Chapter 4
How Do Switches and Router Find
Paths
Metrics and Cost
 Both Routers and Bridges use concept of
metric, or cost.
 Metric-number that makes a path more or
less preferable.
 Dynamic routing may or may not be optimal
when we speak of quality of service
requirements.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Metrics and Cost
 Routers
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Depending on path determination method, can actually
evaluate the metric of the entire route.
Can compute a metric as a value that applies to an endto-end path.
Cost associated with outgoing interfaces. The end-to-end
metric is usually the sum of the interface mechanisms.
 Bridges

Use metrics to establish their position in the spanning
tree topology.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 RIB receives information about potential routes
from a variety of sources.
 Routing protocols can provide information for
forwarding the packets of the routed protocol.
 Dynamic Routing comes from
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RIP
IGRP(Interior Gateway Routing Protocol)
Enhanced IGRP(EIGRP)
OSPF(Open Shortest Path First)
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths


IS-IS(Intermediate System to Intermediate
System) Intra-Domain Routing Exchange
Protocol.
BGP(Border Gateway Protocol).
 Router goes through series of decisions that
lets it select the best paths from the larger set
of possible ones and installs them in the RIB.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principle of Layer 3 Paths

Before a router can forward, it must:
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Obtain information about potential routes, usually through static
configuration or dynamic information exchange that populates
the RIB.
If dynamic information exchange is used, participate in
exchanges with other routers.
Install the best of these routes in the RIB, and , if there is a
separate FIB, populate the FIB.
Look up the destination address of incoming packets in the FIB.
RIB process optionally applies filtering rules to the
information it learns.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Potential sources of routes include the following:
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Hardware status and configuration of local interfaces,
with local address configuration
Software-defined static and quasi-static routes. A quasistatic route is a static route with a preference factor that
lets it be more or less preferable to another route
Dynamic routing processes.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
 Classic Route Lookup Algorithm

RFC 1812 is really an algorithm for route lookup rather
than route installation.

Basic match:
 Prefix part of destination address is extracted. Any routes that do not
include that prefix are rejected.
 Destination address 10.1.0.1/16. 10.2.0.0/16 rejected; 10.0.0.0/8 and
0.0.0.0/0 are accepted.

Longest match:
 If there are multiple routes that meet the basic match, routes with the
longest number of matching bits is selected.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Weak Type of Service(TOS)
 Best Metric

 Routes can have some numerical attribute associated with
them.
 Best is defined for each routing protocol, but is usually the
arithmetically lowest value.
 No one metric is appropriate for all situations.
 Depending on the implementation, static routes may or may
not have metrics.

Vendor Polices: Implementation specific
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Default Routes
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The least specific route possible, but one that
matches all destinations not matched by a more
specific route.
By convention is a route to 0.0.0.0/0
Layer 3 address have at least 2 levels of
hierarchy:

Prefix and host
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths

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Fully meshing a network does not increase its reliability.
If anything it can decrease it.
One of the most common ways to keep the routing table
to a reasonable size is to use hierarchical addressing
structures and the tree organizations they make possible.
Size of the routing table is a matter for the CPU load to
re-compute it than looking up individual entries for
forwarding.
The greater the number of routes, the greater the
probability one of them is going to flap(oscillate),
requiring frequent recomputation.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Without a hierarchy, it is necessary to search tables
containing every address in the domain.
 Hierarchy can break the search into a small number
of sub-searches.
 Default routing is a way to keep the routing tables
small.
 Default routing also is administratively convenient.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Distribution router can participate full
routing with other backbone routers, but
should filter out non-default routes from
being advertised to edge routers.
 Default, or hierarchical, routing is convenient
administratively because only the higher
level core routers need to be updated to know
about new networks.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principle of Layer 3 Paths
 More complex default routing schemes can
associate a metric with multiple instances of
a default route.
 The router can select the best default route,
or possibly load-share among multiple
default routes.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Installing Routes in the RIB: Administrative
Preference Factors:


The route installation process next checks to see
if the destination of this rout is not already in the
RIB. If it is new, the route is installed.
Tables 4.1-4.4
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Preferences in Changing Routing Protocols:
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One situation in which you might want to set a
non-default administrative distance is when
converting from IGRP to either OSPF or IS-IS.
On most implementations, you can override the
preference defaults, either for all information
from a given source or for specific routes.
Steps in changing a routing protocol….
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Load Sharing:


Routes must have the same administrative
distance even to be considered for load sharing.
OSPF external routes or BGP routes are really
not intended to be load balanced.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Metrics in Route Selection:

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Real routing protocols use different means of computing
the metric.
Factors used in computing metrics include the following:

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Link bandwidth. IGRP, EIGRP, OSPF.
Link delay. IGRP and EIGRP, manually compute interface cost
for OSPF.
Administrative preferences, such as monetary costs.
 Can be manually computed as an OSPF int cost. Bandwidth for
IGRP or EIGRP sometimes is artificially raised or lowered to
provide a preference.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
Link error rate. Optionally used by EIGRP and IGRP
 Link utilization. Optionally use by EIGRP and IGRP.
Not recommended.

Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Quasi-Static Routes
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Floating static routes
Set of static routes with different/or same
preference when you need to load balance.
Need to be configured manually.
Useful for dial backup of dedicated facilities
when routing updates fail on dedicated link.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
 Automating the Generation of Static Routes.
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Network administrator need to administer the subnets
assigned to the edge routers.
General principles of address administration.
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Try to avoid entering an address more than once.
Automate the updating of your router configurations created from
your user address assignment database and loaded into routers
with tftp or telnet/expect.
Generate human-readable reports at the same time, so you have
records for troubleshooting and for justifying address space
allocations.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Principles of Layer 3 Paths
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths
 Layer 2 path determination often specifies
where not to go to reach a destination.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths
 Spanning Trees

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Spanning Trees act more to block looping paths from
forming than to identify paths to a destination.
Frames are forwarded over interfaces that are not
blocked by the spanning tree algorithm.
IEEE 802.1D Transparent Bridge Topologies

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Based on Spanning Tree
Bridge with minimum number of active links
Real Bridge implementations usually provide a
mechanism of giving preference to certain nodes and
certain links in Designing
forming
the tree.
Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths

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One bridge is selected as the root, using the rule of the
bridge with the highest priority.
Second, selecting bridge with the lowest unique address.
All other bridges determine their least-cost path and root
port leading to that path. Lowest metric. To the root
bridge.
One bridge is made the designated bridge(least cost path
to root bridge) of all the bridges on that LAN.
All bridge ports being neither a Designated Port nor a
Root port are blocked.
Pruning-optimizing
bridge topology after initial setup.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths
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Bridges negotiate with each other and decide a
root.
End systems do not participate.
Forwarding is done in the direction/or through
the root.
IEEE 802.1D establishes a method in which a
root is selected automatically.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths

Two parts to 802.1D
Initial tree topology definition
 Continued maintenance on routing tables.

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Tree grows from single node, the root bridge
Blocking
Only one bridge can forward traffic onto the links
between 2 bridges.
 One path to one destination.

Designing Routing and Switching
Architectures
Howard C. Berkowitz
Bridge 1 can forward traffic
onto the links btwn
port 5 of bridge 1 and port
5 of bridge 5
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Layer 2 Paths

Designer selects a physical topology
So traffic move through the least number of bridges.
 Over the fastest links.

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Active topology

Set of bridges, paths and ports interconnected by ports
in the forwarding state.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Virtual and Emulated LANs
 VLANs and ELAN’s

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No special path determination process.
Encapsulation mechanisms that encapsulate a payload
protocol. Ie. 802.3 or 802.5 and associate them with a
VLAN or ELAN.
Delivery protocol

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Fast E or Gig E or ATM
Extend the spanning tree model by multiplexing
spanning trees on a system of shared trunks.
Tunneling mechanism.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Virtual and Emulated LANs
 ELANs (LANE)

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Uses ATM trunking.
LANEv2
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Coupled with Multiprotocol over ATM(MPOA)
Hybrid layer 2/3 technology capable of controlled QoS.
Tunneling Components
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Original payload information
Tunneling information-receiver knows that incoming PDU
contains tunneled information
Delivery protocol-used to carry the tunneled information over the
infrastructure.
Designing Routing and Switching
Architectures
Howard C. Berkowitz
Virtual and Emulated LANs
 2 kinds of VLAN trunking

Tunneling method

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VLAN header
IEEE 802.1Q

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Inserts a VLAN tag into the existing header
Source and destination MAC addresses are not changed.
 LANEv1

Runs over ATM links
Designing Routing and Switching
Architectures
Howard C. Berkowitz