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CCNP Routing
Semester 5
Chapter 4
OSPF
Using OSPF in a Single Area
OSPF is more complex than RIP. To harness
its power, it is important to spend time in the
analysis and design of the network.
Sometimes, it is necessary to redesign the
addressing scheme to support the hierarchical
structure that OSPF requires.
Hello Packet Fields
Field
Function
Router ID
Identifies the router within the AS
Hello/Dead
Intervals
Hello maintains presence of router in its
neighbor databases; works like a
keepalive
Neighbor
Another router with whom updates will
be exchanged to synchronize database
Area ID
Hello packet must come from a router
within same area to be valid
Hello Packet Fields
Field
Function
Router Priority Used to manually select DR and BDR
DR IP
Address
Used to allow the router to create unicast
traffic to DR
BDR IP
Address
Allows router to create unicast traffic to
the BDR
Authentication
Password
Used for security
Stub Area Flag
Identifies which type of LSA will be
transmitted and accepted
OSPF Packets
Hello
Protocol
Used to find neighbors and to determine
designated router and backup designated
router – continued propagation of Hello
protocol maintains the transmitting router
in the topology database of those that hear
the message
Database
Descriptor
Used to send summary info to neighbors to
synchronize topology databases
OSPF Packets
Link State
Request
Works as a request for more detailed info
which is sent when router receives a
database descriptor that contains new info
Link State
Update
Works as LSA packet issues in response to
request for database info in LSA request
packet
Link State Acknowledges link-state update
Acknowledge
OSPF Databases
Adjacency
Database
List of all neighbor routers to which a
router has established bidirectional
communication. Unique for each router
Link-State
Database
(topological
database)
List of info about all other routers in the
network. This database shows the network
topology. All routers within an area have
identical link-state databases
Forwarding
List of routes generated when an algorithm
Database
is run on the link-state database. Each
(routing table) router’s routing table is unique and holds
info on how / where to send packets
Types of OSPF Networks
DR
Election?
Type
Determining Characteristic
Broadcast
Multiaccess
Ethernet, Token Ring, or
FDDI
Yes
Nonbroadcast
Multiaccess
Frame Relay, X.25, SMDS
No
Point-to-Point
PPP, HDLC
Yes
Point-toMultipoint
Configured by an
administrator
No
Designated Router
(DR)
For every multiaccess IP network, one router will be
elected the DR. This DR has two main functions:
first, to become adjacent to all other routers on the
network, and second, to act as a “spokesperson” for
the network. As spokesperson, the DR sends other
IP networks network LSAs that list all local routers.
Because the DR becomes adjacent to all other
routers on the IP network, it is the focal point for
collecting routing information (LSAs).
Backup Designated Router
(BDR)
The DR could represent a single point of failure, so a
second router is elected as the BDR to provide fault
tolerance. Thus, the BDR must also become
adjacent to all routers on the network and must serve
as a second focal point for LSAs. But, unlike the
DR, the BDR is not responsible for updating the
other routers or sending network LSAs. Instead, the
BDR keeps a timer on the DR’s update activity to
ensure that it is operational. If the BDR does not
detect activity from the DR before the timer expires,
the BDR takes over the role of DR and a new BDR
is elected.
Steps in the Operation of OSPF
OSPF routers progress through five distinct
steps of operation:
1.
2.
3.
4.
5.
Establish router adjacencies
Elect a DR and BDR (if necessary)
Discover routes
Select the appropriate routers to use
Maintain routing information
Message-Digest Authentication
Router(config)#ip ospf message-digest-key key-id md5 [encryption-type] password
key_id
An identifier in the range 1 to 255, which allows
for multiple keys. The key ID configuration on
each router must match to authentication
md5
A required value that specifies the MD5 algorithm
will be used
encryption-type
Optional value that specifies the type of
encryption to use (0 to 7). Type 7 indicates Cisco
proprietary encryption. Type 0 is the default
password
An alphanumeric password to be used as the
message-digest key
Learning a New Route
(2500 Series)
• Router takes first entry from update – first
network with info about state of its link
• Router verifies that type of LSA is one that can be
accepted by this router
• Router issues a lookup to its topological database
when the LSA is valid
• Entries NOT in topological database will be
flooded immediately out all OSPF interfaces
except receiving interface
• Further questions are required when LSA entry is
in topological database
Learning a New Route
(2500 Series)
• If sequence numbers are the same, router
calculates checksum for LSAs and uses the LSA
with the higher checksum
• When checksum numbers are the same, router
checks MaxAge field to ascertain which is more
recent update
• Router determines whether the LSU has arrived
outside the wait period before another
computation is allowed
• When new LSA entry passes the tests, it is flooded
out all OSPF interfaces except for the receiving
interface
Learning a New Route
(2500 Series)
• Current copy replaces old LSA entry. If there was no
entry, current copy is placed in database
• Received LSA is acknowledged
• If LSA entry was in database, but LSA just received
has an older sequence number, the process asks
whether info in database is the same
• If info is different and new LSA has an older
sequence number, receiving router discards the LSA
update and issues its own LSA
• After initial flood, updates are sent only when there
are changes in the area or when the 30 min. timer
goes off
Glossary
Adjacency
Formed when two neighboring routers
have exchanged info and share the same
topology table. Database are
synchronized and see same networks
Area
A group of routers that share the same
area ID. Each router in the area has the
same topology table. Each router is an
internal router. The area is defined on an
interface basis in the OSPF configuration
Autonomous
System
Routers that share the same routing
protocol within the same organization
Glossary
Backup
Designated
Router
(BDR)
Backup to the designated router in case
the DR fails. BDR performs none of the
DR functions while the DR is operating
correctly
Cost
A metric for OSPF. Not defined in
standard with a value. Cisco uses default
of inverse of bandwidth – higher the
speed, lower the cost. Can be overridden
with manual configuration – do only with
full knowledge of network
Glossary
Database
Descriptor
(DDP)
Referred to as DBD – database descriptor
packet – packets exchanged between
neighbors during the exchange state.
DDPs contain LSAs which describe the
links of every router in the neighbor’s
topology table
Designated
Router (DR)
Responsible for making adjacencies with
all neighbors on a multiaccess network
such as Ethernet or FDDI. DR represents
the multiaccess network in that it ensures
every router on the link has the same
topology database
Glossary
Exchange
State
ExStart
init
Method by which two neighboring routers
discover the map of the network. When
these routers become adjacent, they must
first exchange DDPs to ensure they have
the same topology table
State in which the neighboring routers
determine the sequence number of the
DDPs and establish the master/slave
relationship
State in which hello packet has been sent
from the router, which is waiting for reply
to establish two-way communication
Glossary
Internal
Router
Router that has all its interfaces in the
same area
Link-State
Adv LSA
Packet describing router’s links and
state of those links
Link-State
Database
Topology map. Map of every router, its
links, state of the links. Also has a map
of every network and every path to each
network
Glossary
Link-State
Request LSR
Router requests additional info when LSA
entry is not present or is older than the
DDP when comparing the topology
database
Link-State
Update LSU
Update sent in response to the LSR – it is
the LSA that was requested
Neighbor
Router on the same link with whom routing
info is exchanged
Neighbor
Table
Table built from hello received from
neighbor
Glossary
Priority
Cisco tool by which designated router can
be manually elected or prevented from
taking part in a DR/BDR election
SPF Tree
Tree of topological network. Drawn after
SPF algorithm has been run. Algorithm
prunes database of alternative paths and
creates loop-free shortest path to all
networks
Topology
Table
Same as link-state database
More Glossary
Flood
Refers to network info – sent to every device in
domain
Fully
Adjacent
When routing tables of two neighbors are fully
synchronized
Loading
State
State where router will request more detail
using LSR
Loopback
Interface
Virtual interface that does not exist physically.
If it doesn’t exist, it can’t go down
Setup
Script
Question and answer dialogue offered by Cisco
router
Two-way
State
State during process in which two routers are
creating an adjacency. This is stage before
routing info is exchanged