OSPF Fundamental

Fundamental of
IGP Protocols-OSPF
by : Reza Farahani
1

Routing Protocol
 Interior Gateway Protocol (IGP): A routing protocol that was designed and intended for
use inside a single autonomous system (AS).
 Exterior Gateway Protocol (EGP): A routing protocol that was designed and intended for
use between different autonomous systems.
2

IGP ALGORITHMS
 Distance vector (sometimes called Bellman-Ford after its creators)
 Link-state
 Balanced hybrid (sometimes called enhanced distance vector)
5

DISTANCE VECTOR
 A distance-vector routing protocol sends a full copy of its routing table to
its directly attached neighbors.
 This is a periodic advertisement, even if there have been no topological
changes, a D.V routing protocol will, re-advertise its full routing table to
its neighbors.
 Layer 3 LOOP
 Split Horizon
 Route poisoning
 Poison Reverse
 Triggered update
 Hold-down timer
7

LINK-STATE
 Routers use this information, stored in RAM inside a data structure called the link-
state database (LSDB), to calculate the currently best routes to each subnet.
 The updates containing information called link-state advertisements (LSA).
8

LINK-STATE(CO)
9
 This process uses something called the Dijkstra Shortest Path First (SPF)
algorithm.
 LSDB works like the map, and the SPF algorithm works like the person reading
the map.
 Each router uses itself as the starting point.

CLASSLESS & CLASSFUL
11
 Some routing protocols must consider the Class and other routing protocols can
ignore Class A,B, and C rules altogether.

OSPF LINK STATE CONCEPTS
 OPSF uses link state (LS) logic, which can be broken into three major branches.
 RFC2328
 The first step, neighbor discovery
 The second step, topology database exchange
 Each router stores topology information in its topology database (LSDB).
 The third major step, route computation
 OSPF uses SPF algorithm to analyze the data, choose the best route for each reachable subnet,
and add the correct information for those routes to the IP routing table
12

Router ID
16
 Step 1:
 Use the router ID defined in the router-id x.x.x.x OSPF router subcommand.
 Step 2:
 Use the highest IP address of any up/up loopback interface.
 Step 3:
 Use the highest IP address of any up/up non-loopback interface.

OSPF VERIFICATION
 Command: show ip ospf
 Show information on a variety of general OSPF and area state and configuration information.
 show ip ospf interface [INTERFACE]
 Show state and configuration of OSPF the specified interface, or all interfaces if no interface is given.
 Command: show ip ospf route
 Show the OSPF routing table, as determined by the most recent SPF calculation.
18

OSPF VERIFICATION
19
 Command: show ip ospf neighbor
 Command: show ip ospf neighbor INTERFACE
 Command: show ip ospf neighbor detail
 Command: show ip ospf neighbor INTERFACE detail

OSPF VERIFICATION
 Command: show ip ospf database
 Command: show ip ospf database max-age
20

ADJACENCIES ON LANS
25
 OSPF sends multicast OSPF Hello messages on LAN interfaces, to discover OSPF
neighbors, when two requirements are met:
 OSPF has been enabled
 The interface has not been made passive by the passive-interface router subcommand

Passive Interface
 When a router configures an interface as passive to OSPF, OSPF quits sending OSPF Hellos, so
the router will not discover neighbors.
 The router will still advertise about the interface’s connected subnet if OSPF is enabled on the
interface
26

HELLO MESSAGE FIELDS
 OSPF Router ID
 Hello interval
 Dead interval
 Subnet mask
 List of neighbors reachable on the interface
 Area ID
 Router priority
 Designated Router (DR) IP address
 Backup DR (BDR) IP address
 Authentication digest
27

OPTIMIZING CONVERGENCE
 The Dead interval defines how long a router should wait, without hearing any Hello
messages from that neighbor, before deciding that the neighbor failed.
 The neighbor resets its downward-counting Hold timer to 40 upon receiving a Hello from
that neighbor
 To tune for faster convergence, you can configure OSPF to set a lower Hello and Dead
timer.
 If the interface fails, OSPF will immediately realize that all neighbors reached through that
interface have also failed and not wait on the Dead timer
29

30
 Interface Command: ip ospf dead-interval <1-65535>
 Interface Command: ip ospf dead-interval minimal hello-multiplier <2-20>

31
ip ospf hello-interval <1-65535>
Interface Command: no ip ospf hello-interva

Regular Routing Method bottle-necks
32
 Interface Command: ip ospf retransmit-interval <1-65535>
 Interface Command: no ip ospf retransmit interval

OSPF AUTHENTICATION
 Enabling per interface
 Enabling on all interfaces in an area
 Clear text per area
 OSPF Command: area <0-4294967295> authentication
 Enable on interface by : ip ospf authentication-key AUTH_KEY
33

OSPF AUTHENTICATION
 MD5 per area :
 area #n authentication message-digest
 Enable on interface by : ip ospf message-digest-key KEYID md5 KE
 Per Interface
 ip ospf authentication-key AUTH_KEY
 ip ospf authentication message-digest
 Note: The maximum length of the key is 16
 Show ip ospf interface
34

DR AND BDR
 The end result of the DR election is that topology information is
exchanged only between special neighbors not all.
35

DR AND BDR
 The DR concept prevents overloading a subnet with too much OSPF traffic when many
routers are on a subnet.
 Because the DR is so important to the exchange of routing information, the loss of the
elected DR could cause delays in convergence
 OSPF includes the concept of a Backup All routers except the DR and BDR are typically
called “DROther” in IOS show command output.
 on each subnet, so when the DR fails or loses connectivity to the subnet, the BDR can
take over as the DR.
36

DR AND BDR Election
 The router sending the Hello with the highest OSPF priority setting becomes the
DR.
 If two or more routers tie with the highest priority setting, the router sending the
Hello with the highest RID wins.
 The router with the second-highest priority becomes the BDR.
 After the DR and BDR have been elected, the new better candidate does not
preempt the existing DR/BDR
37

DR AND BDR Command
38
 Interface Command: ip ospf priority <0-255>
 The range of priority values that allow a router to be a candidate are 1 ~ 255

STATIC DISCOVERY
 For example in NBMA Networks : Frame Relay …
 Interface Command: ip ospf network (broadcast|non-broadcast|point-to-
multipoint|point-to-point)
 In our device like cisco LoopBack advertise by /32 and we can use above
command for change real define subnet loopback interface
42

LSA TYPE 1
 An LSA type 1, called a router LSA, identifies an OSPF router based on its OSPF
router ID (RID).
 Each router creates a Type 1 LSA for itself and floods the LSA throughout the
same area.
 The Type 1 LSA in one area will list only interfaces in that area and only neighbors
in that area.
46

LSA TYPE 2
47
 OSPF defines the Type 2 network LSA, used as a pseudonode

LSA TYPE 3
 ABRs do not forward Type 1 and Type 2 LSAs from one area into another area, and
vice versa
 This convention results in smaller per-area LSDBs, saving memory and reducing
complexity for each run of the SPF algorithm, which saves CPU and improves
convergence time.
 OSPF advertises inter-area routes using the Type 3 summary LSA.
 Type 3 summary LSAs do not contain all the detailed topology information, so in
comparison to Types 1 2, these LSAs summarize the information–hence the name
summary LSA.
48

PERIODIC FLOODING
 OSPF does reflood each LSA every 30 minutes based on each LSA’s age variable.
 If 30 minutes pass with no changes to an LSA resets the timer to 0, and refloods
the LSA.
 Note also that when a router realizes it needs to flush an LSA from the LSDB for an
area, it actually sets the age of the LSA to the MaxAge setting (3600) and refloods
the LSA.
49

BEST OSPF ROUTES
 Analyze the LSDB to find all possible routes to reach the subnet.
 Note that OSPF supports equal-cost load balancing.
 Lower cost but SPECIAL RULES :
 Step 1.When choosing the best route, an intra-area route is always better than a
competing interarea route, regardless of metric
 If an ABR learns a Type 3 LSA inside a nonbackbone area, the ABR ignores that LSA
when calculating its own routes
50

METRIC TUNING
51
OSPF Command: auto-cost reference-bandwidth <1-4294967>
Interface Command: ip ospf cost <1-65535>

OSPF Virtual Links
52
 In some cases two backbone areas exist; in other cases, a non-backbone area may not have a point of
connection to the backbone area.

OSPF Virtual Links
53
 Extend Backbone Area but
 Change use unicast packet
 It’s temporary

TYPE 3 LSA FILTERING
 On ABR1, filter subnet 3 from being advertised
 On ABR2, filter both subnet 2 and 3
54

 When “in” is configured, IOS filters prefixes being created and flooded into the
configured area
 When “out” is configured, IOS filters prefixes coming out of the configured area.
55

ROUTE SUMMARIZATION
 OSPF allows summarization at both ABRs and ASBRs but not on other OSPF
routers
 The main reason is again that the LSDB must be the same for all routers in a single
area.
56

END OF PART 1
Next session :
Redistribution
Other LSA Types
OSPF V3
Thanks for your Attention
57

OSPF Fundamental

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to OSPF Fundamental

Similar to OSPF Fundamental (20)

More from Reza Farahani

More from Reza Farahani (14)

Recently uploaded

Recently uploaded (20)

OSPF Fundamental