Chapter 4 How Do Switches and Router Find Paths. Designing Routing and Switching Architectures Howard C. Berkowitz Metrics and Cost Both Routers and

Chapter 4

How Do Switches and Router Find Paths

Designing Routing and Switching Architectures

Howard C. Berkowitz

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.


Howard C. Berkowitz

Metrics and Cost

Routers 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 end-to-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.


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 RIP IGRP(Interior Gateway Routing Protocol) Enhanced IGRP(EIGRP) OSPF(Open Shortest Path First)


Howard C. Berkowitz


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.


Howard C. Berkowitz

Principle of Layer 3 Paths

Before a router can forward, it must: 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.


Howard C. Berkowitz


Potential sources of routes include the following: Hardware status and configuration of local interfaces,

with local address configuration Software-defined static and quasi-static routes. A quasi-

static route is a static route with a preference factor that lets it be more or less preferable to another route

Dynamic routing processes.


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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.


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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


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Default Routes 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


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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.


Howard C. Berkowitz


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.


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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.


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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.


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Howard C. Berkowitz


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


Howard C. Berkowitz


Preferences in Changing Routing Protocols: 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….


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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.


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Metrics in Route Selection: Real routing protocols use different means of computing

the metric. Factors used in computing metrics include the following:

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.


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Link error rate. Optionally used by EIGRP and IGRPLink utilization. Optionally use by EIGRP and IGRP.

Not recommended.


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Quasi-Static Routes 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.


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Automating the Generation of Static Routes. Network administrator need to administer the subnets

assigned to the edge routers. General principles of address administration.

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.


Howard C. Berkowitz



Howard C. Berkowitz

Layer 2 Paths

Layer 2 path determination often specifies where not to go to reach a destination.


Howard C. Berkowitz

Layer 2 Paths

Spanning Trees 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

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 forming the tree.


Howard C. Berkowitz

Layer 2 Paths

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.


Howard C. Berkowitz

Layer 2 Paths

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.


Howard C. Berkowitz

Layer 2 Paths

Two parts to 802.1D Initial tree topology definitionContinued maintenance on routing tables.

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.


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Bridge 1 can forward trafficonto the links btwn port 5 of bridge 1 and port5 of bridge 5


Howard C. Berkowitz

Layer 2 Paths

Designer selects a physical topology So traffic move through the least number of bridges.Over the fastest links.

Active topologySet of bridges, paths and ports interconnected by ports

in the forwarding state.


Howard C. Berkowitz

Virtual and Emulated LANs

VLANs and ELAN’s 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 Fast E or Gig E or ATM

Extend the spanning tree model by multiplexing spanning trees on a system of shared trunks.

Tunneling mechanism.


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ELANs (LANE) Uses ATM trunking. LANEv2

Coupled with Multiprotocol over ATM(MPOA) Hybrid layer 2/3 technology capable of controlled QoS.

Tunneling Components Original payload information Tunneling information-receiver knows that incoming PDU

contains tunneled information Delivery protocol-used to carry the tunneled information over the

infrastructure.


Howard C. Berkowitz


2 kinds of VLAN trunking Tunneling method

VLAN header IEEE 802.1Q

Inserts a VLAN tag into the existing header Source and destination MAC addresses are not changed.

LANEv1 Runs over ATM links

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Chapter 4 How Do Switches and Router Find Paths. Designing Routing and Switching Architectures Howard C. Berkowitz Metrics and Cost Both Routers and