Routing primer

Routing Architecture

Module 2
Routing Fundamentals

Basic Problems
Principles, Classification
Operation

Author: Rolf Augstein
raugstein@rolfaugstein.com
January 2006

Feel free to use this publication for private, non-commercial purposes.

Objectives

1. Basic understanding of routing graphs
2. Describe the process of routing through a given network
3. Identify problems with Distance-Vector and Link-State protocols
4. Understand the solution for different routing problems
5. Outline different routing classifications
6. Describe the process of route summarization
7. Understand the relationship IP addressing scheme - routing functionality

Rolf Augstein © 2006 All rights reserved Page 1

Key terms:

• Aggregate Route
• Classless Inter-Domain Routing (CIDR)
• Classless Routing
• Convergence
• Count-to-Infinity
• Distance Vector (DV)
• Exterior Routing Protocol EGP)
• Flapping Route
• Floating Static
• Fixed Length Subnet Mask (FLSM)
• Interior Routing Protocol (IGP)
• Link State (LS)
• Metric
• Poison Reverse
• Preference Value
• Prefix Routing
• Route Summarization
• Routing Hierarchy
• Routing Loops
• Smart Router
• Split Horizon
• Variable Length Subnet Mask (VLSM)


Routing Principles
Routing in general is a method of finding the best way through a given network
of roads or rail-tracks, for example. The term “best way” depends on individual
parameters. It could mean the fastest, cheapest, or most comfortable one.
Mathematical algorithms like “Dijkstra”, are used to find out the “best” way
through a given network. The discipline dealing with this kind of problems is
called the graph theory.

Graph

Graphs are used to show all possible ways from a source to a destination. Not
all combinations of ways are possible in the typical graph below.

Example:

• It is not possible to go directly from node C to node B
• You can go from node B to node F, but not same way back

Theory of graphs

8 H
D
3 3
3 E
13
C
11
F 1
7
4

6 5
3
2

2 B G
A

- Where are the possible paths ?
From A to H:
- What´s the cost for each path ?
Find the best way
- What´s the best path ?


Further, there are different metric values through certain paths between two
nodes. The metric value from node A to node C is 6. The opposite direction,
node C to node A has a metric value of 5 only.

Different elements are used to draw relations between certain nodes.

Elements of Graphs

Examples:
3

Serial Links, Shared Medium, etc.
- both directions, equal cost

5

Special Links (Satellite)
- one direction

10

3 ADSL

- both directions, unequal cost


Important Terms

A graph consists of vertices (nodes) and edges. Two vertices are adjacent, if
they are connected by an edge.

Example: This is a graph with 6 vertices (nodes) and 7 edges.

A graph is called a complete graph, if each edge is connected to each of the
others in the graph. Below are the first 5 complete graphs.

In data networking, this kind of graph is often called a “fully meshed network”.
The number of edges in a complete graph is increasing dramatically with each
new node. The formula to calculate the number of edges (possible ways) in a
fully meshed network is:

n * ( n-1)

2

One more important term is directed graph. A digraph (directed graph) is a
graph where edges are directed. This means that there are only certain possible
ways through the graph.

The arrows mark the direction from which the graph is determined. In this
example, we have a complete graph but there is no direct path from C to B.


Basic Routing Topologies

Because of the size of modern data networks, it is not possible to connect each
node with all other nodes. So, fully meshed networks are normally not subject of
a network design.

Figures

Partially meshed

Fully meshed

Partially meshed, Hub-and-Spoke

Fully meshed networks can be found in parts of Wide Area Networks like ATM,
Frame Relay or X.25. On the other hand a meshed network is more reliable
because of the redundancy. But the routing becomes very complex.

In IP data networks each node is represented by an IP Router or a Switch with
Layer 3 capabilities. From this point of view, the graphs draw the network
topology from the IP layer.

In most cases the design is based on partially meshed networks. This means
not all nodes are connected to each other. The design is more based on
geographic issues or available bandwidth etc.

The Hub-and-Spoke architecture is often used where smaller locations like
SOHO or ROBO are connected to a centralized node.


Metric

Criteria for finding the best way

? ?

Path length Reliability
Metric Cost factor ....

Bandwidth

The value metric is used in all routing procedures or protocols. In most cases
the metric represents nothing more than an abstract value. Depending on the
routing procedures, the metric has different meanings. Sometimes the metric
counts the number of hops between two nodes. In other cases the metric is
calculated out of the available bandwidths on the path, the delay, the MTU, the
load, or the communication cost.

Note

The smaller the calculated metric, the better the way is. This is true for all
dynamic and static routing procedures.

Different routing protocols use different metric calculations. For this reason
there is no compatibility between the metric values of dynamic routing protocols.
To overcome this problem, it is possible to use route redistribution. This is
covered later in this module.


Each routing protocol uses a default method to calculate the metric between the
nodes. For the network administrator it is possible to influence and manipulate
the metric calculation and the way these information are passed between
neighbours.

It is possible to alter the entire routing behaviour in a given network disregard of
real physical structure and cabling.

Note

Therefore the administrator must clearly understand all aspects of the routing
protocol and it’s behaviour. Do not change metric values in a complex network
structure just to “find out”.

You can the force the entire data flow through a network to take different paths
for special settings.

Example: Asymmetric Routing

Packets to the destination use a different path than the packets back from the
destination. This is called asymmetric routing. With manipulation of the routing
metrics, a router becomes an altered directed graph, a new logical topology of
the data network.


Routing Classification

There are some different ways to make a routing classification. Three are
covered in the following.

Static Routing vs. dynamic Routing

Static vs. Dynamic

Hmm. Which
networks do I need
to reach ?
Tell me, which
networks are
available

Mr. Administrator

Defined by Administrator Learned from Network

Developed by Media-Learning.com © 2005 All rights reserved 2-8

Static Routing

With static routing all destination networks and useable paths must be defined
in the router. These definitions are a big administrative challenge.

All routers at the remote side must have a route back the originating network.
The router does not learn which data packets of a session were routed earlier.
The return packet within a session must be routed back – therefore a back route
has to be defined as well.

Nevertheless, static routing still plays a role even in big networks.


Table: Static Routing

Advantage Disadvantage

No routing updates, less traffic No adaptation when links change
Compatible with each router-system Complexity in bigger networks
No flapping routes

Dynamic Routing

Dynamic routing or adaptive routing uses protocol updates to propagate all
known networks to all adjacent nodes. All possible paths through the data
network are explored and learned. So the routers can take advantage of
redundant links and react automatically whenever a link between two nodes is
lost.

Even the back routes are learned through this dynamic mechanism. There are
various dynamic routing protocols with different level of complexity.

Table: Dynamic Routing

Advantage Disadvantage

All paths are propagated dynamically Routing Updates cause traffic
Adaptation when links change Convergence, “ugly” route effects*
More administrative knowledge

An administrator must have enough knowledge regarding the update behavior
between the routing nodes. They are quite different and come with tricky
problems and solutions. Examples are “Count-to-Infinity”, Split Horizon etc.*

* Problems and effects with Routing Protocols are covered later in detail.


Destination Routing vs. Source Routing

Routing: Destination vs. Source

Destination Routing Source Routing

Routing decision based on Routing decision based on
IP Network to go the Source of the IP packet

Examples: RIP, OSPF Examples: Policy Routing

Arriving IP Packet Destination IP Source IP Data

Whenever a data packet arrives at the router, the destination IP address is
checked against the routing table. If the destination network address is not
defined in the routing table, the packet will be dropped.

When working with static routing or dynamic routing protocols, this is the default
procedure for the IP router in most cases. Routing protocols like RIP, OSPF etc.
are based on destination routing.

It is also possible to use the source part of an IP data packet to make a routing
decision. For this to make work, an administrator must define special route
maps.

Example:

A route map defines to forward all data packets from the network 10.12.5.0/24
to the Ethernet interface 3, and all data packets from 10.12.6.0/24 to the next
hop gateway 10.10.45.1.


The example shows no use of any destination IP addresses to make the routing
decision. Routing decisions are not longer based on best paths with low metric.
Routing becomes a matter of local policies.

Note

This is also called policy based routing. An administrative policy rules how
routing decisions have to be made.

When using this kind of routing, all rules for routing data traffic are defined
statically in route maps. When the network becomes bigger, it can be very
difficult to avoid “loosing routes somewhere in the network”.

It is possible to combine destination routing with source routing within a routing
node. Source routing is often used in conjunction with Quality of Service (QoS).


Interior Routing vs. Exterior Routing

Interior vs. Exterior Routing Protocols

IGP AS 56

EGP

AS 53 IGP

In larger networks it is necessary to use special routing protocols to handle the
huge amount of routing information.

Interior Gateway Protocols

These protocols are used within an administrative area called Autonomous
System (AS). Within an AS an administrator can decide with routing policy to
use. Two or more Autonomous Systems can be linked together with the help of
border routers.

Typical routing protocols are:

RIP Version 1/ 2 Routing Information Protocol
OSPF Open Shortest Path First
IS – IS Intermediate State – Intermediate State
Cisco IGRP Interior Gateway Routing Protocol
Cisco EIGRP Enhanced EIGRP


Note

Autonomous Systems are identified by a 16 Bit number. This number is
administrated from the Internet Assigned Numbers Authority (IANA).

Two Internet RFCs discuss autonomous systems: RFC 1930 (Guidelines for
creation, selection, and registration of an Autonomous System, March 1996)
and RFC 0975 (Autonomous confederations, February 1986)

According to RFC 1930 , "Without exception, an AS must have only one routing
policy. Here routing policy refers to how the rest of the Internet makes routing
decisions based on information from your AS."

Exterior Gateway Protocols

With an Exterior Gateway Protocol capsulated routing information within one
Autonomous System is send to a second AS. The EGP connects Autonomous
Systems by delivering dynamic procedures to propagate routing changes in a
controlled manner.

Typical routing protocols are:

EGP Exterior Gateway Protocol (Old, barely used)
BGP Border Gateway Protocol

BGP design and configuration can be very complex. It is mostly used in some
internet areas where carriers and internet providers are working together.


Routing Operation

Finding the Way

Routing Tables

1 Net Gateway
1a 1 Direct
2 Direct
a 3 2b
4 2b

2a
2
2b Net Gateway
1 2a
b 2 Direct
3 Direct
4 3c
3b 3
Net Gateway 3c
1 3b
2 3b
3 Direct
c
4 Direct
4c 4

The basic idea behind routing protocols is, to send local routing information to
adjacent routing nodes.

All connected interfaces with a configured IP address, cause an entry in the
local routing table. The routing table consists of information to reachable
destination networks. Local networks are marked as “direct connected” or
“local”.

With routing update packets send in a given time interval, neighbor routers
using the same routing protocol learn possible ways to IP networks. In the next
step, all learned routes from adjacent routing nodes are sent again in the next
update cycle.


If a routing node learns routes via OSPF routing, these routes are not updated
by a different routing protocol like RIP. To make these protocols to interact,
route redistribution is necessary.

Next Hop

1
1a

a

I can reach network 3 and 2a
2
4 through my “Next Hop”, 2b
router 2b
b

3b
3
3c

Next Hop: c

4c
4
Interface IP Address of the
directly connected neighbor router

All reachable IP destination networks are learned in the routing table. But the
router has only a limited number of information sources.

Example:

Router “a” has only one information source, which is the adjacent router “b”.
There is no information, telling router “a” that there is a third router “c”. But
router “a” can reach the network “4” through router “b” as well.

This information source is called the “next-hop gateway”. So after some time, a
router learns all reachable networks, but is not aware of all other routers in the
network. This is sometimes referred to as “routers have a flat view of the
network”.

Routers must have a valid route to the next-hop gateway. So always use the
directly connected interface of the next-hop gateway as IP address.


Bellman Ford Algorithm

The Bellman Ford algorithm is used to find the shortest way in a graph and is
the basis of distance-vector routing protocols.

Distance Vector Routing Protocol

RT RT RT RT

Interval n+2 Interval n+1 Interval n

Broadcast Load

Convergence Problem

Metric Restrictions

Distance Vector (DV) Routing

The principle of DV routing is to send routing updates in a defined interval
through all interfaces. These update packets use broadcast addresses, and
contain information about all the reachable networks.

The vector consists of the source address of the sending router. By this address
receiving routers learn the address of the next-hop gateway over which the
propagated networks can be reached.

The distance describes the metric. In most cases this is simply the number of
hops to a destination network. This number is restricted to a maximum of 15
hops.

A typical DV protocol is the Routing Information Protocol (RIP). It is widely used
and implemented in all UNIX and Windows Servers.


By depending on a fixed time interval to send the routing updates to all
neighbors, routing information need a certain amount of time to travel through
the network. This effect is called “convergence”.

The use of broadcast addresses causes in the WAN part of large network some
problems.

The advantage of DV routing is the simple implementation and the easy way to
use it in networks.

Link State Algorithm

Link-State algorithms are the solution for modern routing protocols. But they
operate in a totally different way than the DV protocols.

Link-State Routing Protocol

LSA CPU
Topology
Database Memory

RT
..................
.................
.................
.................
..........
...............

SPF Algorithm

Shortest Path First Tree


Link State (LS) Routing

The basic concept of link-state routing is that every node receives a connectivity
map of the network, in the form of a graph showing which nodes are connected
to which other nodes. Each node independently calculates the best next hop
from it for every possible destination in the network

Each router builds a relationship with all other routers using a link-state
protocol. Different roles like designated router, area router, border router etc.
are assigned. Each node periodically makes up a short message, the link-state
advertisement, (LSA). The LSA´s are used to identify other nodes which are
directly connected and keep track of changes in routing.

All information concerning other routing nodes and reachable networks are
stored in the topology database. Compared to DV routing, a LS router holds
more information about the entire network and does not have a flat view only.
To find the best way through all the reachable destination networks, LS routing
uses the algorithm SPF.

Shortest Path First (SPF)

A routing node uses the stored graph to calculate all paths to each other routing
nodes. The paths with the best metric values are used to forward IP data
packets. The result is a spanned tree with best paths to all destination networks
instead of a flat view compared to DV routing.

The advantage of LS routing is quick reaction to any changes in the network
topology.


Process Topology Changes

Link Up-Down

router% Line protocol down......
or
Keepalive Timer router% Line protocol up......

Entries in Routing Table
..........
C 194.123.123.16 is directly connected, Ethernet0
R network 123.123.0.0 via Ethernet 0
R network 34.23.0.0 via Ethernet 0
All routes associated with
interface Ethernet 0 are C 193.141.147.0 is directly connected, BRI0
not valid any longer ..........

How does a routing node realize changes in the network topology?

Usually, topology changes cause error states on the connected router interface.
The line protocol goes down or the interface hardware fails.

To control the functionality of the interfaces, the Operating System generates
control packets which are sent through the interface.

If the interface signals a problem, the operation state changes and all
corresponding routes are effected in the routing table.


Routing Timer

Update
Time between Updates

network unreachable ....
Invalid network possibly down .....

Time after the entry is
marked as “invalid”

Flushing
Route is erased from
network 13.2.3.0
Routing table

To avoid flapping interfaces and flapping routes the entire state change process
uses a delay mechanism.

Note

The term “flapping” is often used to describe a failure condition, where i.e. an
interface changes the state between up and down very often in small time
intervals. This can cause a lot of problems and effect the entire network routing.

An invalid timer controls when a route is marked as possibly unreachable or
down. This timer is set 2 – 3 times higher than the update timer. At least two
missed updates are necessary to cause a change in routing.

An additionally flushing timer determines when a routing entry marked as
possibly down is erased out of the routing table.


Using multiple Paths

Load Balancing

Route A

Route B

Packets are “balanced” through multiple ways

More Bandwidth
Advantage:
Higher Availability

When the routing process has two or more paths with equal metric to a
destination network, it is possible to send the data packets along these routes.

The data load is balanced. Some routing protocols can perform unequal cost
load balancing with up to 5 different routes.


Load Balancing (cont.)

Problem:

Route A Route B
Different Trip Times 114 ms 262 ms

- Per Destination Load Balancing

- Per Packet Load Balancing

One of the main problems when performing load balancing exists in the different
trip times of particular routes. This can cause problems for data application
when the packets arrive in a different order then actually sent.

Special care must be taken. Different techniques are available to solve the
negative effects.

Example

A gateway has 2 two different paths to the headquarter network. The first
session initiated is sent through the first known path, the second session is sent
through the second path. The third session must use the first path and so on.
This called “Per Destination Load Balancing”.

When using “Per Packet Load Balancing” all packets regardless of the session
ID are balanced over both paths. In this case the load on the different paths is
balanced in a optimized manner. But the risk of packet delays with a higher rate
of retransmissions is more likely.


Control Packet Lifetime

Time-to-Live

IP-Version 4 Header

TTL 23
TTL 23 TTL 22
TTL 22

Decreasing Time-To-Live Counter
when passing through router

In the header of the IP packet, the field TTL takes care of data packets not
travelling in the network for ever. Whenever a routing node forwards an IP
packet, the TTL counter is decreased by one.

A packet with TTL set to 0 is discarded by the router.


Routing Problems

Each routing protocol has advantages and also disadvantages. There is no
perfect routing protocol. An administrator must deal with the pros and the cons
trying to find the best solution for his needs.

Convergence

Convergence Problem

New Route to
194.200.1.0

194.200.1.0 194.200.1.0 194.200.1.0 194.200.1.0 194.200.1.0

300 secs 240 secs 180 secs 120 secs Next Update
in 60 secs

Worse case scenario

A major problem with DV routing is the convergence problem. New information
like changes in routing take quite some time to get to all members of the routing
process.

The negative effect is increasing when networks become bigger and the
changes occur much more often.


Count to Infinity

Count to Infinity

?
Don´t worry ! I have a
route to 194.200.1.0
No Route to
194.200.1.0

194.200.1.0 194.200.1.0 194.200.1.0 194.200.1.0

Worse case scenario

Slow convergence causes additional problems. Routers update routing
information to neighbours, even if they are the source of this information.

This phenomenon is called count-to-infinity, because it leads to a ping-pong
effect until the maximum value for the metric is reached.

So how can one overcome this kind of effects?


Triggered Updates

Solution: Triggered Updates

interface down

Network unreachable
Metric <max. Value>

Neighbor receives Update
with max. Metric
Overcome
Convergence Problem !
Any other Changes are
transmitted immediately

The flow of negative information must be accelerated. Whenever a change in
routing occurs, these changes are transmitted immediately to all adjacencies. If
an entire network is unreachable, the update packets contain the metric value
set to the maximum.

Poison Reverse

This technical term is used to indicate, that a packet with a higher metric or the
maximum metric is set and sent along the reverse path trough the network to
overcome problems like routing loops or count-to-infinity. Poison reverse is a
triggered update to speed up the convergence of the routing protocol.


Interface Hold-down

Update from neighbor
Network
154.34
.23.0 d
own

Route Table
er
Tim Flush network 154.34.23.0 via E0

>entering hold-down for network 154.34.23.0

• Accept no further Information for Network 154.34.23.0
for a certain amount of time

• Avoidance of Routing-Loops

A router should not rely on information arriving on an interface that was sent out
earlier over that interface. When a route is flushed out of the routing table, new
update packets for a particular route from any neighbour are not accepted for
some time.

A router should realize which routes were propagated through the interfaces
and should not accept some routes backward. Again these kinds of problems
occur mainly on DV routing protocols on networks with high convergence.


Loops

Routing Loops

Mrs. Easy Company Intranet with
198.210.25.0
different Administrators

De
fau
… lt R
ou
ute to

te
to
…
o
ult R

200.200.45.0
Def a

195.22.5.0

Default Route to …

Mr. Brainbox

Mr. Theory

Another problem coming up sometimes is a loop in the routing information table.
A routing loop can be caused by a lack of communication between different
routing administrators, for example.

This is a very tricky problem. It looks ridicules – but it is configured very quickly.

Another source for routing loops is the way DV-protocols like RIP are working
as seen in previous chapter. The solution to avoid loops is the Split Horizon.


Split Horizon

Split-horizon is a common solution to avoid routing loops. A cause for the route
loop is that the router propagates routing information learned from a neighbour
to that neighbour back. The idea of the split-horizon is not to send the routing
information over the interface that has received this routing information.

Split Horizon

Hub and Spoke

Is not propagated by RIP

Propagated by RIP

Dynamic Routing
with RIP
Can not access
network 173.25.0.0 ! Network 173.25.0.0

Problem: Point-to-Multipoint Interfaces

The Split Horizon problem comes up in switched wide area networks. In a
switched network, one physical interface is configured with several instances of
logical interfaces. The logical interfaces deal with the different IP networks. The
routing process deals with the physical interface. So information learned from
the way in on this physical interface is not sent out over the same physical
interface. This is to avoid routing loops.

So something that was designed to solve a problem now causes another
problem.

Administrators must be aware of the split horizon effect in point-to-multipoint
interfaces to avoid routing misconfiguration.


Routing Interoperability

Many administrators use more than one routing protocol in their network to
manage various needs. This chapter covers how different routing protocols can
configured to interact with each other.

The Routing Order

The Routing Preference

OSPF

Static

Priority ?
RIP
OSPF 2
Static 1
RIP 3

Choice: Which routing method
should be used ?

Different routing protocols can be configured and activated in parallel on a
router. But there is no interaction between each other. This means RIP gets all
routing information for the network and a second routing protocol like OSPF
calculates the best path through the some network as well.

Question: So what routing paths are preferred by a data packet ?

Each routing protocol including static routing methods do have an assigned
priority value by default. This value is called the preference.


Note:

Cisco uses the same mechanism for routing interaction. This priority value is
called Administrative Distance.

Working with Preference

100.0.0.0

Entries in Routing-Table

Network By Metric Preference

Route to 100.0.0.0 OSPF 3 5

Route to 100.0.0.0 Static 1 8

Route to 100.0.0.0 RIP 3 10

Route with best preference value

If there are several routes to a destination network, the first value checked is the
preference value. This means the routing procedure with the highest priority is
checked first. A lower preference value means more trust for the routing source.
Again, within a routing procedure like OSPF, RIP, or Static, the metric value is
used to define the best path.

For customization purposes the preferences can be manually configured. If an
administrator wants to trust a RIP derivate route more than an OSPF route, the
default preference must be changed.

Different manufacturers have different specifications on the preferences/
administrative distance of the routing protocols.


The following table shows the default preferences of the routers of Quidway
series produced by Huawei. In the table, a value of “0" denotes the direct route,
and a value of "255" denotes any route from an untrustworthy source.

Table: Default Preference Values for Quidway Series

Routing Protocol Preference
DIRECT 0
OSPF 10
STATIC 60
RIP 100
Internal BGP 130
OSPF AS External 150
External BGP 170
UNKNOWN 255

Except the direct route, preferences of all dynamic routing protocols can be
configured manually according to the users' requirement.


Floating Static

Floating Static Route

100.0.0.0

Use preference values to make
static routes “interactive”
ISDN Link, Serial Link,
Backup 128 KB

Entries in Routing-Table

Network By Metric Preference Via

Route to 100.0.0.0 RIP 3 10 Serial Link

Route to 100.0.0.0 Static 1 20 ISDN

If serial links goes down, ISDN backup is triggered by static route

With the help of the preference one can make a static route more “dynamic”.
By default, a static route has higher priority than all other dynamic routing
procedures. One can change the behaviour, so as long as a dynamic route is
present in the routing table, these routes are preferred. When for some reason
the dynamic route disappears, the defined static route takes precedence.

Floating static routes are often used as part of routing concepts with ISDN
backup links.


Route Redistribution

Route Redistribution

Routing with OSPF Routing with RIP

Metric 117
? Metric 2
Metric 139 Metric 5
not compatible

As mentioned earlier, each routing procedure uses proprietary metric
calculations. To make them working together and exchange routing information,
Route Redistribution can be used.

With Route Redistribution, basically each routing procedure can be transferred
in each other.

There are a lot of considerations to make, when using redistribution. This entire
technique is covered in detail in a later chapter. Administrators should have
deeper understanding of the single routing procedures before using
redistribution between them.


Redistribution Policy

Define rules for redistribution

Convert OSPF Routes to
RIP: Starting Metric 4

Convert RIP Routes to
OSPF: Starting Metric 9

The basic principle with route redistribution consists in the choice for special
routing nodes in the network, where redistribution should be established.

Example:

A set of definitions rule the way, a RIP route is converted and transferred in an
OSPF route and vice versa.

OSPF Metric 230 is converted to RIP Metric 4
RIP Metric 3 is converted to OSPF Metric 9

All metric conversions must be set with care, so the entire routing information
context makes sense. Also, the choice of the position of the router in the
network redistributing routes is relevant.


Routing Design

A structured network design is the fundament for implementing a useful routing
strategy. Without a proper IP addressing, there is no way for scalable and
stable networks.

Routing Hierarchy

Building
Internet Areas, Domains, AS

Core

Edge/ Convergence

Access

In larger networks with structured network design, some routers will take special
control and handling of routing updates. To take advantage of different routing
mechanisms it is very important to have a well administrated IP address
scheme.

Smart Router:

From the routing perspective, some routers are smarter than others. Because of
the routing information they hold in their routing table, some routers may have a
more detailed knowledge about the network.

This is common technique to control the amount of routing information. Small
routers in the access zone do not need all information about the entire network.


A good IP address plan implemented in a well-designed network has the
following characteristics:

• Scalability

Allows for large
increases in the number
of supported sites

• Predictability

Exhibits predictable
behavior and
performance

• Flexibility

Minimizes the impact of
routers, additions,
changes, or removals


The Prefix

Prefix

Prefix Host

“Classfull” Routes

Class A 10.0.0.0/8 10.0.0.0 255.0.0.0

Class B 129.12.0.0/16 129.12.0.0 255.255.0.0

Class C 201.12.23.0/24 201.12.23.0 255.255.255.0

“Classless” Routes

Class C 201.12.112.0/21 201.12.112.0 255.255.248.0

For routing purposes, an IP address without a given subnet mask is “worthless”.
To make routing decisions the subnet mask must always be considered.
Instead of using the subnet mask in the dotted decimal format, a more
convenient format is used.

The Prefix points out, how many bits within the 32 bits of the IP address are
used as the network part.

So a prefix of 20 bits for an IP address like 144.37.99.34 means, you deal with a
class B network 144.37.0.0 performing 4 bit subnetting.


Summarize Routes

Route Summarization

Prefix Host Subnetting

- Gain more routable networks

- Search common network bits for summarization

Prefix Host Summarization

The process of divide a network in smaller sub-networks is done by shifting the
network bits to the right. (see TCP/IP fundamentals).

When dealing with large networks, it is important to minimize the amount of
routing information. Less routing information means less routing update traffic
and less RAM (memory) needed in the router.

So the process of summarize many sub-networks to one network is called
Route Summarization. This is done by shifting the network bits to the left.


IP Address Management

Route Summarization
Route Aggregation

132.17.25.0
132.1
7.0.0
/16 132.17.26.0

Only 1 update necessary 132.17.27.0
132.17.28.0

132.17.29.0

IP subnetworks are auto-summarized
based on Class A, B, C addresses

By default, most routers perform auto summarization for class A, B, or C
networks. Instead of propagating up to 254 subnets of the network 132.17.0.0
(132.17.1.0 to 132.17.254.0) the summarized route 132.17.0.0/16 is used. This
means an enormous improvement for the amount of routing traffic sent to the
neighbour router.

Note:

Sometimes the term Route Aggregation is used. An aggregate route includes
different sub-networks by using appropriate subnet masks.


Relevant Bits

IP Address: 10.20.35.5/ 16

10.20.35.5 00001001 00010100 00100011 00000101

16 Bits Prefix: Marks the relevant bits for all routing decisions

10.20.35.5 00001001 00010100 00100011 00000101
Logical “AND”

255.255.0.0 11111111 11111111 00000000 00000000
What the Router does !

Network: 00001001 00010100 00000000 00000000

Bits to care Don´t care

Routing nodes need the IP address and the corresponding subnet mask to
make routing decisions. Each interface needs this information as part of the
configuration. With the help of the subnet mask and the logical “AND” operation,
it is a simple process to read out the network part of the IP address.

The prefix bits define the network relevant bits within an IP address. This is the
reason for sending the prefix in each routing update when using routing
protocols like OSPF or RIP version 2, so different subnet masks can be used
within a single IP network.

The older RIP version 1 is not capable of using different subnet masks in one
class A, B, or C network. RIP updates are not aware of network prefix.


Sub-Subnet

Host A IP Address: 10.20.35.5/ 16

10.20.35.5/16 00001001 00010100 00100011 00000101 Subnet 10.20

10.20.35.5/19 00001001 00010100 0010 0011 00000101 Subnet 10.20.32

Host B IP Address: 10.20.35.5/ 19

Without the subnet mask information, it is not possible to determine the location
of a given host in the network. VLSM is like using an additional subnet for a
“main subnet”. A “sub subnet” describes how many subnets are used within a
defined subnet. Working with VLSM is simple math, but can be complex in real
live.

Interesting: Only routing nodes with appropriate routing procedures must be
“aware” of VLSM. So, not all routing protocols can be used. End systems like
hosts or servers do not have to deal with VLSM. They just have to be
configured with a proper IP address and mask.


VLSM Routing

VLSM

172.16.56.0/24

172.16.13.4/30

172.16.11.0/24 172.16.13.8/30

172.16.0.0/16

Optimization with use of various prefix subnets

Variable Length Subnet Mask is often used, to optimize the address space for a
given class A, B, or C network. There are lots of small networks with few hosts.
Using large subnets like 8 bits prefix actually wastes a lot of address space.

Worse case is a PPP link with the need of two valid IP addresses only. With a 8
bits subnet, there are 252 wasted IP addresses !


Variable-Length Subnet Mask

172.16.0.0/24
172.16.1.0 Aggregate Route
172.16.2.0
254 . . . . 172.16.14.0/30
Subnets 172.16.14.0
. . . .
172.16.254.0 172.16.14.4
172.16.14.8
62
. . . .
Subnets
172.16.14.252

Use one subnet to split into smaller VLSM subnets

A proven way of using VLSM is, to take a certain subnet out of the group of
available subnets. Apply the new subnet mask i.e. 30 bits, so 62 new subnets
are addressable. Each new subnet can address two hosts.

The benefit is the gain of new smaller routable networks, which can be used to
address PPP links.

Note:

VLSM does not mean an increasing of IP addresses at all. As a matter of fact,
lots of addresses are lost because of broadcasts and network addresses.


Table: Prefix Calculation

CIDR Netmask Hosts / Class Typical usage
subnet
/8 255.0.0.0 16777216 A Largest block allocation
made by IANA
/9 255.128.0.0 8388608
/10 255.192.0.0 4194304
/11 255.224.0.0 2097152
/12 255.240.0.0 1048576
/13 255.248.0.0 524288
/14 255.252.0.0 262144
/15 255.254.0.0 131072
/16 255.255.0.0 65536 B
/17 255.255.128.0 32768 ISP / large business
/20 255.255.240.0 4096 Small ISP / large
business
/21 255.255.248.0 2048 Small ISP / large
business
/22 255.255.252.0 1024
/23 255.255.254.0 512
/24 255.255.255.0 256 C Large LAN
/25 255.255.255.128 128 Large LAN
/26 255.255.255.192 64 Small LAN
/27 255.255.255.224 32 Small LAN
/28 255.255.255.240 16 Small LAN
/29 255.255.255.248 8
/30 255.255.255.252 4 "Glue network" (point to
point links)
/31 255.255.255.254 2 "Useless Network",
proposed for point to
point links (RFC 3021)
/32 255.255.255.255 1 Host route


Classless Routing

Contains block of:

Defined Summary Route:
200.16.168.0
200.16.168.0/21 200.16.169.0
200.16.170.0
200.16.171.0
200.16.172.0
200.16.173.0
200.16.174.0
200.16.175.0

CIDR:
Classless Inter-Domain Routing

The IP address space was divided into three main network classes, where each
class had a fixed network size. The class, the length of the subnet mask and the
number of hosts on the network, could always be determined from the most
significant bits of the IP address. Without any other way of specifying the length
of a subnet mask, routing protocols necessarily used the class of the IP address
specified in route advertisements to determine the size of the routing prefixes to
be set up in the routing tables.

CIDR uses VLSM to allocate IP addresses to subnets according to individual
needs. Thus the network/host division can occur at any bit boundary in the
address. The process can be recursive, with a portion of the address space
being further divided into even smaller portions, through the use of masks which
cover more bits.

Because the normal class distinctions are ignored, the new system is called
classless routing.


Prefix aggregation

Another benefit of CIDR is the possibility of routing prefix aggregation. For
example, sixteen contiguous /24 networks could now be aggregated together,
and advertised to the outside world as a single /20 route (if the first 20 bits of
their network addresses match). Two contiguous /20s could then be aggregated
to a /19, and so forth. This allows a significant reduction in the number of routes
that had to be advertised over the Internet, preventing 'routing table explosion'
from overwhelming routers, and stopping the Internet from expanding further.

When dealing with aggregate routes within the internet the term “Supernet” is
used sometimes.

These kinds of routing mechanisms are part of BGP routing. The Border
Gateway Protocol is discussed more detailed in a later module.

CIDR is described in:
RFC 1519 (http://www.ietf.org/rfc/rfc1519.txt)
Classless Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy.

RFC 1518 (http://www.ietf.org/rfc/rfc1518.txt)
Architecture for IP Address Allocation with CIDR


Discontinuous Use of Subnets

Routing with RIP, Auto-summarization

155.10.34.0/24
198.23.24.0/24 155.10.35.0/24

?
Oh fine: I have two routes
by RIP to 155.10.0.0

Another interesting effect comes up with the discontinuous use of a class A, B,
or C network, which is important to understand for routing administrators.

Because routers perform auto-summarization on IP network address borders,
the above situation arises for a router between two networks using
discontinuous IP address spaces. From the routing perspective, there are two
paths to the network 155.10.0.0 – with fatal consequences !

It is not recommended to split IP networks and use them on different
discontinuous locations.

To solve the above problem, auto-summary must be disabled on both routers.
But then, too many routes are propagated through the network cloud, which
could lead to additional problems.


Prefix Matching

Priority
192.16.3.33 / 32 Host
192.16.3.32 / 27 Subnet
192.16.3.0 / 24 Net
192.16.0.0 / 16 Block Network
0.0.0.0 / 0 Default

Rule: “best prefix matches”

When using subnetting and VLSM in a network, the routing table has various
entries for a network with different prefix lengths.

Longest prefix match or best prefix match refers to an algorithm used to decide
for the best routing entry.

Because each entry of a routing table may specify a range of addresses, one
destination address may match more than another routing table entry. The most
specific table entry, this means the one with smallest host address range, is
called the longest prefix match.


Module Review

Summary

Static routing is still as important as adaptive routing protocols.

Adaptive routing protocols are divided in Distance-Vector and Link-State
protocols.

Routing decisions are based on preferences and metric calculations.

Network administrators must be aware of different routing problems like Split
Horizon, Convergence, Loops, or other effects depending on the routing
protocol.

Different routing protocols can interact with the help of routing redistribution.

Network design and appropriate IP addressing schemes are important for fast
and stable routing.

The ability for route summarization and aggregation is the key for adaptive
routing in larger networks.


Review Question

1. Outline the difference between metric and preference?

2. What are common problems of D-V routing protocols?

3. Build a small table and outline the advantages and disadvantages of
L-S routing protocols.

4. What is the meaning of an adjacent router?


5. What is meant by a “Floating Static Route” ?

6. Describe the rule “best prefix matches” and the relevancy to routing
protocols.

7. What kind of topology is Hub-and-Spoke?

8. Describe the problems arising with a slow convergence.

9. What is the preference for a direct connected network ? Why ?

10. What is the meaning of asymmetric routing ?


Routing primer

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