chapter 4: Data link layer switching

what is the name used for the switch connecting two LANs

a bridge

what is the main purpose of a bridge

to connect two or more LANs whilst maintaining their separate network capacity

which layer do bridges operate in

data link layer (L2) since they examine frame addresses to be forwarded to destination

how are bridges different to routers

routers examine the addresses in packets and route based on them, hence they can only work with protocols they are designed to handle, whereas bridges can work with any kind of packet

what does each port represent connecting to a switch

each port represents a collision domain

true or false: bridges have different kind of cables connected to it

true

what technique is used to decide which port to output a received frame to

backward learning used to populate an internal hash table

how does backward learning work

* associate frame source address with input port from MAC header
* frames with unknown port destinations sent to all ports except input port
* unused entries are forgotten to allow address reuse

what happens when the source of frame and destination of frame are connected to the same port

frame is discarded as each port is a broadcast domain so destination would’ve already seen frame

why are entries on the hash table periodically forgotten

to handle dynamic topologies where devices may be unplugged and moved around a building for example

what is cut-through switching/ wormhole routing + what is its benefit

as soon as the destination address has arrived in the header, the bridge can start forwarding, provided the output line is available

this reduces latency of bridges

how many instances of MAC layers and physical layers does a bridge with n ports have

n MAC layers and physical layers

what are redundant links

redundant links are additional links added between bridges

what is the purpose of redundant links

they increase reliability because if one link is cut, there is still another available

what problems could redundant links introduce and how is this avoided

they may result in infinite loops of duplicate frames

the work around is to have bridges communicate an overlay of the topology with a spanning tree, so there are no loops

what is a spanning tree

a loop free subset of a topology with a unique path from each source to destination

how is a spanning tree built

bridges run a distributed algorithm:


1. bridges send configuration message that is not forwarded
2. once they have all agreed on root (lowest MAC address), shortest path selected from root to all bridges, otherwise lowest address is used
3. loop around until all tables are updated
4. ports not part of shortest paths switched off
5. cycle continues periodically for network lifetime

what does a bridge configuration message contain

* its own MAC address
* address of bridge it thinks should be root
* suspected distance from bridge to root

what layer do repeaters work with and how do they work

physical layer (L1)

analog devices that work with signals on the cables to which they’re connected to amplify and ‘clean’ the signal, they understand symbols that encode bits as volts

what layer do hubs work with and how do they work

physical layer (L2)

electrically joins a number of input lines that all operate at the same speed

how are bridges/switches different to hubs

each port in a bridge/switch is isolated and allow lines running at different speeds to be connected using buffering

what happens if frames come into a switch faster than they can be retransmitted

if buffer space is out, frames will have to be discarded

what layer do routers work with and how do they work

network layer (L3)

software inspects packet header to choose an output line, it does not see the anything from L2

what layer do transport gateways work with and how do they work

transport layer (L4)

these connect two computers that may use different connection-oriented transport protocols, e.g. connecting a computer using TCP/IP to one using SCTP

what layer do application gateways work with and how do they work

application layer (L5)

understands the format + contents of data so it can be translated to another format