Download Class Seven

IP Routing

Two types: direct and indirect.

Routing provides for efficient network topologies.

Flat networks cannot scale.

Protocols used today are the same ones that were used back in the
shared network environment.

Two types of protocols IGP and EGP.

IGP provides for routing within a single AS

EGP provides for routing between ASs
73
Direct Routing
Direct Routing
Direct
Routing
Station B
140.1.2.1
Station A
140.1.1.1
Station C
140.1.3.1
Indirect Routing
Station D 140.2.1.1

Network numbers must match for direct routing.

Different network numbers for indirect routing.

Remote nodes may use a combination of both direct and indirect
routing.
74
Indirect Routing

Occurs when the source and destination network or subnet do not
match.

Source will ARP for a router and send the datagram to the router.

The router will either forward the packet directly to the destination or
it will forward it to another router in the path to the destination.

Routers decrement the TTL field.

Routers forward the packet based on the IP address and not the
MAC address.
75
A Flowchart
Packet Received
NO
Header and
checksum valid?
If route is available,
search for MAC
address in ARP
cache
NO
Received
ARP
Reply?
YES
YES
Decrement TTL;
TTL >= 0?
NO
Send ICMP
error message
to originator
YES
Discard original
packet
NO
YES
MAC address
found?
Send ARP request
and wait for a
response
YES
Route Table lookup
based on
destination address
Route
found?
NO
NO
Build new packet with
MAC address and
route packet through
port found in routing
table.
Received ARP reply,
insert MAC and IP
address into
ARP table
Default route
available?
YES
76
Routing Protocols - Distance Vector
134.4.0.0
1
2
134.3.0.0
Network Metric Port Age
134.4.0.0
1
1
xxx
134.3.0.0
1
1
xxx
134.5.0.0
2
2
xxx
134.5.0.0
77
Updating Other Routers (Distance Vectors)

Upon initialization, each router reads its preconfigured IP address
and metric (cost in hops) of all its active ports.

Each router transmits a portion of its routing table (network ID,
metric) to each “neighbor” router.

Each router uses the most recent updates from each neighbor.

Each router uses the update information to calculate its own
“shortest path” (distance in hops) to a network.

Tables are updated only:

If the received information indicates a shorter path to the
destination network.

If the received update information indicates a network is no
longer reachable.

If a new network is found.
78
A Bigger Update
Z
Y
Router B
Route Hop
Route Hop
1
1
X
Y
Z
1
1
2
Router A
Z
Y
Network Hop Router Port
W
1
Local
2
X
1
Local
1
Y
2
B
1
Z
3
B
1
X
Route
Hop
Route
Hop
Route
Hop
Router C
W
X
Y
Z
1
1
2
3
W
79
IP Routing Tables
Port IP address
(i.e., 132.2.2.2)
132.2.0.0
2.2
133.3.0.0
134.4.0.0
3.3
4.5
3.4
1.1
130.1.0.0
Routing Table
Network Number Next Hop
Hops
Learned from
Port
132.2.0.0
Direct
1
RIP
1
133.3.0.0
Direct
1
RIP
2
130.1.0.0
Direct
1
RIP
3
134.4.0.0
Direct
1
RIP
2
80
The Routing Information Protocol (Version
1)
RIP Header
UDP Header
IP Header
DA
SA
TF
RIP Data
UDP Data
IP Data
Data
CRC
81
RIP Operational Types

RIP can operate in either ACTIVE or PASSIVE mode.

Active means that it builds routing tables and responds to RIP
requests.

Passive means that it can build a routing table for its own use, but it
does not respond to any RIP requests.

Most workstations (PCs) use a default gateway (i.e., router) and not a
routing update protocol like RIP.
82
RIP Field Descriptions
0
31
Command
Version
Reserved
Family of Net 1
Reserved
Net 1 address
Set to 0
Set to 0
Distance of network 1
Reserved
Family of Net 2
Net 2 address
Set to 0
Set to 0
Distance of network 2
Up to 25 entries
DA SA
TF
IP Hdr UDP Hdr
UDP Data
CRC
83
Default Router and Gateways
130.1.1.1
Default Route
0.0.0.0
129.1.1.2
Default Route
130.1.1.1
129.1.1.1
129.1.1.2
Default Route
129.1.1.1
84
Disadvantages of the RIPv1 Protocol

RIPv1 only understands the shortest route to a destination, based
on a simple count of router hops.

It depends on other routers for computed routing updates.

Routing tables can get large and these are broadcasted every 30
seconds.

Distances are based on hops, not real costs (such as the speed of
a link).

Patched with split horizon, poison reverse, hold-down timers,
triggered updates.


It continues to be a router-to-router configuration. One router is
fully dependent on the next router to implement the same options.
Fix one problem and others appear.
85
Scaling with RIP
Z
Y
1
1
W
X
Y
Z
W
X
Y
Z
2
1
1
2
Router B
2
1
1
1
Router A
Z

Y
Router A previously sent its table
X
W
X
Y
Z
W
X
Y
Z
1
1
2
3
1
1
2
3
Router C
W
86
Routers and Subnet Masks
150.1.0.0
160.1.0.0
160.1.1.0
255.255.255.0
150.1.1.0
255.255.255.0
150.1.3.0
150.1.3.0
255.255.255.0
87
RIP Fixes

Split Horizon—Rule states that a router will not rebroadcast a
learned route back over the interface from which the route was
learned.

Hold-Down Timer—Rule states that when a router receives
information about a network that is unreachable, the router must
ignore all subsequent information about that network for a
configurable amount of time.

Poisoned Reverse and triggered updates—Rule states a router is
allowed to rebroadcast a learned route over the interface from which
it learned it, but the metric is set to 16. A triggered update allows a
router to broadcast its table when a network is found to be down.
88
Split Horizon Demonstrated
Z
Y
1
1
X
Y
W
X
Y
Z
W
1
1
2
Router B
1
1
2
2
Router A
Z
Y
X
W
X
W
X
Y
Z
1
1
1
1
2
3
Router C
W
89
RIP Version 2
Command
Version
Unused
Route Tag
Address Family Identifier
Net 1 address
Subnet mask
Next-Hop IP Address
Metric
Route Tag
Address Family Identifier
Net 2 address
Subnet mask
Next Hop
Metric
DA SA
TF
IP Hdr UDP Hdr
UDP Data
CRC
90
Authentication
0
31
Command
Version
Unused
Authentification Type
OxFFFF
Password
Password
Password
Password
Address Family Identifier
Route Tag
Net 2 address
Subnet mask
Next Hop
Metric
91
Subnet Mask Field
0
31
Command
Version
Unused
Authentification Type
OxFFFF
Password
Password
Password
Password
Address Family Identifier
Route Tag
Net 2 address
Subnet mask
Next Hop
Metric
92
Route Tag and Next-Hop Fields
0
31
Command
Version
Unused
Authentification Type
OxFFFF
Password
Password
Password
Password
Address Family Identifier
Route Tag
Net 2 address
Subnet mask
Next Hop
Metric
93
Multicast Support

RIPv2 uses the multicast address of 224.0.0.9 to multicast, does
not broadcast its table.

MAC address of 01-00-5E-00-00-09.

Details of this conversion are covered in RFC 1700 and the
multicast section of this book

RIPv1 uses a broadcast address in both the IP header and the MAC
header.

IGMP is not used for this multicast support.
94
RIPv2 Compatibility with RIPv1

Configuration parameters on the router for:

RIPv1 only – version 1 messages will be sent

RIPv1 compatibility – RIP 2 messages as broadcast

RIPv2 – Messages are multicast

None – No RIP messages are sent
95
Open Shortest Path First (OSPF, RFC 2178)

Shortest-path routes based on true metrics, not just a hop count.

Computes the routes only when triggered to or every 30 minutes
(whichever is less).

Pairs a network address entry with a subnet mask.

Allows for routing across equal paths.

Supports ToS.

Permits the injection of external routes (other ASs).

Authenticates route exchanges.

Quick convergence.

Direct support for multicast in both the IP header and the MAC
header.
96
An OSPF Network
Other Autonomous
Systems
Backbone Area 0.0.0.0
Router
Router
Router
Host
Router
PC
PC
Area 1
Area 4
PC
PC
Area 2
Area 5
97
A Routing Protocol Comparison
Function/Feature
RIPv1
RIPv2
OSPF
Standard Number
Link State Protocol
Large Range of
Metrics
RFC 1058
No
Hop Count
(16=Infinity)
RFC 1723
No
Hop Count (16 =
Infinity)
RFC 2178
Yes
Yes, based on 1- 65535
Update Policy
Route Table every 30
sec
Broadcast
300 secs total
Route Table every 30
sec
Broadcast, Multicast
300 seconds total
No
Yes
Link state changes or
every 30 minutes
Multicast
Up to 300 seconds
total. Usually shorter
Yes
Variable based on
(number of routers x
dead interval)
No
Variable based on
(number of routers x
dead interval)
Yes
Media Delay + Dead Interval
No
Yes
Yes
No
No
15 hops
Yes
No
No
15 hops
Yes
Yes
Yes
N/A but up to 65535
No
Update address
Dead Interval
Supports
authentication
Convergence Time
Variable Length
Subnets
Supports
Supernetting
Type of Service (TOS)
Multipath routing
Network Diameter
Easy to use
Yes
98
OSPF Overview







Upon initialization, each router records information about all its
interfaces.
Each router builds a packet known as the Link State Advertisement
(LSA).
 Contains a listing of all recently seen routers and their cost
 LSAs are restricted to being forwarded only in the orginated area
Received LSAs are flooded to all other routers.
 Each router makes a copy of the most recently “seen” LSA
Each router has complete knowledge of the topology of the area to
which it belongs.
Adjacencies are formed between a Designated Router (and Backup
DR) and other routers on a network.
Shortest Path Trees are constructed after routers exchange their
databases.
Router algorithm only when changes occur (or every 30 minutes,
whichever is shorter).
99
OSPF Media Support

Broadcast - Networks such as Ethernet, Token Ring, and FDDI.

Non-broadcast Multiaccess (NBMA) - access that does not support
broadcast but allows for multiple station access such as ATM, Frame
Relay, and X.25.

Point-to-Point - Links that only have two network attachments, such
as two routers connected by a serial line.
100
Router Types
Other Autonomous
Systems
Autonomous System
Border Router
Backbone Area 0.0.0.0
Internal Router
Area Border
Router
Router
Router
Backbone
Router
Backup
DR
Designated
Router
Host
PC
Area 1
PC
Area 2
Area 3
Internal Router
PC
Area 4
101
Router Names and Routing Methods

Three types of routing in an OSPF network:

Intra-Area routing - Routing within a single area

Inter-Area routing - Routing within two areas of the same AS

Inter-AS routing Routing between AS systems
102
Message Types


OSPF routers communicate by sending Link State Advertisement
(LSAs) to each other.

Type 1 - Router Links Advertisement

Type 2 - Network Links Advertisement

Type 3 - Summary Links Advertisement

Type 4 - AS Boundary Router Summary Link Advertisement

Type 5 - AS External Link Advertisement

Type 6 - Multicast Group Membership LSA
LSAs contain sequence numbers to detect old and duplicate LSAs.
103
Metrics (Cost)

Reference RFC 1253

Metric = 10n8 / interface speed

Examples:

=> 100 Mbps
1

10 Mbps
10

E1
48

T1
65

64 kbps
1562

19.2 kbps
5208

9.6 kbps
10416
104
Generic Packet Formula
Version
Type
Packet Length
Router ID
Area ID
Checksum
Authentication Type
Authentication
LSA Specific
1 – Hello, 2 – DB Description, 3 – LS Request,
4 – LS Update, 5 – LS Ack
DA SA
TF
IP Header
Protocol ID 89
IP Data
CRC
105
The Hello Protocol
C
B
A



30 15 C
B
89 A
Designated B
Router
MC
Backup
DR
C
D
Routers send periodic Hello messages to each other.
The packet contains:
 The router’s selection of the DR and BDR
 Router’s priority used to determine the DR and BDR
 Configurable timers that include:
 Hello Interval – To determine when you should hear from a
neighbor
 RouterDeadInterval – The period before a router is declared down
 A list of neighbors the router has heard from
This can be turned off by setting the network to an NBMA.
 This is useful when there is only one router on the cable segment
106
Adjacency
Router 1
Down
ExStart
Hello
Hello DR = RT2
D-D Seq = x M, Master
Router 2
Designated Router
Down
ExStart
D-D Seq = y M, Master
Exchange
D-D Seq = y M, Slave
D-D Seq = y+1 M, Master
Exchange
D-D Seq = y+1 M, Slave
Loading
Full
D-D Seq = y+n, Master
D-D Seq = y+n, Slave
LS Request
LS Update
LS Request
LS Update
LS Ack
LS Ack
Loading
Full
107
Maintaining the Database

After Dykstra runs, the database is checked for consistency.

Uses the flooding procedure:

Receive an LSA

Check for the information in the database

Determine whether or not to forward this LSA to an adjacency

Reliability checked using an acknowledgment procedure.

Each LSA contains an age entry.

Sequence numbers are generated for every LSA.
108
OSPF Areas
AS 1
ASBR
Area 0
Backbone
Router
Backbone
Router
Could be a RIP
network within the
same domain as OSPF
Backbone
Router
Area 1
Area 2
Internal
Router
Area
Border
Router
109
The Backbone Area

There must be at least one area in an OSPF network.

It is called the backbone area.

Designated by area ID of 0.0.0.0.

Primarily responsibility to propagate information between areas.

Has the same attributes as any other area.

Any network topology may make up the backbone.

It can be used as a real network with attachments.
110
The Area Border Router (ABR)

Connects an area (or areas) to the backbone.

Summarizes its area topology to the backbone.

Propagates summarized information from the backbone into its area.

Final router that receives an area’s LSA.

ABRs do not flood LSA information into the backbone

Only produces summaries to the backbone for the backbone to
propagate to other areas

Uses the network summary LSA.

Summarized information is propagated in an area by the DR and its
adjacencies.
111
Virtual Link
Area 2.2.2.2
Area 1.1.1.1
ABR
Backbone Area
ABR
Virtual Link
112
Inter-Area Routing
ASBR
Area 0
Backbone
Router
Backbone
Router
AS 1
Could be a RIP
network within the
same domain as OSPF
Backbone
Router
Area 1
Area 2
Area
Border
Router
113
Information from other Autonomous
Systems

Uses the ASBR.

Other ASs according to OSPF may simply be a RIP network within
the same OSPF domain.

External LSA used.

Type 1 – The preferred route and used when considering the
internal cost of the AS.

Type 2 – Advertising the same metric as was advertised by the
ASBR.

These are used to calculate the shortest path to the ASBR.
114
Stub Areas
Area 0
AS 2
Does not
contain AS2
route entries
Area 1
Contains AS2
route entries
Area 2

An area that has only one entry and one exit point (must be the same
area).

Used to reduce the number of external advertisements.

A stub area blocks AS external link advertisements.
115
RFCs Related to OSPF
2178 DS: J. Moy, “OSPF Version 2,” 07/22/97 (211 pages) (.txt format) (obsoletes RFC 1583).
2154 ES: M. Murphy, B. Badger, A. Wellington, “OSPF with Digital Signatures,” 06/16/97 (29 pages) (.txt format).
1850 DS: F. Baker, R. Coltun, “OSPF Version 2 Management Information Base,” 11/03/95. (80 pages)
(.txt format) (Obsoletes RFC 1253).
1793 PS: J. Moy, “Extending OSPF to Support Demand Circuits,” 04/19/95 (31 pages) (.txt format).
1765 E: J. Moy, “OSPF Database Overflow,” 03/02/95 (9 pages) (.txt format).
1745 PS: K. Varadhan, S. Hares, Y. Rekhter, “BGP4/IDRP for IP—OSPF Interaction,” 12/27/94 (19 pages) .txt format).
1587 PS: R. Coltun, V. Fuller, “The OSPF NSSA Option,” 03/24/94 (17 pages) (.txt format).
1586 I: O. deSouza, M. Rodrigues, “Guidelines for Running OSPF Over Frame Relay Networks,”
03/24/94 (6 pages) (.txt format).
1585 I: J. Moy, “MOSPF: Analysis and Experience,” 03/24/94 (13 pages) (.txt format).
1584 PS: J. Moy, “Multicast Extensions to OSPF,” 03/24/94 (102 pages) (.txt, .ps formats).
1403 PS: K. Varadhan, “BGP OSPF Interaction,” 01/14/93 (17 pages) (.txt format) (obsoletes RFC 1364).
1370 PS: Internet Architecture Board, “Applicability Statement for OSPF,” 10/23/92 (2 pages) (.txt format).
116
Static versus Dynamic Routing

Entries in a routing table can be static (manually entered by the network administrator) or
dynamic (learned through a routing protocol such as RIP).

Static entries:


In the workstation for either:

Default Gateway (router) - used by indirect routing

Place a static route in for one that is not learned through RIP, etc.
In the router:

Entered as 0.0.0.0 and the next hop (no subnet) to indicate a default route

Routers can broadcast this information to their networks to let everyone know which
is the default router

A default router is one that all other look to for networks that are not in their tables

Static routes can be used to increase security on the network

Any IP network address can be manually entered into the routing table

The router administrator supplies:

IP Network address

Subnet mask

Next hop interface (the IP address of the next routers interface to get to the
network)
117
Remote Networks
Virginia
T3
T3
T1
California
Texas
Z
A
T1
T3
=
=
1.544Mbps
45Mbps
118
Datagram Routing
Host - 129.1.1.1
Host - 129.1.1.2
E
D
129.1.1.3
C
IP Header
Router
129.2.1.1
C D
0800 129.2.1.2 129.1.1.2 IP Data
CRC
B
IP Header
129.2.1.2 A
B A
0800 129.2.1.2
129.1.1.2 IP Data
CRC
PC
DA SA TF
Data
CRC
119