Download Selecting Bridging, Switching, and Routing Protocols

CIS460 – NETWORK
ANALYSIS AND DESIGN
CHAPTER 7 Selecting Bridging, Switching, and
Routing Protocols
Introduction
• In this chapter we are going to look at bridging,
switching, and routing protocol attributes of:
– Network Traffic characteristics
– Bandwidth, memory, and CPU usage
– The approximate number of peer routers or switches
supported
– The capability to quickly adapt to changes in an
internetwork
– The capability to authenticate route updates for security
reasons
Making Decisions as Part of the TopDown Network Design Process
• Factors involved in making sound
decisions:
– Goals must be established
– Many options should be explored
– The consequences of the decisions should be
investigated
– Contingency plans should be made
• Use a decision to match options with goals
Making Decisions as Part of the TopDown Network Design Process (Cont’d)
• Table 7-1 shows a decision table
• Once decision is made look at it to determine:
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What could go wrong
Hs it been tried before
How will customer react
Contingency plans if customer disapproves
• Can use during both logical and physical design
phase
Selecting Bridging and Switching
Methods
• Decision making is simple because of few options
– If includes Ethernet bridges and switches most likely use
transparent bridging with spanning-tree protocol
– Might also need a protocol for connecting switches that
support virtual LANs
– With Token Ring networks options include source-route
bridging (SRB), source-route transparent (SRT) bridging
and source-route switching (SRS)
Characterizing Bridging and
Switching Methods
– Bridges operate at Layers 1 and 2 of OSI
– Determine how to forward a frame based on
information in Layer 2 header
– Bridge does not look at Layer 3 information
– Bridge segments bandwidth domains so that devices
do not compete with each other for media access
control
– Bridge does forward Ethernet collisions or MAC
frames in a Token Ring network
Characterizing Bridging and
Switching Methods (Cont’d)
– Bridge does not segment broadcast domains. It sends
broadcast packets out all ports
– Bridges normally connect like networks but can be a
translation or encapsulating bridge
– A switch is like a bridge only faster
– Switches take advantage of fast integrated circuits to
offer very low latency
– Switches usually have a higher port density and a
lower cost per port
Characterizing Bridging and
Switching Methods (Cont’d)
• Bridges do store and forward
• Switches can be store and forward or cutthrough
• Cut-through is faster but more prone to
letting runts or error packets through
• On a network that is prone to errors do not
use cut-through processing
• Adaptive cut-through switching
Transparent Bridging
• Most common Ethernet environments
• A transparent bridge (switch) connects one
or more LAN segments so that end systems
on different segments can communicate
with each other transparently
• Looks at the source address in each frame to
learn location of network devices
• It develops a switching table (Table 7-2)
Transparent Bridging (Cont’d)
• Receives a packet look sup address in switch table
• If no address it sends the frame out every port like
a broadcast frame
• Send Bridge Protocol Data Unit (BPDU) frames to
each other to build and maintain the spanning tree
• Sends BPDU to a multicast address every two
seconds
Source-Route Bridging
• Developed for Token Ring networks in the 80s by
IBM
• Uses a source-routing-transparent (SRT) standard
• An SRT bridge can act like a transparent bridge or
a source-routing bridge depending on whether
source-routing information is included in a frame
• Not transparent if pure SRB is used
Source-Route Bridging (Cont’d)
• Uses explorer frames
– All-routes explorer - take all possible paths, take just
one route back
– Single-route explorer - takes just one path and
response take all paths or just one back
– With single-route explorer frames the spanning-tree
algorithm can be used to determine a single path
– Scalability is impacted by amount of traffic when
all-routes explorer frames are used
Source-Route Switching
• SRS is based on SRT bridging
• SRS forwards a frame that has no routing
information field
• Learns the MAC addresses of devices on the ring
• Also learns source-routing information for devices
on the other side of SRB bridges
Source-Route Switching (Cont’d)
• Benefits
– Rings can be segmented without adding new ring
numbers
– can be incrementally upgraded to transparent bridging
with minimal disruption or reconfiguration
– does not need to learn the MAC addresses of devices on
the other side of source-route bridges
– can support parallel source routing paths
– can support duplicate MAC addresses
Mixed-Media Bridging
– Mixture of Token Ring, FDDI and Ethernet bridging
– Encapsulating bridging is simpler than translation
bridging but is only appropriate for some network
topologies
– Encapsulating bridge encapsulates an Ethernet frame
inside an FDDI or Token ring frame for transversal
across a backbone network that has no end systems
Mixed-Media Bridging (Cont’d)
• Support for end systems on a backbone then
need to use translation bridging which translates
from one data-link-layer protocol to another
– Problems
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Incompatible bit ordering
Embedded MAC addresses
Incompatible maximum transfer unit (MTU) sizes
Handling of exclusive Token Ring and FDDI functions
No real standardization
Mixed-Media Bridging (Cont’d)
• While FDDI is a common choice for
backbone networks in campus network
designs to avoid translating Ethernet and
FDDI frames should use 100-Mbps Ethernet
or Gigabit Ethernet on backbone segments
Switching Protocols for
Transporting VLAN Information
• When VLANs are implemented in a switched network
the switches need a method to make sure intra-VLAN
traffic goes to the correct segments
• Accomplished by tagging frames with VLAN
information
• two tagging methods:
– adaptation of the IEEE 802.10 security protocol
– Inter-Switch Link (ISL) protocol
IEEE 802.10
• A security specification used as a way of placing
VLAN identification (VLAN ID) in a frame
• Inserted between the MAC and LLC headers of
the frame
• The VLAN ID allows switches and routers to
selectively forward packets to ports with the same
VLAN ID
• VLAN ID removed from frame when forwarded
to destination segment
Inter-Switch Protocol
• Another method for maintaining VLAN
information as traffic goes between switches
• Developed to carry VLAN information on a 100Mbps Ethernet switch-to-switch or switch-torouter link. Can carry multiple VLANs
• ISL link is call a trunk. A trunk is a physical link
that carries the traffic of multiple VLANs between
two switches or between a switch and a router.
Allows VLANs to extend across switches
VLAN Trunk Protocol
• Some networks have a combination of different
media types
• VLAN trunk protocol (VTP) allows a VLAN to
span the different technologies by automatically
configuring a VLAN across a campus network
regardless of media type
• VTP is a switch-to-switch and switch-to-router
VLAN management protocol that exchanges
VLAN configuration changes as they are made to
the network
Selecting Routing Protocols
• A routing protocol lets a router dynamically
learn how to reach other networks and
exchange this information with other routers or
hosts
• Selecting routing protocols is harder than
selecting bridging protocols because there are
so many
• Made easier using a table such as 7-1 to pick
the best one
Characterizing Routing Protocols
• General goal to share network reachability
information among routers
• Some send complete other only an update
• Differ in scalability and performance
characteristics
– Many are designed for small networks
– Static environment
– Some are meant for connecting interior campus
networks
Distance-Vector Versus LinkState Routing Protocols
• Two major classes: distance-vector and link-state
• Distance-vector protocols
– IP Routing Information Protocol (RIP) Version 1 and 2
– IP Interior Gateway Routing Protocol (IGRP)
– Novell NetWare Internetwork Packet Exchange Routing
Information Protocol (IPX RIP)
– AppleTalk Routing Table Maintenance Protocol (RTMP)
– AppleTalk Update-Based Routing Protocol (AURP)
– IP Enhanced IGRP
– IP Border Gateway Protocol (BGP) (path-vector)
Distance-Vector Versus LinkState Routing Protocols (Cont’d)
• Vector means distance or course. A distancevector includes information on the length of the
course. Many use hop count
• A hop count specifies the number of routers that
must be traversed
• Maintains a distance-vector routing table that
lists know networks and the distance to each.
• Sends table to all neighbors, or an update after
first transmission
Distance-Vector (Cont’d)
– Split Horizon, Hold-Down, and Poison-Reverse
Features
• Split-horizon technique - sends only routes that are
reachable via other ports
• Hold-down timer - new information about a route to a
suspect network is not believed right away. A standard
way to avoid loops
• Poison-reverse messages - way of speeding convergence
and avoiding loops. When a router notices a problem it
can immediately send a route update that specifies the
destination is no longer reachable
Link-State Routing Protocols
– Do not exchange routing tables
– Exchange information about the status of their
directly connected links using periodic multicast
messages
– Each router builds its own routing table
– Protocols
• IP Open Shortest Path First (OSFP)
• IP Intermediate System-to-Intermediate System (IS-IS)
• NetWare Link Services Protocol (NLSP)
Link-State Routing Protocols
(Cont’d)
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Converge more quickly
Less prone to routing loops
Require more CPU power and memory
More expensive to implement and support
Harder to troubleshoot
Routing Protocol Metrics
• Used to determine which path is preferable
when more than one path is available
• Vary on which metrics are supported
• Distance-vector use hop count
• Newer protocols take into account delay,
bandwidth, reliability and other factors
• Metrics can effect scalability
Hierarchical Versus NonHierarchical Routing Protocols
– Some routing protocols do not support hierarchy
– Normally all routers perform same tasks
– Hierarchical protocols assign different tasks to
different routers and group routers in areas
– Some routers communicate with local routers in
the same area and other routers have the hob of
connecting areas, domains, or autonomous
systems
Interior Versus Exterior Routing
Protocols
• Interior protocols, such as RIP, OSPF, and
IGRP are used by routers within the same
enterprise or autonomous
• Exterior such as BGP perform routing
between multiple autonomous systems.
Classful Versus Classless Routing
Protocols
– A classful routing protocol always considers the
IP network class
– Address summarization is automatic by major
network number and discontiguous subnets are
not visible to each other
– Classless protocols transmit prefix-length or
subnet mask information with IP network
addresses. The IP address can be mapped so
that discontinuous subnets and VLSM are
supported
Dynamic Versus Static and
Default Routing
• Static routes are often used to connect to a
stub network
• A stub network is a part of an internetwork
that can only be reached by one path
• Internal routers can simply be configured
with a default route that points to the ISP
Scalability Constraints for
Routing Protocols
• Consider customer’s goals for scaling the
network to a larger size
• There are a number of questions that relate
to scalability that should be answered
• They can be answered by watching routing
protocol behavior with a protocol analyzer
and by studying the relevant specifications
Routing Protocols Convergence
• Convergence is the time it takes for routers
to arrive at a consistent understanding of the
internetwork topology after a change takes
place
• Understand the frequency of changes, links
that fail often, etc
• Convergence time is a critical design
constraint
Routing Protocols Convergence
(Cont’d)
– Convergence starts when a router notices a link
has failed
– If a serial link fails it can start immediately. If
it uses keepalive frames it starts convergence
after it has been unable to send two or three
keepalive frames
– If use hello packets and the hello timer is
shorter than the keep alive timer then routing
protocol it can start convergence sooner
IP Routing
• Most common protocols are RIP, IGRP,
Enhanced IGRP, OSPF, and BGP
Routing Information Protocol
– The first standard routing protocol developed for
TCP/IP environments
– It is a distance-vector protocol that features
simplicity and ease-of-troubleshooting
– Uses a hop count to measure the distance to a
destination. Cannot be more than 15 hops
– RIPv2 developed to address some of the
scalability and performance problems with
Version 1
Interior Gateway Routing
Protocol
• Meet needs of customers requiring a robust
and scalable interior routing protocol
• Uses composite metric based on:
bandwidth, delay, reliability, and load
• Load balances over equal-metric paths and
non-equal-metric paths. (3 to 1)
• Has a better algorithm for advertising and
selecting a default rout than RIP
Enhanced Interior Gateway
Routing Protocol
– Meet the needs of enterprise customers with
large, complex, multiprotocol internetworks
– Goal is to offer quick convergence on large
networks. Diffusing update algorithm (DUAL)
guarantees a loop-free topology
– The router develops a topology table that
contains all destinations advertised by
neighboring routers. It can scale to thousands
of nodes
Open Shortest Path First
– Open standard supported by many vendors
– converges quickly
– authenticates protocol exchanges to meet
security goals
– supports discontiguous subnets and VLSM
– sends multicast frames vice broadcast frames
– does not use a log of bandwidth
– can be designed in hierarchical areas
Open Shortest Path First (Cont’d)
– Propagates only changes
– accumulate link-state information to calculate
the shortest path to a destination
– all routers run the same algorithm in parallel
– Allows sets of networks to be grouped into
areas
– A contiguous backbone area, called Area ) is
required
– Assign network numbers in blocks that can be
summarized
Border Gate Protocol
• iBGP used at large companies to route
between domains
• EBGP is often used to multihome an
enterprise’s connection to the Internet
• Main goal is to allow routers to exchange
information on paths to destination
networks
Apple Talk Routing
• Three options:
• Routing Table Maintenance Protocol (RTMP)
• AppleTalk Update-Based Routing Protocol
(AURP)
• Enhanced IGRP for AppleTalk
• RTMP is most common because it is easiest to
configure and is supported by most vendors
Routing Table Maintenance
Protocol
• Routing table sent every 10 seconds using
split horizon
• Works closely with Zone Information
Protocol (ZIP)
• Checks routing table updates and sends ZIP
query
Using Multiple Routing and
Bridging Protocols
• Important to realize you do not have to use
the same routing and bridging protocols
throughout the internetwork
• To merge old networks with new networks
it is often necessary to run more than one
routing or bridging protocol
• Solutions include source-route transparent
bridging, external routes in OSPF and RIP2
Redistribution between Routing
Protocols
– Redistribution allows a router to run more than
one routing protocol and share routes among
routing protocols
– Network administrator must configure
redistribution by specifying which protocols
should insert routing information into other
protocol’s routing tables
– A router can learn about a destination from
more than one protocol
Integrated Routing and Bridging
• CISCO offers support for IRB which
connects VLANs and bridged networks to
routed networks within the same router
• One advantage of IRD is that a bridged IP
subnet or VLAN can span a router
Summary
• Deciding on the right bridging, switching,
and routing protocols for your customer will
help you select the best switch and router
products for the customer