Open Shortest Path First
description
Transcript of Open Shortest Path First
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 StateThe 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 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
OSPF as a Link-State Protocol
– 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 Data Structures
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 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
OSPF Areas
Backbone Area
Area1 Area2Area3
OSPF Terminology
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
Backbone Area
Area1 Area2Area3
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
Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to link-state 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
x
C
BA
F
DE
HG
x
C
BA
F
DE
HG
Shortest PathLink-state DB
Dijkstra’s algorithm
Assume all links are Ethernet, with an OSPF cost of 10
OSPF Calculation(cont.)
OSPF Packets Type
OSPF Packet Types
1.Hello
2.Destination Description
5.Link-State Acknowledgement
4.Link-State Update
3.Link-State Request
Neighborship: The Hello Packet
Hello
hello
•Router ID•Hello and dead intervals•Neighbors•Area ID•Router priority•DR IP address•BDR IP address•Authentication password•Stub area flag
•Entry must match on neighboring routers
I am router id 172.16.5.2, and I see 172.16.5.1
I am router id 172.16.5.1, and I see no one
BA
172.16.5.1/24
172.16.5.2/24
helloTo 224.0.0.5
Down state
Initial State
Port1
Port2
Router B neighbor List 172.16.5.1/24,in Port2
hello
Router A neighbor List 172.16.5.2/24,in Port1
Two-way State
Unicast to A
Establishing Bidirectional Communication
Here is a summary of my LSDB
No, I’ll start exchange because I have a higher RID
I will start exchange because I have router id 172.16.5.1
BA
172.16.5.1/24
172.16.5.2/24
DBD
Exstart state
exchange State
Port1
Port2
DBD
Discovering the Network Routes
Here is a summary of my LSDBDBD
DBD
Thanks for the information!
BA
172.16.5.1/24
172.16.5.2/24
LSAck
Loading state
Full State
Port1
Port2
Adding the Link-State Entries
LSAck
I need complete entry for network 172.16.6.0/24LSR
Here is the entry for network 172.16.6.0/24LSU
LSAckThanks for the information!
int1192.168.1.1/24
System10.1.1.2/24
int2192.168.2.1/24
int1172.17.1.1/24
int2172.17.2.1/24
int110.1.1.3/24
Area 0
Area 2
Case Study: OSPF Packets
Router ID:2.2.2.2
Router ID:1.1.1.1
NO
NO
YES
YES
NO
YES
Link-State Data Structures: LSA Operation
IS entry in LSDB?
Is seq# the
same?
Ignore LSA
Add to DB
Send LSAck
Flood LSA
Run SPF to calculate new routing table
END
LSA
Is seq# higher?
Send LSU with newer information to source
END
Maintaining Routing Information
B
DR
A
12
3
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 1-255>] | 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)
int1192.168.1.1/24
System10.1.1.2/24
int2192.168.1.1/24
int1172.17.1.1/24
int2172.17.2.1/24
System10.1.1.3/24
Area 0
Area 2
Router ID:2.2.2.2
Router ID:1.1.1.1
DES-3852
DES-3828P
DES-3852# OSPFconfig 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 2.2.2.2enable ospf
DES-3828P# OSPFcreate ospf area 0.0.0.2 type normalconfig 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 enableconfig ospf router_id 1.1.1.1enable ospf
System10.1.1.2/24
System10.1.1.3/24
int1172.17.1.1/24
int2172.17.2.1/24
int1192.168.1.1/24
int2192.168.1.1/24
DES-3852# OSPFconfig 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 2.2.2.2enable ospf
DES-3828P# OSPFcreate ospf area 0.0.0.2 type normalconfig 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 enableconfig ospf router_id 1.1.1.1enable ospf
System10.1.1.2/24
System10.1.1.3/24
int1172.17.1.1/24
int2172.17.2.1/24
int1192.168.1.1/24
int2192.168.1.1/24
DES-3852# OSPFconfig 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 2.2.2.2enable ospf
DES-3828P# OSPFcreate ospf area 0.0.0.2 type normalconfig 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 enableconfig ospf router_id 1.1.1.1enable ospf
System10.1.1.2/24
System10.1.1.3/24
int1172.17.1.1/24
int2172.17.2.1/24
int1192.168.1.1/24
int2192.168.1.1/24
SW1SW2
•X is the Switch Number•Each Switch has a loopback: X.X.X.X/32 except SW3
Area 0
Area 1
SW3
SW4
Fa1/310.1.3.1/24
Fa1/210.1.2.1/24
Fa1/210.1.2.1/24
Fa1/310.1.3.2/24
Fa1/1172.31.1.1/24
Fa1/1172.31.1.2/24
Fa1/188.88.88.88/24
Area 3
Fa1/299.99.99.99/24
Case Study: OSPF Configuration(Cisco)
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
Hello
Electing the DR and BDR
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 ModeNBMA Preferred
TopologySubnet Address
Hello Timer
AdjacencyRFC or Cisco
BroadcastFull or partial
meshSame 10 sec
Automatic, DR/BDR elected
Cisco
Nonbroadcast (NBMA)
Full or partial mesh
Same 30 secManual
configuration, DR/BDR elected
RFC
Point-to-multipoint
Partial-mesh or star
Same 30 SecAutomatic, no DR/BDR
RFC
Point-to-multipoint
nonbroadcast
partial-mesh or star
Same 30 secManual
configuration, no/DR/BDR
Cisco
Point-to-pointPartial-mesh or
star, using subinterface
Different for Each Subinterface
10 secAutomatic, no DR/BDR
Cisco
OSPF Network Type summary
ABR and Backbone
Router
ASBR and Backbone
Router
OSPF Router Type
ExternalAS
Area1
Area0
Area2
Internal routers
Internal routers
Backbone/Internal routers
ABR and Backbone
Router
OSPF Virtual Link
Virtual Link
Area010.0.0.0
Area010.0.0.0
Area1172.16.0.0
•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
A B
Case Study: OSPF Virtual Link (D-Link)
Case Study: OSPF Virtual Link (Cisco)
Q&A
END