Download OSPF

Intro to OSPF
© 1998, Cisco Systems, Inc.
1
Introduction to OSPF
Presented by: Andrew Short
Session Number
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Agenda
• Distance Vector vs Link State Routing protocols
• OSPF basics
• OSPF issues
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OSPF – Open Shortest Path First
• “Open”
• “Shortest Path First”
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Distance Vector Routing Protocols
• Learn about network from neighbors only
• Keep a table entry for all destinations
Contains the distance and first gateway
• Periodically send routing table updates to neighbors
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5
Link State Routing Protocols
• Flood Link State Advertisements (LSA) within Autonomous System
(AS).
LSA is state of each link from current router
LSA must reach all routers in AS
• Run Shortest Path First (SPF) algorithm to create an SPF tree for the
complete AS.
SPF tree is rooted at the current router.
Use Next-Hop entries from SPF tree to populate routing table.
• Triggered LSAs
when a link goes up or down, the attached router sends updated
LSAs out all remaining interfaces.
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OSPF vs RIP ( link-state vs distance vector )
• RIP
•Does not scale (15 hop count limit)
•Flat networks, no hierarchy
•Slow convergence
•(RIP 1) Does not support VLSM (variable length subnet mask)
•Periodic broadcast of route table leads to waste of bandwidth
(periodic offload of routing table to the network, re: ISDN links!)
•No concept of link-delay or cost
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OSPF vs RIP ( link-state vs distance vector )
• OSPF
•No hop count limitation
•Scalable - hierarchical network design
•“Areas” limit route information flooding, short convergence time
•supports VLSM
•link bandwidth determines metric
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8
OSPF – Design, Design, Design
• A healthy OSPF implementation is the product of a
well thought out….
• DESIGN.
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OSPF Terminology
Area 0
0.0.0.0
Backbone
Area 2
Area 1
0.0.0.1
0.0.0.2
To Another AS
Presentation_ID
Internal Router
Area Border Router (ABR)
Backbone Router
Autonomous System Boundary Router (ASBR)
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OSPF Terminology contd.
• Router ID (RID)
Must be unique
Highest loopback address or Highest interface address if no loopback
• LSA (Link State Advertisement)
LSA describes state of all the routers links
OSPF routers flood LSAs to create a picture of the network
Each router uses these LSAs to create shortest-path tree rooted at
itself
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OSPF Terminology contd.
• Area
Logical group of routers which have complete picture of network within
grouping.
If multiple areas exist in the network, there must be an ‘area 0’, which is the
backbone area.
All routing information is disseminated from one area to another through area 0.
• Network
A single broadcast domain within the OSPF Autonomous System
Frame-Relay PVC
Ethernet VLAN
Point to Point T1
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OSPF Areas
Area 0
0.0.0.0
Backbone
Area 1
Area 2
0.0.0.2
0.0.0.1
To Another AS
Presentation_ID
Internal Router
Area Border Router (ABR)
Backbone Router
Autonomous System Boundary Router (ASBR)
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OSPF Terminology contd.
• Neighbors
OSPF routers connected to the same network
Must agree on:
Area ID
Authentication
Network Mask
Hello Interval
RouterDeadInterval
Options
• Adjacencies
Neighboring OSPF routers that proceed to the database exchange process
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Configuring OSPF
Router(config)#router ospf 1
Router(config-router)#net 10.0.0.0 0.255.255.255 area 1
- enable OSPF on router and specifies interfaces in OSPF domain
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Forming adjacencies
• OSPF has several network types
- Broadcast
- Non-broadcast Multi-access (NBMA)
- Point-to-multipoint
- Point-to-point
• DR/BDR election only performed on broadcast and NBMA networks
• Hellos sent to AllSPFRouters address (224.0.0.5).
DR/BDR listen to AllDRRouters address (224.0.0.6)
• On broadcast and NBMA networks, routers only become fully
adjacent with DR and BDR.
• On a NBMA network the DR/BDR need to have a static list of all
OSPF routers in that network segment.
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Forming adjacencies contd.
• OSPF neighbors progress through the following stages:
• Down: No information has been received from anybody on the segment.
• Attempt: no recent information has been received. Attempt to contact
neighbor.
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Forming adjacencies contd.
• Init: The interface has detected a Hello packet from a neighbor.
• Two-way: Router sees itself in neighbors Hello packet
• Exstart: Negotiating parameters before exchanging LSA information
• Exchange: Routers will describe their entire link-state database by
sending database description packets.
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Forming adjacencies contd.
• Loading: Finalizing information exchange.
• Full: LSA exchange complete. Both routers now have identical link-state
databases
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OSPF Stub Areas
• Stub Area
• Totally Stubby Area
• Not So Stubby Area (NSSA)
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OSPF – Stub Area
• External LSAs are NOT flooded into stub areas
• No type 4 or type 5 LSAs in this area.
• No virtual links
• “area 1 stub” on ALL area 1 routers
• Route lookups
Intra-area routes
Inter-area routes
Default route
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OSPF – Totally Stub Area
• Type 1 and Type 2 internally
• Only the single ABR produces a single Type 3
Network to advertise the default route.
• “area 1 stub no-summary”
(at the abr of a stub area)
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OSPF – Not So Stubby Area
• Basically, an ospf stub area with an ASBR
• “area 1 nssa”
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LSA Types
Type
1
Router
2
Network
3
Summary Network
4
Summary ASBR
5
External
NSSA
7
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LSA
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LSA Types contd.
• Router LSA (Type 1)
Produced by every SPF Router
Describes the state and cost of the router’s links to the area
All of the router’s links in an area must be described in a single
LSA
Flooded throughout the particular area and no more
Router indicates whether it is an ASBR, ABR, or end point of
virtual link
• “show ip ospf network database router”
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LSA Types contd.
• Network LSA (Type 2)
Sent by DRs
Generated for every transit broadcast and NBMA network
Describes all the routers attached to the network
Only the designated router originates this LSA
Flooded throughout the area and
no more
• “show ip ospf network database network”
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LSA Types contd.
• Summary Network LSA (Type 3) and ASBR
Summary LSA (Type 4)
–Both are
•originated by an ABR
•Flooded throughout a single area
–3 - Describes networks (including default routes) external to the area
but still in the AS
•“show ip ospf database summary”
–4 - Describes ASBRs external to the area.
•“show ip ospf database asbr-summary”
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LSA Types contd.
• External LSA (Type 5)
Originated by ASBRs
Defines routes to destination external
to the AS
Default route is also sent as external
Flooded to all non-stub areas
Two types of external LSA:
E1: (E bit = 0) Consider the total cost up to the external
destination
E2: (E bit = 1) Considers only the cost of the outgoing interface
to the external destination
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LSA Types contd.
• NSSA External LSA (Type 7)
Originated by ASBRs in NSSA (not-so-stubby-area) area
Flooded only within NSSA area
ABR to NSSA area converts it to a Type 5 LSA
Information DOES leave the Area, but not as a Type 7, because
type 7 MUST remain in the area.
Outside the area, it is seen as a Type 5
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Show commands
• show ip ospf
• show ip ospf neighbors
• show ip ospf interface
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Different Types of LSAs
Sample Network for Each LSA
192.1.2.0
3
R1
192.1.1.0
R4
8
8
Area 0
1
R2
R3
8
2
192.1.3.0
External Route
140.10.0.0
195.12.1.1
18.10.0.6
192.1.4.0
2
4
1
Area 1
Router id
131.108.1.1
R7
NSSA
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External LSA
External Type 1
To N1
External Cost = 1
R3
Cost = 10
R1
To N1
External Cost = 2
Cost = 8
R2
Network
N1
N1
Presentation_ID
Type 1
11
10
Next Hop
R1
R2
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Selected Route
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External LSA
External Type 2
To N1
External Cost = 1
R3
Cost = 10
R1
To N1
External Cost = 2
Cost = 8
R2
Network
N1
N1
Presentation_ID
Type 2
1
2
Next Hop
R1
R2
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Selected Route
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Type 5 Details
• Forwarding address
Must be known via intra or inter area route
router ospf 1
redistribute rip subnets
RIP Between the Routers
ASBR
Rest of the OSPF Network
OSPF Is Enabled on this Interface
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Show IP OSPF
GSR-3#sh ip ospf
Routing Process "ospf 100" with ID 10.10.128.3
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x0
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 5
Area has no authentication
SPF algorithm executed 2773 times
Area ranges are
Number of LSA 97. Checksum Sum 0x2B19E3
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
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Show IP OSPF (Cont.)
Area 8
Number of interfaces in this area is 2
It is a stub area, no summary LSA in this area
Area has no authentication
SPF algorithm executed 11 times
Area ranges are
Number of LSA 5. Checksum Sum 0x22812
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
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OSPF Database
3600-g1#sh ip ospf data
OSPF Router with ID (30.8.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID
30.8.1.1
ADV Router
30.8.1.1
Age
1592
Seq#
Checksum Link count
0x800001D0 0xA180
0
Router Link States (Area 8)
Link ID
ADV Router
Age
Seq#
Checksum Link count
30.8.1.1
30.8.1.1
1592
0x8000023F 0xC782
1
30.8.1.2
30.8.1.2
298
0x800003D1 0x2967
2
30.8.3.2
30.8.3.2
666
0x800002B8 0xE52B
1
Net Link States (Area 8)
Link ID
ADV Router
Age
30.8.1.2
30.8.1.2
299
30.100.1.2 30.8.3.2
666
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Seq#
Checksum
0x80000203 0x4153
0x8000027A 0x10AB
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OSPF Database
GSR-3#show ip ospf database database-summary
OSPF Router with ID (10.10.128.3) (Process ID 100)
Area 0 database summary
LSA Type
Count
Router
63
Network
4
Summary Net
30
Summary ASBR
0
Type-7 Ext
0
Opaque Link
0
Opaque Area
0
Subtotal
97
Presentation_ID
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Delete
0
0
0
0
0
0
0
0
Maxage
0
0
0
0
0
0
0
0
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OSPF Neighbor
GSR-3#sh ip ospf neighbor
Neighbor ID
10.64.1.2
10.10.128.1
10.10.128.2
10.10.128.4
GSR-3#
Presentation_ID
Pri State
Dead Time Address Interface
1 FULL/DR
00:00:38 10.128.16.2 GigabitEthernet3/0
1 2WAY/DROTHER 00:00:31 10.128.1.1 Ethernet 6/0
1 FULL/BDR
00:00:36 10.128.1.2 Ethernet 6/0
1 FULL/DR
00:00:38 10.128.1.4 Ethernet 6/0
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OSPF Adjacency Changes
r4-1#
router ospf 1
ospf log-adjacency-changes
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.16.2 on Serial0.1 from FULL to
DOWN, Neighbor Down
%OSPF-5-ADJCHG: Process 1, Nbr 0.0.0.0 on Serial0.1 from DOWN to
ATTEMPT, NBMA Start
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Neighbor Details
r4-1#sh ip ospf nei det
Neighbor 172.16.133.5, interface address 172.16.133.5
In the area 0 via interface Ethernet0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 172.16.133.29 BDR is 172.16.133.6
Options is 0x2
Dead timer due in 00:00:37
Neighbor is up for 00:01:24
Index 2/2, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 4 msec, maximum is 4 msec
Neighbor 172.16.133.29, interface address 172.16.133.29
In the area 0 via interface Ethernet0
Neighbor priority is 1, State is FULL, 6 state changes
DR is 172.16.133.29 BDR is 172.16.133.6
Options is 0x2
Dead timer due in 00:00:31
Neighbor is up for 00:01:58
Index 1/1, retransmission queue length 0, number of retransmission 2
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
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Show IP OSPF Interface
r4-1#show ip ospf int
Serial0.1 is up, line protocol is up
Internet Address 172.16.7.1/24, Area 0
Process ID 1, Router ID 172.16.14.1, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DR, Priority 100
Designated Router (ID) 172.16.14.1, Interface address 172.16.7.1
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
Hello due in 00:00:12
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 172.16.30.1
Adjacent with neighbor 172.16.16.2
Suppress hello for 0 neighbor(s)
Serial0.2 is up, line protocol is up
Internet Address 172.16.14.1/24, Area 33
Process ID 1, Router ID 172.16.14.1, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DR, Priority 100
Designated Router (ID) 172.16.14.1, Interface address 172.16.14.1
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
Hello due in 00:00:11
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.16.100.2
Suppress hello for 0 neighbor(s)
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Other show Commands
R6-2500#sh ip ospf database self-originate (shows the LSAs this router is generating)
OSPF Router with ID (192.168.6.1) (Process ID 100)
Router Link States (Area 0)
Link ID
ADV Router
Age
Seq#
773
0x8000005B 0xC608
1
Checksum Link count
192.168.6.1
192.168.6.1
Net Link States (Area 0)
Link ID
135.4.1.2
ADV Router
192.168.6.1
Age
773
Seq#
0x80000056
Checksum
0x4D14
Seq#
0x80000053
Checksum Link count
0xC745
0
Router Link States (Area 5)
Link ID
192.168.6.1
ADV Router
192.168.6.1
Age
264
Summary Net Link States (Area 5)
Link ID
135.4.1.0
ADV Router
192.168.6.1
Age
774
Seq#
Checksum
0x8000005C 0xDDFC
Summary ASB Link States (Area 5)
Link ID
5.5.5.5
ADV Router
192.168.6.1
Age
774
Seq#
Checksum
0x8000002B 0x681B
Type-5 AS External Link States
Link ID
1.1.1.0
Presentation_ID
ADV Router
192.168.6.1
Age
266
Seq#
Checksum Tag
0x8000002C 0x31D0
0
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Other show Commands
R6-2500#sh ip ospf database adv-router 5.5.5.5
(shows the LSAs neighbor is
generating)
OSPF Router with ID (192.168.6.1) (Process ID 100)
Router Link States (Area 0)
Link ID
ADV Router
Age
Seq#
Checksum Link count
5.5.5.5
5.5.5.5
1479
0x80000034 0xA52A
1
Type-5 AS External Link States
Link ID
ADV Router
Age
Seq#
Checksum Tag
1.1.1.0
5.5.5.5
1478
0x80000032 0xACCB
5.5.5.0
5.5.5.5
1478
0x80000030 0x204E
0
135.4.1.0
5.5.5.5
1478
0x80000032 0xB33B
0
0
R6-2500#
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Show IP OSPF stat
r4-1#sh ip ospf stat
Area 0: SPF algorithm executed 16 times
Area 33: SPF algorithm executed 8 times
SPF calculation time
Delta T Intra D-Intra Summ
3d18h
4
0
4
3d18h
8
0
4
3d18h
4
0
4
3d18h
4
0
4
3d18h
4
0
4
3d18h
4
0
4
3d17h
4
0
4
00:04:45 4
0
4
00:02:51 4
0
4
00:02:41 4
0
8
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D-Summ
4
0
0
0
0
0
0
8
0
8
Ext
8
8
8
8
8
8
8
8
8
8
D-Ext Total Reason
0
20
R, SN,
0
20
R, N, SN,
0
16
N,
0
20
N, SN,
0
16
R, N,
0
16
R, SN,
0
28
R, N, SN, SA, X
0
24
N, SN,
0
20
R, N,
0
28
R, SN,
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OSPF issues
• Number of routes in area
• Number of adjacencies per router
• Size of link-state database
Managed with summarization and use of areas
• Route summarization
Pros and Cons
• Reference bandwidth
Ospf auto-cost Reference-bandwidth [#]
• SPF calculation frequency and duration
• Partitioned areas
• Virtual links
Transistional only!
Presentation_ID
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OSPF resources
• RFCs
2328: OSPF Version 2
1587: The OSPF NSSA Option
2370: Opaque LSA
• Books
CCIE Professional Development: Routing TCP/IP Vol 1 by Jeff Doyle
• Cisco online OSPF resources
OSPF Design Guide
http://www.cisco.com/warp/customer/104/1.html
OSPF: Frequently Asked Questions
http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.s
html
• Dijkstra Algorithm Applet:
http://www.dgp.toronto.edu/people/JamesStewart/270/9798s/Laffra/DijkstraApplet.html
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Intro to OSPF
© 2001, Cisco Systems, Inc.