Download ospf_1 - D-Link

Open Shortest Path First
Pedro Tsao
E-mail:[email protected]
Agenda
Link state
OSPF Areas
OSPF behavior
OSPF Packets Type
OSPF Network Type
OSPF LSA Type
OSPF Route Summarization
Configuring OSPF Special Area types
Configuring OSPF Authentication
Link State
Link State
The information available to a distance vector router has been compared to the information
available from a road sign. Link state routing protocol are like a road map.
Link State routing include the following:
• Open Shortest Path First (OSPF) for IP
•The ISO’s Intermediate System-to-Intermediate System (IS-IS) for CLNS and IP
•DEC’s DNA Phase V
•Novell’s NetWare Link Services Protocol (NLSP)
OSPF as a Link-State Protocol
•OSPF propagates link-state advertisements rather than routing table updates
•LSAs are flooded to all OSPF routers in the area
•The OSPF link-state database(LSDB) is pieced together from the LSAs generated by
the OSPF routers
•OSPF use the SPF algorithm to calculate the shortest path to a destination
Link = router interface
State = description of an interface and its relationship to neighboring routers
Link State Data Structures
– Neighbor table:
 Also known as the adjacency database
 Contains list of recognized neighbors
– Topology table:
 Typically referred to as LSDB
 Contains all routers and their attached links in the area or network
 Identical LSDB for all routers within an area
– Routing table:
 Commonly named a forwarding database
 Contains list of best paths to destinations
Link State Routing Protocol
Link-state routers recognize more information about the network
than their distance vector counterparts.
Each router has a full picture of the topology.
Consequently, link-state routers tend to make more accurate decisions.
OSPF Areas
Link State Data Structure: Network Hierarchy
Link-state routing requires a hierachical network structure
that is enforced by OSPF.
This two-level hierarchy consists of the following:
•Transit area (backbone or area 0)
•Regular areas (non-backbone areas)
OSPF Areas
Backbone Area
OSPF area characteristics:
Minimizes routing table entries
Localizes impact of a topology
change within an area
Detailed LSA flooding stops at the
area boundary
Requires a hierarchical network
design
Area1
Area2
Area3
OSPF Terminology
Backbone Area
Router A and B are backbone routers
Backbone routers make up area 0
Router C, D and E are known as Area Border
Routers (ABRs)
ABRs attach all other areas to area 0
Area1
Area2
Area3
OSPF Behavior
OSPF Adjacencies
Hello
Routers discover neighbors by exchange hello packets
Routers declare neighbors to be up after checking certain
parameters or options in the hello packet
Forming OSPF Adjacencies

Point-to-point WAN links:

Both neighbors become full adjacent

LAN Links

Neighbors form a full adjacency with the DR and BDR

Routers maintain two-way state with the other routers (DROTHERs)

Routing updates and topology information are passed only between adjacent routers

Once an adjacency is formed, LSDBs are synchronized by exchanging LSAs

LSAs are flooded reliably through the area (or network)
OSPF Router ID
The Router is Known to OSPF by the OSPF router ID number
LSDBs use the OSPF router ID to differentiate one router from the next
In descending other of specificity, the Router-id may be one of following:
•Router-id command
•Highest loopback address
•Highest Active IP address
OSPF DR/BDR Election
DR/BDR will be electing by the following rules:
• The router With Highest priority value is the DR
• The router with the second highest priority value is BDR
• In case of a tie. The highest Router ID is DR, the second is BDR
• A router with priority of 0 cannot be the DR or BDR
• A router that’s not DR or BDR is a DROther
•If a router with higher priority comes into the network, it does not preempt the DR or BDR
OSPF Calculation
Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to linkstate database as follows:
Every router in an area has the identical link-state DB
Each router in the area places itself into the root of the tree that is built
The best path is calculated with respect to the lowest total cost of links to a specific
destination
Best routes are put into the forwarding database(routing table)
OSPF Calculation(cont.)
Link-state DB
Shortest Path
x
x
B
A
B
A
C
Dijkstra’s algorithm
C
D
D
E
E
F
G
H
F
Assume all links are Ethernet, with an OSPF cost of 10
G
H
OSPF Packets Type
OSPF Packet Types
1.Hello
2.Destination Description
3.Link-State Request
4.Link-State Update
5.Link-State Acknowledgement
Neighborship: The Hello Packet
Hello
hello
•Entry must match on neighboring routers
•Router ID
•Hello and dead intervals
•Neighbors
•Area ID
•Router priority
•DR IP address
•BDR IP address
•Authentication password
•Stub area flag
Establishing Bidirectional Communication
Port2
172.16.5.1/24
A
Port1
172.16.5.2/24
B
Down state
hello
I am router id 172.16.5.1, and I see no one
To 224.0.0.5
Initial State
Router B neighbor List
172.16.5.1/24,in Port2
Unicast to A
I am router id 172.16.5.2, and I see 172.16.5.1
Router A neighbor List
172.16.5.2/24,in Port1
Two-way State
hello
Discovering the Network Routes
Port2
172.16.5.1/24
A
Port1
172.16.5.2/24
B
Exstart state
DBD
I will start exchange because I have router id 172.16.5.1
No, I’ll start exchange because I have a higher RID
DBD
exchange State
Here is a summary of my LSDB
DBD
Here is a summary of my LSDB
DBD
Adding the Link-State Entries
Port2
172.16.5.1/24
A
LSAck
Port1
172.16.5.2/24
Thanks for the information!
B
LSAck
Loading state
LSR
I need complete entry for network 172.16.6.0/24
Here is the entry for network 172.16.6.0/24
LSAck
Thanks for the information!
Full State
LSU
Case Study: OSPF Packets
Area 0
int1
172.17.1.1/24
int2
172.17.2.1/24
Router ID:
2.2.2.2
int1
10.1.1.3/24
System
10.1.1.2/24
Router ID:
1.1.1.1
int1
192.168.1.1/24
Area 2
int2
192.168.2.1/24
Link-State Data Structures: LSA Operation
LSA
IS entry
in LSDB?
YES
YES
NO
NO
Add to DB
Is seq#
the
same?
YES
Is seq#
higher?
Send LSAck
NO
Flood LSA
Send LSU with newer
information to source
Run SPF to calculate
new routing table
END
END
Ignore
LSA
Maintaining Routing Information
3
DR
1
2
A
B
Router A notifies all OSPF DRs on 224.0.0.6
DR notifies others on 224.0.0.5
Command (OSPF)
Command
Parameters
Enable ospf
Disable ospf
create ospf area
<area_id> type [normal | stub {stub_summary [enable |
disable] | metric <value 0-65535>}
create ospf
host_route
<ipaddr> {area <area_id> | metric <value 1-65535>}
create ospf
aggregation
<area_id> <network_address> lsdb_type summary {advertise
[enabled | disabled]}
config ospf ipif
[ipif <ipif_name 12> | all] {area <area_id> | priority <value> |
hello_interval <sec 1-65535> | dead_interval <sec 1-65535> |
authentication [none | simple <password 8> | md5 <key_id 1255>] | metric <value 1-65535> | state [enable | disable] |
active | passive}
create ospf
virtual_link
<area_id> <neighbor_id> {hello_interval <sec 1-65535> |
dead_interval <sec 1-65535> | authentication [none | simple
<password 8> | md5 <key_id 1-255>]}
Case Study: OSPF Configuration(D-Link)
Area 0
DES-3852
int1
172.17.1.1/24
int2
172.17.2.1/24
Router ID:
2.2.2.2
System
10.1.1.3/24
DES-3828P
System
10.1.1.2/24
Router ID:
1.1.1.1
int1
192.168.1.1/24
Area 2
int2
192.168.1.1/24
int1
172.17.1.1/24
int2
172.17.2.1/24
DES-3852
# OSPF
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.0 state enable
config ospf ipif int2 area 0.0.0.0 state enable
config ospf router_id 2.2.2.2
enable ospf
System
10.1.1.3/24
System
10.1.1.2/24
DES-3828P
# OSPF
create ospf area 0.0.0.2 type normal
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.2 state enable
config ospf ipif int2 area 0.0.0.2 state enable
config ospf router_id 1.1.1.1
enable ospf
int1
192.168.1.1/24
int2
192.168.1.1/24
int1
172.17.1.1/24
int2
172.17.2.1/24
DES-3852
# OSPF
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.0 state enable
config ospf ipif int2 area 0.0.0.0 state enable
config ospf router_id 2.2.2.2
enable ospf
System
10.1.1.3/24
System
10.1.1.2/24
DES-3828P
# OSPF
create ospf area 0.0.0.2 type normal
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.2 state enable
config ospf ipif int2 area 0.0.0.2 state enable
config ospf router_id 1.1.1.1
enable ospf
int1
192.168.1.1/24
int2
192.168.1.1/24
int1
172.17.1.1/24
int2
172.17.2.1/24
DES-3852
# OSPF
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.0 state enable
config ospf ipif int2 area 0.0.0.0 state enable
config ospf router_id 2.2.2.2
enable ospf
System
10.1.1.3/24
System
10.1.1.2/24
DES-3828P
# OSPF
create ospf area 0.0.0.2 type normal
config ospf ipif System area 0.0.0.0 state enable
config ospf ipif int1 area 0.0.0.2 state enable
config ospf ipif int2 area 0.0.0.2 state enable
config ospf router_id 1.1.1.1
enable ospf
int1
192.168.1.1/24
int2
192.168.1.1/24
Case Study: OSPF Configuration(Cisco)
Fa1/3
10.1.3.2/24
Fa1/1
88.88.88.88/24
SW3
Area 0
Fa1/2
10.1.2.1/24
Area 1
SW2
Fa1/1
172.31.1.2/24
Fa1/3
10.1.3.1/24
Fa1/2
10.1.2.1/24
SW1
Fa1/1
172.31.1.1/24
SW4
Fa1/2
99.99.99.99/24
Area 3
•X is the Switch Number
•Each Switch has a loopback: X.X.X.X/32 except SW3
OSPF Network Type
OSPF Network Type
The three types of networks defined by OSPF are:
Point-to-point: A network that joins a single pair of routers
Broadcast: A multiaccess broadcast network, such as Ethernet
Non-Broadcast multiaccess(also called NBMA): A network that interconnects more than
two routers but that has no broadcast capability. Frame Relay, X.25 and ATM are examples
of NBMA networks
Point-to-point Links
Usually a serial interface running either PPP or HDLC
May also be a point-to-point interface running Frame Relay or ATM
No DR or BDR election required
OSPF autodetects this interface type
OSPF packets are send using multicast 224.0.0.5
Broadcast Network
Generally these are LAN technologies like Ethernet and Token Ring
DR and BDR election are required
All neighbor routers form full adjacencies with the DR and BDR only
Packets to the DR and BDR use 224.0.0.6
Packets from DR to all other routers use 224.0.0.5
Electing the DR and BDR
Hello
Hello packets are exchange via IP multicast
The router with the highest priority is selected as the BR. The second-highest one is the
BDR
Use the OSPF RID as the tie breaker
The DR election is nonpreemptive
OSPF Network Type summary
OSPF Mode
NBMA Preferred
Topology
Subnet Address
Hello
Timer
Adjacency
RFC or
Cisco
Broadcast
Full or partial
mesh
Same
10 sec
Automatic,
DR/BDR elected
Cisco
Nonbroadcast
(NBMA)
Full or partial
mesh
Same
30 sec
Manual
configuration,
DR/BDR elected
RFC
Point-tomultipoint
Partial-mesh
or star
Same
30 Sec
Automatic,
no DR/BDR
RFC
Point-tomultipoint
nonbroadcast
partial-mesh
or star
Same
30 sec
Manual
configuration,
no/DR/BDR
Cisco
Point-to-point
Partial-mesh or
star, using
subinterface
Different for Each
Subinterface
10 sec
Automatic,
no DR/BDR
Cisco
OSPF Router Type
Area0
ABR and
Backbone
Router
Backbone/Internal
routers
Internal
routers
Internal
routers
ABR and
Backbone
Router
Area1
External
AS
Area2
ASBR and
Backbone
Router
OSPF Virtual Link
Area1
172.16.0.0
Area0
10.0.0.0
A
Virtual Link
Area0
10.0.0.0
B
•Virtual links are used to connect a discontinuous area to area 0
•A logical connection is built between router A and router B
•Virtual links are recommended for backup or temporary connections
Case Study: OSPF Virtual Link (D-Link)
Case Study: OSPF Virtual Link (Cisco)
Q&A
END