Sarah L. Sischo
September 30, 2002
CPT 426 – Homework 3
Homework 3
Chapter 3: How Do Switches and Routers Forward?
1. Why are layer 2 address used in bridges and switches.
First, a switch is a beefed up version of a bridge, so the two essentially perform
the same task.
Relay protocols which do not have network headers such as NetBIOS and LAT
Simple point to point topologies.
Reduction of bandwidth contention, compensate for interconnecting media delay,
and other performance related problems.
2. List major reasons to use a bridge.
Connect two networks using different media (token vs. Ethernet)
Permits equal bandwidth to the connected media
Permit non-routable protocols to be moved via bridge (gateway) – NetBeui
Permits piggybacking without ever converting to another protocol – Fig 3.23
3. What is a transparent bridge?
Has the ability to make necessary relaying decisions in the bridge itself and is thus
“transparent” to the communicating workstations. It involved frame forwarding,
learning workstation addresses, and ensuring no topology loops exist.
4. How does a transparent bridge decide where to send frames.
Each interface records the source address field of every frame that it hears in the
promiscuous mode. If the interface hears a frame, it can assume that the source
address of that frame is physically present on the medium to which the interface is
connected.
Learning, Flooding, Filtering, Aging
5. What is a remote bridge?
A bridge between two ore more similar networks on remote sites. Dial up or
leased lines typically require a local bridge or gateway and a remote bridge or
gateway at each end, in order to network.
6. What is a Source-Route bridge?
Token-ring technique for establishing communications between devices on
different LANs.
A method used by a bridge for moving data between two networks. Relies on
information contained within the token to route the packet between the two
networks. Since the information in the token is supplied by the computer that sent
the data packet, that computer must know on which network the destination
Sarah L. Sischo
September 30, 2002
CPT 426 – Homework 3
computer is located. For source-routing tot work, every computer and every
bridge on all networks must support this protocol. If some computers do not use
this protocol, they will not receive packets from bridges that use source routing.
7. What is a Mixed-Media bridge?
A method for bridging two networks off of two different protocols. For example,
a bridge between 802.5 and 802.3 must face several incompatibilities between the
two protocols. One of these incompatibilities is bit ordering within the octets of
frames, which is different in the two protocols. According to the text, mixed
media bridging is fundamentally flawed and obsolete approach.
8. Follow/List the steps to forward with VLANS.
Short Answer: A multiplexing system that maps multiple bridging domains onto a
common trunk medium.
Long Answer: VLAN schemes need to build translation tables that map edge
LAN MAC addresses to units in the multiplexed trunk system. In the most
common implementation, physical ports on a switch are configured to specific
VLANs. As frames arrive, their real MAC addresses are stored in a table
associated with that VLAN.
VLAN trunks are LAN media to which switches connect. Each switch interface
has a MAC address appropriate for that medium, and this MAC address
conceptually appears as the source address in VLAN headers, when the VLAN
trunking protocol encapsulate edge frames as does ISL. The trunk frames can be
sent as multicasts or as unicasts to a trunk port on a specific destination VLAN
bridge.
9. What is Hybrid Forwarding?
Short Answer: bleeding edge of technology. Separate the path in the forwarding
function. Uses layer 2 with layer 3 as an exception – switching
Long answer: Information is downloaded to a device that might be called a
switching, but is given information that lets it look up destinations. The device
might recognized the destination from a Layer 2 address, from a shim field
preceding the Layer 3 header, or from a pattern-matching rule that lets the
forwarding engine recognize a flow. Shim fields often define tunnel, in which a
packet has two layer 3 headers, one for transmission system and one for user
network mapped onto it.
10. As processing moves higher in the OSI model, what happens?
As a process moves higher in the OSI model, chisp are likely to become custom
integrated circuits.
11. List/Describe the 3 major families of fabric.
Crossbar – multiple paths among input and output paths. They can be any-to-any
or have some smaller number significantly greater than one. ATM switches
usually have a crossbar design and fall into speed ranges.
Sarah L. Sischo
September 30, 2002
CPT 426 – Homework 3
Shared Bus – can either use a central or distributed forwarding engine. The shared
bus is very fast, but is accessed serially. Forwarding and interface processors must
wait for the bus.
Shared Memory- includes single processor and multiple processors; can offer
better performance than shared bus because the memory can be accessible
simultaneously to more than one interface. For multicasting forward, where a
single input unit of data is sent to more than one interface, shared memory can be
especially good because the output interface can retrieve the information from a
single location. Crossbar approaches can require explicit data copying for each
path.
12. What does blocking mean?
The fabric is too slow for full noninterfering transmission
13. What does nonblocking mean?
The fabric can keep up with a setup of input ports, each of which outputting to
unique output ports of the same or greater speed.
14. Head-of-line refers to:
The data unit bas to wait to be transmitted. Something is in front of it and block
the rest of the line.
15. Describe Shared buses.
Can only be used for one transfer at a time; half or full duplex. Forwarding
engines cannot transfer in parallel across the bus.
CISCO 7x00, Bay’s BCN and BLN routers;, Cabletron MMAC Plus
16. What is Forwarding Engine?
As performance requirements drop, there can be a single forwarding processor
separate from the main management processor. For yet lower performance
requirements, it can be very cost-effective to use a single CPU for both bath
determination and packet forwarding, although is generally makes sense to use
hardware-level 1 and 2 processing.
17. List describe the major tiers that routers and switching products can be broken down to.
Core Devices – high performance transfer; emphasizes forwarding speeds
Distribution Devices – optimizes for conversion between different networking
environments; CPU intelligence for protocol processing; works with trusted and
non-trusted; firewall; LAN to WAN; low-speed to high-speed; tunneling pay/load
protocols aver directory protocols; network access servers are also distribution
their devices.
Access Products – closer to users; frame relay; dedicated access devices;
workgroup switches; dial access routers.
18. List some observations on availability.
Sarah L. Sischo
September 30, 2002
CPT 426 – Homework 3
Different platforms have different amounts of internal redundancy.
Switches
Power Supplies
Mission-Critical
19. Describe all the Cisco Platform Examples(this should have a plethora of information).
Cisco Platform Examples – When a packets enters a router interface, its protocol
type is determined and it is sent either to bridging or routing for that protocol
type. Based on the hardware type and software configuration, the data unit is sent
to a forwarding information base (FIB). Forwarding information base can be on
the interface processor card. Special purpose central switching engine. On the
CPU
Platform Design Examples – CPU based FIB can be separate from the RIB. Or
RIB and FIB can be the same. Higher-end boxes implement a fast forwarding
engine separate form the management and interface processors. Applicationspecific integrated circuit (ASICs) and has a hardware-enabled cache.Catalyst
5000 uses the NetFlow Feature Card (NFFC). Daughter card or applique enables
the switch to forward on Layer 3 address at less cost per unit of forwarding.
Platform Design Examples
NetFlow – Make layer 4 decisions.
Switch – way to add Layer 3 to forwarding decision
Router – way to offload forwarding decisions from the main processor to a
distributed forwarding processor
Classical Routers – Bus Switching.Fast Switching. CPU Hold Queue Forwading,
or Process SwitchingNetFlow. CEF. Multiprotocol Label Switching.Tag/Label.
High-Performance “Switches”. Platform Design Examples
Fore Platform Examples – PowerHub products. Distributed switching
architecture. Shared memory model. Network controller stores a packer in RAM.
Forwarding processor has access.
Platform Design Examples – PowerHub has seven different processes (Packet
reception, Receive polling, Forward polling takes place in the main
processor(MCPU), Management functions, ie SNMP and routing updates, done in
MCPU, Transmit polling, Packet transmission, Transmit clean-up)