Transcript BSCI 2.0 - College of DuPage

OSPF (Single Area OSPF)
Routing Protocols and Concepts – Chapter 11
Modified by Tony Chen
07/01/2008
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Notes:

If you see any mistake on my PowerPoint slides or if
you have any questions about the materials, please
feel free to email me at [email protected].
Thanks!
Tony Chen
College of DuPage
Cisco Networking Academy
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Introduction
•In this chapter, you will learn basic, single-area OSPF implementations
and configurations.
•More complex OSPF configurations and concepts (multi-areas OSPF)
are reserved for CCNP-level courses.
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Introduction to OSPF
Background of OSPF
 Began in 1987
 1989 OSPFv1 released in RFC 1131
This version was experimental & never deployed
 1991 OSPFv2 released in RFC 1247
 1998 OSPFv2 updated in RFC 2328
 1999 OSPFv3 published in RFC 2740
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Introduction to OSPF
OSPF Message Encapsulation
 OSPF packet type
– There exist 5 types (next slide)
 OSPF packet header
–Contains - Router ID an area ID
and Type code for OSPF packet
type
 IP packet header
– Contains - Source IP address,
Destination IP address, & Protocol
field set to 89. the destination
address is set to one of two
multicast addresses: 224.0.0.5 or
224.0.0.6.
 Data Link Frame Header
–Contains - destination MAC address is
also a multicast address: 01-00-5E-0000-05 or 01-00-5E-00-00-06.
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Introduction to OSPF
5 OSPF Packet Types:
 1. Hello - Hello packets are used to establish and
maintain adjacency with other OSPF routers.
 2. DBD - The Database Description (DBD) packet
contains an abbreviated list of the sending router's
link-state database and is used by receiving
routers to check against the local link-state
database.
 3. LSR - Receiving routers can then request more
information about any entry in the DBD by sending
a Link-State Request (LSR).
 4. LSU - Link-State Update (LSU) packets are
used to reply to LSRs as well as to announce new
information.
–LSUs contain 7 different types of Link-State
Advertisements (LSAs).
–LSUs and LSAs are discussed in a later topic.
 5. LSAck - When an LSU is received, the router
sends a Link-State Acknowledgement (LSAck) to
confirm receipt of the LSU.
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OSPF: Hello Protocol
 Purpose of Hello Packet
 Discover OSPF neighbors & establish adjacencies
 Advertise parameters on which routers must agree to become
neighbors
 Used by multi-access networks to elect a Designated Router and
a Backup Designated Router
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
Type: OSPF Packet Type: Hello (1), DD (2), LS
Request (3), LS Update (4), LS ACK (5)

Router ID: ID of the originating router

Area ID: area from which the packet originated

Network Mask: Subnet mask associated with the
sending interface

Hello Interval: number of seconds between the
sending router's hellos

Router Priority: Used in DR/BDR election (discussed
later)

Designated Router (DR): Router ID of the DR, if any

Backup Designated Router (BDR): Router ID of the
BDR, if any

List of Neighbors: lists the OSPF Router ID of the
neighboring router(s)
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OSPF: Hello Protocol
Also need to have the
same Area ID.
 Establish adjacencies:
– They must agree on three values: Hello
interval, Dead interval, and network type.
 OSPF Hello Intervals
Why 10 second hello interval
communications consider better than
the 30 second routing update for RIP?
–Hello interval indicates how often an OSPF
router transmits its Hello packets
–Usually multicast (224.0.0.5) for
ALLSPFRouters
–sent every 10 seconds on multiaccess and
point-to-point segments
–Sent every 30 seconds for NBMA segments
 OSPF Dead Intervals
–This is the time that must transpire before the
neighbor is considered down
–Default time is 4 times the hello interval
–For multiaccess and point-to-point segments,
this period is 40 seconds.
–For NBMA networks, the Dead interval is 120
seconds.
–If the Dead interval expires before the routers
receive a Hello packet, OSPF will remove that
neighbor from its link-state database.
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OSPF: Hello Protocol
 To reduce the amount of OSPF traffic on
multiaccess networks, OSPF elects a
Designated Router (DR) and Backup
Designated Router (BDR).
 Hello protocol packets contain information that
is used in electing DR and BDR
–The DR is responsible for updating all other
OSPF routers (called DROthers) when a change
occurs in the multiaccess network.
–The BDR monitors the DR and takes over as
DR if the current DR fails.
 In the figure, R1, R2, and R3 are connected
through point-to-point links. Therefore, no
DR/BDR election occurs.
–The DR/BDR election and processes will be
discussed in a later topic and the topology will
be changed to a multiaccess network.
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More detail discussion
on the DR, BDR,
DROther later. You
need to know this for
CCNA exam.
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Introduction to OSPF
OSPF Link-state Updates
 Purpose of a Link State Update (LSU)
–Used to deliver link state advertisements
 Purpose of a Link State Advertisement (LSA)
–Contains information about neighbors & path costs
–An LSU packet can contain 11 different types of LSAs,
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Introduction to OSPF
OSPF Algorithm

OSPF routers build &
maintain link-state
database containing LSA
received from other
routers
1. Information found in
database is utilized upon
execution of Dijkstra SPF
algorithm
2. SPF algorithm used to
create SPF tree
3. SPF tree used to populate
routing table
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Introduction to OSPF
Administrative Distance
 Default Administrative Distance for OSPF is 110
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Introduction to OSPF
 OSPF Authentication
–It is good practice to authenticate transmitted
routing information.
–This is an interface specific configuration
–This practice ensures that routers will only accept
routing information from other routers that have been
configured with the same password or authentication
information
MD5 authentication
uses a key ID that
allows the router to
reference multiple
passwords, making
password migration
easier and more
secure.
Note: Authentication
does not encrypt the
router's routing table.
?
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Basic OSPF Configuration
Lab Topology
 Topology used for this chapter
–Discontiguous IP addressing
scheme
–Since OSPF is a classless
routing protocol the subnet mask
is will be configured as part of our
OSPF configuration.
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Basic OSPF Configuration
The router ospf command
 To enable OSPF on a router use the following
command
R1(config)#router ospf process-id
ID cannot be 0
Process id
 A locally significant number between 1 and 65535
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Basic OSPF Configuration
 OSPF network command
–Requires entering:
•network address
•wildcard mask - the inverse of the subnet mask
•area-id - area-id refers to the OSPF area. OSPF area
is a group of routers that share link state information
Router(config-router)#network network-address wildcard-ask area area-id
255.255.255.255
- 255.255.255.240
-------------------0. 0. 0. 15
255.255.255.255
- 255.255.255.252
-------------------0. 0. 0. 03
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Subtract the
subnet mask
Wildcard mask
Subtract the
subnet mask
Wildcard mask
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Basic OSPF Configuration
 Cisco IOS now properly handles overlapping network ... area configuration commands.
 Consider the following example:
fw#conf t
Enter configuration commands, one per line. End with CNTL/Z.
fw(config)#router ospf 100
fw(config-router)#network 0.0.0.0 255.255.255.255 area 0
fw(config-router)#network 10.0.0.0 0.0.3.255 area 1
13:06:57: %OSPF-6-AREACHG: 10.0.0.0 255.255.252.0 changed from area 0 to area 1
fw(config-router)#network 10.0.0.0 0.0.0.7 area 2
13:07:10: %OSPF-6-AREACHG: 10.0.0.0 255.255.255.248 changed from area 1 to area 2
fw(config-router)#^Z
 I've entered overlapping network statements, each one with a smaller address range. Not
only does IOS detect that they overlap, it also prints nice syslog messages and reorders
the commands in the running configuration. Well done !
fw#show run | begin router ospf
router ospf 100
log-adjacency-changes
network 10.0.0.0 0.0.0.7 area 2
network 10.0.0.0 0.0.3.255 area 1
network 0.0.0.0 255.255.255.255 area 0
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http://blog.ioshints.info/2006/11/
network-statements-in-ospfprocess-are.html
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Basic OSPF Configuration
 ospf network definition for adding all interfaces / default route
What’s the difference?
router ospf 1
network 0.0.0.0 0.0.0.0 area 0
vs.
router ospf 1
network 0.0.0.0 255.255.255.255 area 0
 Both add all existing interfaces into area 0 and all later added interfaces
also. Both statements are valid.
http://blog.sazza.de/?p=427
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Basic OSPF Configuration
 Area area-id
An OSPF area is a group of routers that share link-state
information.
 In this chapter, we will configure all of the OSPF routers within a
single area. This is known as single-area OSPF.
Multi-area OSPF is covered in CCNP.
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Basic OSPF Configuration

Router ID
– This is an IP address used to identify a router
– 3 criteria for deriving the router ID
1. Use IP address configured with OSPF router-id command
-Takes precedence over loopback and physical interface
addresses
2. If router-id command not used then router chooses highest
IP address of any loopback interfaces
3. If no loopback interfaces are configured then the highest IP
address on any active physical interface is used
 The interface does not need to be enabled for OSPF,
meaning that it does not need to be included in one of the
OSPF network commands.
 However, the interface must be active - it must be in the
up state.
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However!!!!!!
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Basic OSPF Configuration
 Router ID
– If you are the king when the
kingdom is built, you are the
KING for life
– It means when ID is elected, it
is the ID for the router, unless
…………..
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Basic OSPF Configuration
OSPF Router ID
 Commands used to verify current router ID
–Show ip protocols
–Show ip ospf
–Show ip ospf interface
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Basic OSPF Configuration
OSPF Router ID
 Router ID (not configured) & Loopback addresses
(configured)
–Highest loopback address will be used as router ID
–Advantage of using loopback address the loopback
interface cannot fail  OSPF stability
 The OSPF router-id command
–Introduced in IOS 12.0
–OSPF router-id command, which is a fairly recent
addition to IOS, it is more common to find loopback
addresses used for configuring OSPF router IDs.
–Command syntax
Router(config)#router ospf process-id
Router(config-router)#router-id ip-address
 Modifying the Router ID
–Use the command Router#clear ip ospf process
This command does not work in PT.
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Basic OSPF Configuration
Modifying the Router ID

The router ID is selected when OSPF is
configured with its first OSPF network command.
– If the OSPF router-id command or the loopback
address is configured after the OSPF network
command, the router ID will be derived from the
interface with the highest active IP address.

Modifying the Router ID
The router ID can be modified with
1. the IP address from a subsequent OSPF router-id
command by reloading the router or
2. by using the following command:
Router#clear ip ospf process
3. Modifying a router ID with a new loopback or
physical interface IP address may require
reloading the router
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Basic OSPF Configuration
Duplicate Router IDs
 When two routers have the same router ID in
an OSPF domain, routing may not function
properly.
– If the router ID is the same on two
neighboring routers, the neighbor
establishment may not occur.
 When duplicate OSPF router IDs occur, IOS
will display a message similar to:
– %OSPF-4-DUP_RTRID1: Detected router
with duplicate router ID
 To correct this problem, configure all routers
so that they have unique OSPF router IDs.
 Because some IOS versions do not support
the router-id command, we will use the
loopback address method for assigning
router IDs.
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Quick Review
We just went over 3 different types of ID
 ospf process-id.
– OSPF process.
– Cannot be 0
 Area ID:
– OFPS area
– If it is the first, and the backbone area, it is 0
 Router ID
– Router ID
– 1 IP address is elected per router,
• Highest physical address (or)
• Highest logical address (loopback)
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Basic OSPF Configuration
Verifying OSPF
 Use the show ip ospf command to verify &
trouble shoot OSPF networks:
 Neighbor adjacency
Adjacency indicated by
The OSPF state of the interface is
“full state”
No adjacency indicated by -
Neighboring router’s Router ID is not
displayed
A state of full is not displayed
-Consequence of no adjacencyNo link state information exchanged
Inaccurate SPF trees & routing tables
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•Neighbor ID - The router ID of the neighboring router.
•Pri - The OSPF priority of the interface..
•State - The OSPF state of the interface. FULL state
means that the router and its neighbor have identical
OSPF link-state databases.
•Dead Time - The amount of time remaining that the
router will wait to receive an OSPF Hello packet from the
neighbor before declaring the neighbor down. This value
is reset when the interface receives a Hello packet.
•Address - The IP address of the neighbor's interface to
which this router is directly connected.
•Interface - The interface on which this router has formed
adjacency with the neighbor.
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Basic OSPF Configuration
Note:
 On multiaccess networks such as Ethernet,
two routers that are adjacent may have their
states displayed as 2WAY.
–This will be discussed in a DR and BDR
section.
 Two routers may not form an OSPF
adjacency if:
–The subnet masks do not match, causing
the routers to be on separate networks.
–OSPF Hello or Dead Timers do not match.
–OSPF Network Types do not match.
–There is a missing or incorrect OSPF
network command.
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•Neighbor ID - The router ID of the neighboring router.
•Pri - The OSPF priority of the interface..
•State - The OSPF state of the interface. FULL state
means that the router and its neighbor have identical
OSPF link-state databases.
•Dead Time - The amount of time remaining that the
router will wait to receive an OSPF Hello packet from the
neighbor before declaring the neighbor down. This value
is reset when the interface receives a Hello packet.
•Address - The IP address of the neighbor's interface to
which this router is directly connected.
•Interface - The interface on which this router has formed
adjacency with the neighbor.
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Verifying OSPF
 Show ip protocols
–OSPF process ID,
–the router ID,
–networks the router is advertising,
–the default administrative distance, 110 for OSPF.
 Show ip ospf
–OSPF process ID
–router ID.
–OSPF area information
–the last time the SPF algorithm was calculated.
•R1 has participated in during the past 11 and a half hours is to
send small Hello packets to its neighbors.
–SPF schedule delay
•The router waits 5000 msecs after receiving an LSU before
running the SPF algorithm.
•There is an additional Hold Time of 10000 msecs between 2
SPF calculations.
 Show ip ospf interface
–The quickest way to verify Hello and Dead intervals
• for OSPF routers to become neighbors, their OSPF Hello and
Dead intervals must be identical.
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Configuring OSPF loopback address and router priority
The command show ip ospf interface will display the
interface priority value as well as other key information.
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Basic OSPF Configuration
Examining the routing table
 Use the show ip route command to display the routing table
-An “O’ at the beginning of a route indicates that the router source is
OSPF
-OSPF does not automatically summarize at major network
boundaries
•Loopback
interface counts
as a network.
•These loopback
interfaces are
not advertised in
OSPF.
•They function as
router ID.
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OSPF Metric
 OSPF uses cost as the metric for determining the
best route
–A cost is associated with the output side of each
router interface.
–The lower the cost, the more likely the interface is
to be used to forward data traffic
 The Cisco IOS uses the cumulative
bandwidths of the outgoing interfaces from
the router to the destination network as the
cost value.
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
108 / bandwidth
-Reference bandwidth
The 100Mbps (FastEthernet) and higher will have the
same OSPF cost of 1.
This reference bandwidth can be modified using
auto-cost reference-bandwidth command
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OSPF Metric
 COST of an OSPF route is the accumulated value from
one router to the destination network
•For example, in the figure,
the routing table on R1 shows
a cost of 65 to reach the
10.10.10.0/24 network on R2.
•Because 10.10.10.0/24
is attached to a
FastEthernet interface,
R2 assigns the value 1 as
the cost for 10.10.10.0/24.
•R1 then adds the
additional cost value of 64
to send data across the
default T1 link between
R1 and R2.
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64 + 1 = 65
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OSPF Metric
 Sometimes the actual speed of a link is different than
the default bandwidth
–This makes it imperative that the bandwidth value reflects
link’s actual speed
Reason: so routing table has best path information
 The show interface command will display interface’s
bandwidth
–Most serial link default to 1.544Mbps
–However, some serial interfaces may default to 128 kbps.
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Modifying OSPF cost metric
 OSPF uses cost as the metric for determining the
best route.
Cost is calculated using the formula 108/bandwidth,
where bandwidth is expressed in bps. (Cost =
100,000,000/Bandwidth)
 The Cisco IOS automatically determines cost based
on the bandwidth of the interface.
 It is essential for proper OSPF operation that the
correct interface bandwidth is set.
Router(config)#interface serial 0/0
Router(config-if)#bandwidth 64
The default bandwidth for Cisco serial interfaces is
1.544 Mbps, or 1544 kbps.
2A/S
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COD has these 2 types
of serial cards in the lab
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OSPF Metric: Bandwidth
 Remember, this bandwidth value
does not actually affect the speed of
the link; it is used by some routing
protocols to compute the routing
metric.
–It is important that the bandwidth
value reflect the actual speed of the
link so that the routing table has
accurate best path information.
 The figure displays the routing table
for R1.
–R1 believes that both of its serial
interfaces are connected to T1 links,
•one of the links is a 64 kbps link
•the other one is a 256 kbps link.
–This results in R1's routing table
having two equal-cost paths to the
192.168.8.0/30 network, when Serial
0/0/1 is actually the better path.
How to modify the cost of all the links?
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Basic OSPF Configuration
Modifying the Cost of a link
 Both sides of a serial link should be
configured with the same bandwidth
–Commands used to modify bandwidth value
Bandwidth command
–Example: Router(config-if)#bandwidthbandwidth-kbps
ip ospf cost command – allows you to directly specify
interface cost
-Example:R1(config)#interface serial 0/0/0
R1(config-if)#ip ospf cost 1562
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Modifying the Cost of the link
 Difference between bandwidth command & the ip ospf
cost command
–Ip ospf cost command
Sets cost to a specific value
–Bandwidth command
Link cost is calculated
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OSPF and Multiaccess Networks
Challenges in Multiaccess Networks
 OSPF defines five network types:
–Point-to-point
•network there are only two devices on
the network, one at each end.
–Broadcast Multiaccess
•a network with more than two devices on
the same shared media.
•all devices on the network see all
broadcast frames.
–Nonbroadcast Multiaccess (NBMA)
•networks include Frame Relay, ATM,
and X.25 networks.
–Point-to-multipoint
•networks include Frame Relay, ATM,
and X.25 networks.
–Virtual links
•Virtual links are a special type of link that
can be used in multi-area OSPF.
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OSPF in Multiaccess Networks
 2 challenges presented by
multiaccess networks
–Multiple adjacencies
–Extensive LSA flooding
 The creation of an adjacency between
every pair of routers in a network
would create an unnecessary number
of adjacencies.
–This would lead to an excessive
number of LSAs passing between
routers on the same network.
•5 routers in the figure will need 10
adjacencies,
•10 routers would require 45
adjacencies.
•20 routers would require 190
adjacencies
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OSPF in Multiaccess Networks
 Extensive flooding of LSAs
For every LSA sent out there must be an acknowledgement of
receipt sent back to transmitting router.
consequence: lots of bandwidth consumed and chaotic traffic
Solution:
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Steps in the operation of OSPF
 OSPF routers send Hello packets on OSPF enabled interfaces.
 On multi-access networks, the routers elect a DR and BDR. On these networks other routers
become adjacent to the DR.
To reduce the number of adjacencies traffics
To reduce the number of adjacencies each router must form, OSPF calls
one of the routers the designated router. A designated router is elected as
routers are forming adjacencies, and then all other routers establish
adjacencies only with the designated router. This simplifies the routing
table update procedure and reduces the number of link-state records in the
database. The designated router plays other important roles as well to
reduce the overhead of a OSPF link-state procedures. For example, other
routers send link-state advertisements it to the designated router only by
using the all-designated-routers multicast address of 224.0.0.6.
http://www.chebucto.ns.ca/Chebucto/Technical/M
anuals/Max/max6000/isptele/maxospf.htm
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Steps in the operation of OSPF
 OSPF routers send Hello
packets on OSPF enabled
interfaces.
 On multi-access networks, the
routers elect a DR and BDR. On
these networks other routers
become adjacent to the DR.
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Steps in the operation of OSPF
 To reduce the number of adjacencies traffics
http://www.cisco.com/warp/public/104/11.html
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OSPF in Multiaccess Networks
 Solution to LSA flooding issue is the use of
–Designated router (DR)
–Backup designated router (BDR)
•this solution is analogous to electing
someone in the room to go around and learn
everyone's names and then announce these
names to everyone in the room at once.
–DROther
• All other routers become DROthers (this
indicates a router that is neither the DR or the
BDR).
•DROthers only form full adjacencies with the
DR and BDR in the network.
 DR & BDR
–On multiaccess networks, OSPF elects a
Designated Router (DR) to be the collection and
distribution point for LSAs sent and received.
–A Backup Designated Router (BDR) is also
elected in case the Designated Router fails.
–DR & BDR are elected to send & receive LSA
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OSPF in Multiaccess Networks
 DR & BDR & DROther
–Routers on a multiaccess network
elect a DR and BDR.
–DR & BDR are elected to send &
receive LSA
–DROthers only form full
adjacencies with the DR and BDR in
the network.
 Sending & Receiving LSA
–DRothers send LSAs via multicast
224.0.0.6 to DR & BDR
(ALLDRouters - All DR routers)
–DR forward LSA via multicast
address 224.0.0.5 to all other routers
(AllSPFRouters - All OSPF routers).
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OSPF in Multiaccess Networks
DR/BDR Election Process
 DR/BDR elections DO
NOT occur in point-topoint networks
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 DR/BDR elections will take
place on multiaccess
networks as shown below
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OSPF in Multiaccess Networks
 Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF
interface priority.
2. BDR: Router with the second highest
OSPF interface priority.
3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.
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
Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF interface
priority.
2. BDR: Router with the second highest OSPF
interface priority.
3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.

Example:
– The OSPF for all interface priority is 1.
– The OSPF router ID is used to elect the DR
and BDR.
• RouterC with the highest router ID,
becomes the DR
• RouterB, with the second highest router
ID, becomes the BDR.
• Because RouterA is not elected as
either the DR or BDR, it becomes the
DROther.
DROthers only form FULL adjacencies with the DR and BDR, but will still form
a neighbor adjacency with any DROthers that join the network.
When two DROther routers form a neighbor adjacency, the neighbor state is
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as 2WAY.
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You need 4 routers
topology to see this
“2way” adjacency.
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OSPF network types (cont.)
Real DR and BDR election process
The first router up on the network is the
DR.
The second router up on the network is the
BDR.
If the DR fails then the BDR becomes DR
and another router is elected the BDR.
The DR does not change just because
another router comes on line with a higher
priority or a higher router id.
If both the existing DR and BDR fail and a
new DR must be elected, the router with
the highest priority is elected DR.
If there's a tie, the router with the highest
router id is elected DR.
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Timing of DR/BDR Election
(This is really of how the election works)
 Election occurs as soon as 1st router has its OSPF
enabled on multiaccess network. This can happen
when
1. When the routers are powered-on
• it is possible that a router with a lower router ID
will become the DR. This could be a lower-end
router that took less time to boot.
2. when the OSPF network command for that
interface is configured.
 When a DR is elected it remains as the DR until
one of the following occurs
-The DR fails.
-The OSPF process on the DR fails.
-The multiaccess interface on the DR fails.
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Timing of DR/BDR Election
(This is really of how the election works)
 DR Fails
–If the DR fails, the BDR assumes the role of
DR and an election is held to choose a new
BDR.
–In the figure, RouterC fails and the former
BDR, RouterB, becomes DR. The only other
router available to be BDR is RouterA.
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Timing of DR/BDR Election
(This is really of how the election works)
 New Router
–If a new router enters the network after
the DR and BDR have been elected, it will
not become the DR or the BDR even if it
has a higher OSPF interface priority or
router ID than the current DR or BDR.
•If the current DR fails, the BDR will
become the DR, and the new router
can be elected the new BDR.
•After the new router becomes the
BDR, if the DR fails, then the new
router will become the DR.
•The current DR and BDR must both
fail before the new router can be
elected DR or BDR.
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Timing of DR/BDR Election
(This is really of how the election works)
 Old DR Returns
–A previous DR does not regain DR status
if it returns to the network.
•In the figure, RouterC has finished a
reboot and becomes a DROther even
though its router ID, 192.168.31.33, is
higher than the current DR and BDR.
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Timing of DR/BDR Election
(This is really of how the election works)
 BDR Fails
–If the BDR fails, an election is held
among the DRothers to see which router
will be the new BDR.
•In the figure, the BDR router fails.
•An election is held between RouterC
and RouterD.
•RouterD wins the election with the
higher router ID.
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Timing of DR/BDR Election
(This is really of how the election works)
 New DR Fails
–In the figure, RouterB fails. Because
RouterD is the current BDR, it is promoted
to DR. RouterC becomes the BDR.
 So, how do you make sure that the
routers you want to be DR and BDR
win the election? Without further
configurations, the solution is to
either:
–Boot up the DR first, followed by the
BDR, and then boot all other routers, or
–Shut down the interface on all routers,
followed by a no shutdown on the DR,
then the BDR, and then all other routers.
OR: use the priority command set
not desired DR and BDR to 0
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OSPF in Multiaccess Networks
OSPF Interface Priority
 Manipulating the DR/BDR election process continued
–Use the ip ospf priority interface command.
–Example:Router(config-if)#ip ospf priority {0 - 255}
Priority number range 0 to 255
–0 means the router cannot become the DR or BDR
–1 is the default priority value
»router ID determined the DR and BDR
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OSPF in Multiaccess Networks
OSPF Interface Priority
 Modify Priority
–Router(config-if)#ip ospf priority {0 - 255}
 Force Election
–After doing a shutdown and a no shutdown
on the FastEthernet 0/0 interfaces of all three
routers, we see the result of the change of
OSPF interface priorities.
–The show ip ospf neighbor command on
RouterC now shows that RouterA (Router ID
192.168.31.11) is the DR with the highest
OSPF interface priority of 200
–RouterB (Router ID 192.168.31.22) is still
the BDR with the next highest OSPF
interface priority of 100.
–Notice from RouterA's output of show ip
ospf neighbor that it does not show a DR,
because RouterA is the actual DR on this
network.
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DR
BDR
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More OSPF Configuration
Redistributing an OSPF Default Route
 Topology includes a link to ISP
–Router connected to ISP
In this topology, the
Called an autonomous system border router
Loopback1 (Lo1) simulate
the connection to another
Used to propagate a default route
router.
–Example of static default route
R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1
–Requires the use of the default-information originate
command
–Example of default-information originate command
R1(config-router)#default-information originate
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Redistributing an OSPF Default Route
The default route in R2 and R3 with the routing
source OSPF, but with the additional code, E2. For
R2, the route is:
O*E2 0.0.0.0/0 [110/1] via 192.168.10.10, 00:05:34,
Serial0/0/1
E2 denotes that this route is an OSPF External
Type 2 route. the cost of an E2 route is always the
external cost, irrespective of the interior cost to
reach that route. (CCNP)
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More OSPF Configuration
Fine-Tuning OSPF
 Since link speeds are getting
faster it may be necessary to
change reference bandwidth
values
–Do this using the auto-cost
reference-bandwidth command
–Example:
 R1(config-router)#auto-cost
reference-bandwidth 10000
•the default value is equivalent to 100. To
increase it to 10GigE speeds, you would need
to change the reference bandwidth to 10000.
•Again, make sure you configure this command
on all routers in the OSPF routing domain.
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More OSPF Configuration
Fine-Tuning OSPF
 R1(config-router)#autocost reference-bandwidth
10000
•the default value is equivalent to 100. To
increase it to 10GigE speeds, you would need
to change the reference bandwidth to 10000.
R1 Before, the cost to 10.10.10.0/24 is 1172.
After configuring a new reference bandwidth,
the cost for the same route is now 117287.
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More OSPF Configuration
Fine-Tuning OSPF
 Modifying OSPF timers
–Reason to modify timers
Faster detection of network failures
–Manually modifying Hello & Dead intervals
Router(config-if)#ip ospf hello-interval seconds
Router(config-if)#ip ospf dead-interval seconds
–Point to be made
Hello & Dead intervals must be the same between
neighbors
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The End
 Questions?
What will be the result of the DR and BDR elections for this single area
OSPF network? (Choose three.)
*. Decision process:
HQ will be DR for 10.4.0.0/16.
Router A will be DR for 10.4.0.0/16.
HQ will be BDR for 10.4.0.0/16.
Router A will be DR for 10.5.0.0/16.
Remote will be DR for 10.5.0.0/16.
Remote will be BDR for 10.5.0.0/16.
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1.
Which segment will have election?
2.
Priority?
3.
Router ID (each router will only has 1
ID)?
1.
Set using “router-ID” command
2.
Highest Loopback IP address?
3.
Highest physical IP address
(include serial interface)?
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The End
 Questions?
The routers in the diagram are configured as shown. The loopback interface on
router R1 is labeled as lo0. All OSPF priorities are set to the default except for
Ethernet0 of router R2, which has an OSPF priority of 2. What will be the result of
the OSPF DR/BDR elections on the 192.1.1.0 network? (Choose two.)
R1 will be the DR
R1 will be the BDR
R2 will be the DR
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*. Decision process:
1.
Which segment will have election?
2.
Priority?
3.
Router ID (each router will only has 1 ID)?
R2 will be the BDR
1.
Set using “router-ID” command
R3 will be the DR
2.
Highest Loopback IP address?
R3 will be the BDR
3.
Highest physical IP address (include
serial interface)?
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 Questions?
*. Decision process:
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1.
Which segment will have election?
2.
Priority?
3.
Router ID (each router will only has 1 ID)?
1.
Set using “router-ID” command
2.
Highest Loopback IP address?
3.
Highest physical IP address (include serial
interface)?
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 Questions?
Answer?????
*. Decision process:
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1.
Which segment will have election?
2.
Priority?
3.
Router ID (each router will only has 1 ID)?
1.
Set using “router-ID” command
2.
Highest Loopback IP address?
3.
Highest physical IP address (include
serial interface)?
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 Questions?
Answer?????
 HQ will be DR for
10.4.0.0/16
 Router A will be DR for
10.4.0.0/16.
 HQ will be BDR for
10.4.0.0/16.
*. Decision process:
 Router A will be DR for
10.5.0.0/16
1.
Which segment will have election?
2.
Priority?
 Remote will be DR for
10.5.0.0/16.
3.
Router ID (each router will only has 1 ID)?
 Remote will be BDR for
10.5.0.0/16
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1.
Set using “router-ID” command
2.
Highest Loopback IP address?
3.
Highest physical IP address (include
serial interface)?
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Summary
 RFC 2328 describes OSPF link state concepts and
operations
 OSPF Characteristics
–A commonly deployed link state routing protocol
–Employs DRs & BDRs on multi-access networks
DRs & BDRs are elected
DR & BDRs are used to transmit and receive LSAs
–Uses 5 packet types:
1: HELLO
2: DATABASE DESCRIPTION
3: LINK STATE REQUEST
4: LINK STATE UPDATE
5: LINK STATE ACKNOWLEDGEMENT
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Summary
 OSPF Characteristics
–Metric = cost
Lowest cost = best path
 Configuration
–Enable OSPF on a router using the following command
R1(config)#router ospf process-id
–use the network command to define which interfaces will
participate in a given OSPF process
R1(config-router)#network network-address
wildcard-mask area area-id
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Summary
 Verifying OSPF configuration
–Use the following commands
show ip protocol
show ip route
show ip ospf interface
show ip ospf neighbor
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