Chapter 5 Part 1

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Chapter 5 Part 1

Dynamic Interior Routing Information
Mechanisms

Designing Routing and Switching
Architectures

Howard C. Berkowitz


Chapter 4 reviews the internals of routing
protocols to the extent relevant to network
design.


Neighbors
-
share a common transmission
medium.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Dynamic routing is the most common source
of routing information.


In network design you want to balance static
and dynamic routing.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Logical steps in path determination:


Address discovery


Neighbor discovery


Specialization


Forwarding


Routing information exchange


Dead detection

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Router’s Own Identity


Early protocols


IP address of interface sending the packet sufficient


Modern Protocols


Router identifier that is not associated with a physical interface.


Uniquely identifies the router


Determining the router’s identity is the first step of
initialization.


Router builds internal data structures


Starts to load its tables with information about directly connected
media.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Generates a preliminary set of routes and
submits them to the RIB installation process.


After local routes are installed, or in parallel to
their installation, the router attempts to find
neighbors and exchange information with them.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Information to be exchanged


Dynamic routing protocol


The existence of neighboring routers


The existence of reachable destinations.


The desirability of paths to reachable
destinations(might)


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Path Calculation and Routing Information
Flooding.


Two algorithms


Distance Vector


RIP, IGRP(Interior Gateway Routing Protocol),
EIGRP(Enhanced)
-
Diffusing update algorithm(DUAL)


Link
-
state


OSPF(Open Shortest Path First), IS
-
IS(Intermediate System
-
to
-
Intermediate System.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Area


Collection of routers and routes.


Boundary defines functional area.


Distance vector


Each network entity generates a map of its paths to all connected
nodes in the network.


Create cost tables that contain the router’s perception of the
cost to all destinations.


Only sends this map to adjacent neighbors


Link
-
State


Mirror image of distance vector.


Create cost maps only of the paths to their adjacent neighbors


Flood entire routing domain(or area) with these maps.


More resource intensive.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Distance vector


RIP, IGRP


No idea which router generated a route


No idea when route was generated.


Old data to build tables that generate loops.


EIGRP


Measures to prevent loop formation.



Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Link
-
State


OSPF


Large networks


Updates contain originator and age of originator.


Persistent topological database


Copy in each router in an area.


IS
-
IS


Appropriate for some specialized applications


Primarily used in large internet service providers


Telecom operational networks(SONET)

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


RIPv1 and IGRP


Small networks


Converge much more slowly


Cannot handle classless addressing.


RIPv2


Can handle classless routes


No scaling advantages over RIPv2


No real idea of route summarization.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Convergence


Exists when the router has a valid path to all
reachable destinations.


Factors involved in convergence


Time needed to load an initial set of routes.


Failure detection time after which reconvergence can
begin.


Time for announcement of a new route to propagate
from the most distant router to the local router.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Factors involved in convergence(cont.)


Rules that prevent route announcements from being
accepted for a period of time.


Delays before recomputing routing tables.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Problem of overloading Protocol
Mechanisms


Some mechanism that started with a single
function has additional functions assigned to it.


First generation (RIP)
-
mechanism of tracking
hop count two purposes


Loop detection through count
-
to
-
infinity


Route metric


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Second
-
generation of distance vector (IGRP)


Deleted hop count


Link
-
state


Do not use hop count


EIGRP


Does not use hop count.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Hop Count and Loop detection


Infinite loops


Count
-
to
-
infinity


Hop count steadily increments as the routing loop
continues.


Count reaches an infinite value.


RIP value is 16

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Split Horizon


Mechanism to prevent routing loops


Two types


Simple split horizon
-
prevents re
-
advertising to router where it
first learned.


Poisoned reverse
-
sends an update back to the originator, but with
an infinite metric to indicate the sending router believes the route
to be down.


Holddown timers


Reduces loops
-
takes longer to recover from failures.


After router hears that a given route is down, it does not hear
any updates for the given holdown time.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Metrics in Modern Routing Protocols.


IGRP and EIGRP


Primarily base their metrics on bandwidth


OSPF


Primarily base their metrics on bandwidth in most
implications.




Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Periodic vs. Triggered Updates:Dead Routers


Periodic announcements like that are a source of
overhead.


Updates when something changes.


RIP and IGRP


If a router does not send out updates, it can be assumed to be
down.


OSPF, IS
-
IS, EIGRP


Low overhead function called hello subprotocol


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Neighbor and Adjacency Relationships



Newer protocols try to be selective to optimize the use
of bandwidth.


Newer protocols send updates only when they represent a
significant change.


They send updates only to the routers affected by the
changes.


OSPF sends changes only to designated routers (DRs)


DRs only send the most recent significant updates to non
-
designated routers.


EIGRP routers do not propagate updates that do not
change the routing table of the router that received the
update.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Peering and Workload


EIGRP talks to all neighbors, but its diffusing update
mechanism limits propagation of information that is
not useful.


DR mechanisms in OSPF and IS
-
IS are a good
example of traffic reduction optimization in broadcast
networks.



Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Media Issues


4 Basic medium types:



Broadcast


Nonbroadcast multiple access (NBMA)


Packet switching


Point
-
to
-
point


Demand


dialups

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Stub Media


Some routers serve stub networks(networks that do
not have routers)


Load sharing


Do not need to propagate routing updates because
there are no routers to process it.


Default routes and local routing information is all that
is needed.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Sharing Information Among Routing Sources.



Acceptance


General process of receiving routing information


Acceptance policy


Rules used to accept routing information


Must insure there is no feedback between routers


RFC 1812


Router should be relatively suspicious in accepting data from
other routing systems.


Routers must be paranoid about accepting routing data from
anyone and must be especially careful when they distribute
routing information provided by another party.

Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Redistribution(term invented by Cisco)


General acceptance of routing information from sources other
than the current source.


Importing and exporting


One route learns from another when it imports/exports routes.


Inject


Inject foreign routes into a routing process.


Must import and export for actual transfer to take place.


Designing Routing and Switching
Architectures

Howard C. Berkowitz

Dynamic Routing Principles


Importing is logically just before installation in
RIB


Should design the redistribution to minimize the
full mutual exchange of routing information and
make it heirarchical whenever possible to
simplify loop prevention.


Designing Routing and Switching
Architectures

Howard C. Berkowitz