at which layer?

Layer 4–5 (Transport/Application). Converts one protocol into another i.e. TCP ← → ISO

No 1-to-1 function mapping required.

What is the primary design goal of a Bridge?

The primary goal is Transparency, since it makes it possible for stations to communicate/move across segments WITHOUT changing hardware,software or configuration tables

What tasks does a Bridge/Switch fulfill?

Connect different LANs;

+ protocol independence; fire-door function (limits frame propagation).

→ improve scalability, capacity, distance, reliability, security;

How does a Bridge connect two different 802.x networks

Uses LLC(L2 Interface for L3) as the common layer.

→routes frames to the respective MAC;

Requires: bridge contains separate MAC + physical layer implementations for each network type.

What is a Transparent Bridge?How does it work?

A bridge invisible to other network components;

+ Recieves all frames from all connected LANS, maintains a forwarding Table [Station A → output LAN1];

What are the three forwarding decisions of a Transparent Bridge?

• Src and dst on same LAN → drop frame

• Src and dst on different LANs → forward to destination LAN

• Destination unknown → flood to all LANs

How does a Transparent Bridge learn its forwarding table? (Backward Learning)

Reads source address of every received frame; concludes that source station is reachable on the incoming LAN; creates/updates table entry with timestamp.

How does a Transparent Bridge handle topology changes?

Each table entry has a timestamp; entries are periodically purged if not refreshed within a timeout (several minutes) → flooding resumes for that station.

Why do Transparent Bridges need a Spanning Tree?

Parallel bridges between LANs create loops → unknown-destination flooding causes infinite frame duplication.

How does the Spanning Tree algorithm work in bridges?

1) Each bridge has a unique ID (e.g. MAC + priority); 2) All bridges broadcast their ID; 3) Lowest ID becomes root; 4) A loop-free tree is built from root to every bridge/LAN; 5) Unused links blocked. Adapts when topology changes.

What is the drawback of Spanning Tree in bridges?

Some physical links are blocked and not used → bandwidth wasted; not all bridges appear in the active tree.

What is Source Routing and how does it differ from Transparent Bridging?

The sender specifies the full path in the frame header. Bridge only routes flagged frames. (Transparent bridge: bridge decides; sender unaware.)

How does a sender determine the path in Source Routing?

Sends a discovery broadcast; each bridge appends its ID and forwards; the destination returns the complete path → sender embeds path in subsequent frames.

What are the addressing requirements for Source Routing?

Each LAN has a unique 12-bit address; each bridge has a unique 4-bit address per LAN.

Why is Transparent Bridging preferred over Source Routing?

Source routing causes high broadcast traffic during path discovery; Transparent bridging is simpler and more scalable.

Why is Internetworking necessary?

Networks differ in protocols, addressing (flat vs. hierarchical), packet size, QoS, error handling, flow control, security, and accounting. Connecting them enables resource sharing and increased availability.

Name 5 dimensions in which networks can differ.

Service type (connection-oriented vs. connectionless), addressing scheme, packet size, QoS, error/flow/congestion control.

To interconnect 802.11 (WLAN) and 802.3 (Ethernet) which layer and device do you use?

A: Layer 2 (Data Link) → Bridge (or Switch)

What tasks must a Bridge perform when connecting 802.11 and 802.3?

A: Frame format conversion (different header fields, e.g. 802.11 has DURATION field absent in 802.3); re-encapsulation at MAC level; use LLC as common sublayer.

What additional tasks arise when bridging 802.3 and 802.5 (Token-Ring)?

Also handle different frame formats AND different medium access methods (CSMA/CD vs. token-passing); bit-order differences; different max frame sizes may require frame fragmentation or dropping.

Can a Bridge also connect two identical networks? What are the advantages?

Yes. Benefits: separate collision domains (more capacity), increased range, improved reliability ("fire door"), improved security (limits frame propagation).