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Every router that connects to a given OSPF area should learn the exact same topology
data. Each router stores the data, composed of individual link state advertisements (LSA),
in their own copy of the link state database (LSDB). Then, the router applies the Shortest
Path First (SPF) algorithm to the LSDB to determine the best (lowest cost) route for each
reachable subnet (prefix/length).
When a router uses SPF to analyze the LSDB, the SPF process has some similarities to
how humans put a jigsaw puzzle together–but without a picture of what the puzzle looks
like. Humans faced with such a challenge might first look for the obvious puzzle pieces,
such as the corner and edge pieces, because they are easily recognized. You might then
group puzzle pieces together if they have the same color or look for straight lines that
might span multiple puzzle pieces. And of course, you would be looking at the shapes of
the puzzle pieces to see which ones fit together.
Similarly, a router’s SPF process must examine the individual LSAs and see how they fit
together, based on their characteristics. To better appreciate the SPF process, the first section
of this chapter examines the three LSA types OSPF uses to describe an Enterprise
OSPF topology inside the OSPF domain. By understanding the types of LSAs, you can
get a better understanding of what a router might look for to take the LSAs–the pieces of
a network topology puzzle if you will–and build the equivalent of a network diagram.
For reference, Table 6-2 lists the various OSPF LSA types. Note that Chapter 9 explains
three other LSA types, all used when redistributing routes into the OSPF domain.
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Table 6-2 OSPF LSA Types
LSA Type Common Name Description
5 AS External Created by ASBRs for external routes injected into OSPF.
6 Group Membership Defined for MOSPF; not supported by Cisco IOS.
7 NSSA External Created by ASBRs inside an NSSA area, instead of a type 5
LSA.
8 External Attributes Not implemented in Cisco routers.
9–11 Opaque Used as generic LSAs to allow for easy future extension of
OSPF; for example, type 10 has been adapted for MPLS
traffic engineering.
LSA Type 1: Router LSA
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. To flood the LSA, the originating router sends the Type 1 LSA to its neighbors
inside the same area, who in turn send it to their other neighbors inside the same area, until
all routers in the area have a copy of the LSA.
Besides the RID of the router, this LSA also lists information about the attached links. In
particular, the Type 1 LSA lists:
■ For each interface on which no DR has been elected, it lists the router’s interface subnet
number/mask and interface OSPF cost. (OSPF refers to these subnets as stub
networks.)
■ For each interface on which a DR has been elected, it lists the IP address of the DR
and a notation that the link attaches to a transit network (meaning a type 2 LSA exists
for that network).
■ For each interface with no DR, but for which a neighbor is reachable, it lists the neighbor’s
RID.
As with all OSPF LSAs, OSPF identifies a Type 1 LSA using a 32-bit link state identifier
(LSID). When creating its own Type 1 LSA, each router uses its own OSPF RID value as
the LSID.
Internal routers each create a single Type 1 LSA for themselves, but ABRs create multiple
Type 1 LSAs for themselves: one per area. The Type 1 LSA in one area will list only interfaces
in that area and only neighbors in that area. However, the router still has only a single
RID, so all its Type 1 LSAs for a single router list the same RID. The ABR then floods each
of its Type 1 LSAs into the appropriate area.
To provide a better backdrop for the upcoming LSA discussions, Figure 6-1 shows a sample
internetwork, which will be used in most of the examples in this chapter.
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Chapter 6: OSPF Topology, Routes, and Convergence 181
R3
R4
R5
Fa0/0
10.10.34.3/24
Fa0/0
10.10.34.4/24
Fa0/0
5.5/27
S0/0.1
15.5
S0/0.2
25.5
S0/0/0.1
14.4
S0/0/0.2
24.4
S0/0/0.1
13.3
S0/0/0.2
23.2
Fa0/0
17.1
Fa0/1
18.1
Fa0/0.1
12.1
Fa0/0.1
12.2
Fa0/0
27.2
Fa0/1
28.2
Fa0/1
17.7
Fa0/2
27.2
Gi0/1
98.7
Gi0/1
98.8
Data
Center
Subnet
10.10.99.0/24
18.8
Fa0/1
Fa0/2
28.8
SW1
SW2
SW3
R2
R1
Area 34
Area 5 Area 0
Figure 6-1 Sample OSPF Multi-Area Design
All routers that participate in an area, be they internal routers or ABRs, create and flood a
Type 1 LSA inside the area. For example, in Figure 6-1, area 5 has one internal router (R5,
RID 5.5.5.5), and two ABRs: R1 with RID 1.1.1.1 and R2 with RID 2.2.2.2. Each of these
three routers create and flood their own Type 1 LSA inside area 5 so that all three routers
know the same three Type 1 LSAs.
Next, to further understand the details inside a Type 1 LSA, first consider the OSPF configuration
of R5 as an example. R5 has three IP-enabled interfaces: Fa0/0, S0/0/0.1, and
S0/0.2. R5 uses point-to-point subinterfaces, so R5 should form neighbor relationships
with both R1 and R2 with no extra configuration beyond enabling OSPF, in area 5, on all
three interfaces. Example 6-1 shows this baseline configuration on R5.
Example 6-1 R5 Configuration–IP Addresses and OSPF
interface Fastethernet0/0
ip address 10.10.5.5 255.255.255.224
ip ospf 5 area 5
!
interface s0/0.1 point-to-point
ip addr 10.10.15.5 255.255.255.248
frame-relay interface-dlci 101
ip ospf 5 area 5
!
interface s0/0.2 point-to-point
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ip addr 10.10.25.5 255.255.255.248
frame-relay interface-dlci 102
ip ospf 5 area 5
!
router ospf 5
router-id 5.5.5.5
!
R5#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
se0/0.2 5 5 10.10.25.5/29 64 P2P 1/1
se0/0.1 5 5 10.10.15.5/29 64 P2P 1/1
fa0/0 5 5 10.10.5.5/27 1 DR 0/0
R5#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:30 10.10.25.2 Serial0/0.2
1.1.1.1 0 FULL/ - 00:00:38 10.10.15.1 Serial0/0.1
R5’s OSPF configuration enables OSPF, for process ID 5, placing three interfaces in area 5.
As a result, R5’s type 1 LSA will list at least these three interfaces as links, plus it will refer
to the two working neighbors. Example 6-2 displays the contents of R5’s area 5 LSDB, including
the detailed information in R5’s Type 1 LSA, including the following:
■ The LSID of R5’s Type 1 LSA (5.5.5.5)
■ Three links that connect to a stub network, each listing the subnet/mask
■ Two links that state a connection to another router, one listing R1 (RID 1.1.1.1) and
one listing R2 (RID 2.2.2.2)
Example 6-2 R5 Configuration–IP Addresses and OSPF
R5#show ip ospf database
OSPF Router with ID (5.5.5.5) (Process ID 5)
Router Link States (Area 5)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 835 0x80000002 0x006BDA 2
2.2.2.2 2.2.2.2 788 0x80000002 0x0082A6 2
5.5.5.5 5.5.5.5 787 0x80000004 0x0063C3 5
Summary Net Link States (Area 5)
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Link ID ADV Router Age Seq# Checksum
10.10.12.0 1.1.1.1 835 0x80000001 0x00F522
10.10.12.0 2.2.2.2 787 0x80000001 0x00D73C
! lines omitted for brevity
R5#show ip ospf database router 5.5.5.5
OSPF Router with ID (5.5.5.5) (Process ID 5)
Router Link States (Area 5)
LS age: 796
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 5.5.5.5
Advertising Router: 5.5.5.5
LS Seq Number: 80000004
Checksum: 0x63C3
Length: 84
Number of Links: 5
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 10.10.25.5
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.25.0
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 1.1.1.1
(Link Data) Router Interface address: 10.10.15.5
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.15.0
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 64
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Stub
5.5.5.5
Stub
Stub
Subnet
10.10.5.0/27
neighbor 1.1.1.1
10.10.15.5
10.10.15.0/29
neighbor 2.2.2.2
10.10.25.5
1.1.1.1
Stub
neighbor 5.5.5.5
10.10.15.1
Subnet
10.10.15.0/29
2.2.2.2
Stub
neighbor 5.5.5.5
10.10.25.2
Subnet
10.10.25.0/29
Subnet
10.10.25.0/29
R5 R1
R2
Figure 6-2 Three Type 1 LSAs in Area 5
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.5.0
(Link Data) Network Mask: 255.255.255.224
Number of TOS metrics: 0
TOS 0 Metrics: 1
The first command, show ip ospf database, displays a summary of the LSAs known to
R5. The output mainly consists of a single line per LSA, listed by LSA ID. The three highlighted
lines of this command, in Example 6-2, highlight the RID of the three router (Type
1) LSAs, namely 1.1.1.1 (R1), 2.2.2.2 (R2), and 5.5.5.5 (R5).
The output of the show ip ospf database router 5.5.5.5 command displays the detailed
information in R5’s router LSA. Looking at the highlighted portions, you see three stub
networks–three interfaces on which no DR has been elected–and the associated subnet
numbers. The LSA also lists the neighbor IDs of two neighbors (1.1.1.1 and 2.2.2.2) and the
interfaces on which these neighbors can be reached.
Armed with the same kind of information in R1’s and R2’s Type 1 LSAs, a router has
enough information to determine which routers connect, over which stub links, and then
use the interface IP address configuration to figure out the interfaces that connect to the
other routers. Figure 6-2 shows a diagram of area 5 that could be built just based on the
detailed information held in the router LSAs for R1, R2, and R5.
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Chapter 6: OSPF Topology, Routes, and Convergence 185
Note that Figure 6-2 displays only information that could be learned from the Type 1
router LSAs inside area 5. Each Type 1 router LSA lists information about a router but
only the details related to a specific area. As a result, Figure 6-2 shows R1’s interface in
area 5 but none of the interfaces in area 34 nor in area 0. To complete the explanation surrounding
Figure 6-2, Example 6-3 lists R1’s Type 1 router LSA for area 5.
Example 6-3 R1’s Type 1 LSA in Area 5
R5#show ip ospf database router 1.1.1.1
OSPF Router with ID (5.5.5.5) (Process ID 5)
Router Link States (Area 5)
Routing Bit Set on this LSA
LS age: 1306
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x6BDA
Length: 48
Area Border Router
Number of Links: 2
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 5.5.5.5
(Link Data) Router Interface address: 10.10.15.1
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.15.0
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 64
Note: Because the OSPF uses the RID for many purposes inside different LSAs–for
instance, as the LSID of a type 1 LSA–Cisco recommends setting the RID to a stable, predictable
value. To do this, use the OSPF router-id value OSPF subcommand, or define a
loopback interface with an IP address, as discussed in Chapter 5’s section “Using a Unique
OSPF Router ID.”
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LSA Type 2: Network LSA
SPF requires that the LSDB model the topology with nodes (routers) and connections between
nodes (links). In particular, each link must be between a pair of nodes. When a multiaccess
data link exists—for instance, a LAN—OSPF must somehow model that LAN so
that the topology represents nodes and links between only a pair of nodes. To do so,
OSPF uses the concept of a Type 2 Network LSA.
OSPF routers actually choose whether to use a Type 2 LSA for a multiaccess network
based on whether a designated router (DR) has or has not been elected on an interface. So,
before discussing the details of the Type 2 network LSA, a few more facts about the concept
of a DR need to be discussed.
Background on Designated Routers
As discussed in Chapter 5’s section “OSPF Network Types,” the OSPF network type assigned
to a router interface tells that router whether to attempt to elect a DR on that interface.
Then, when a router has heard a Hello from at least one other router, the routers elect
a DR and BDR.
OSPF uses a DR in a particular subnet for two main purposes:
■ To create and flood a Type 2 network LSA for that subnet
■ To aid in the detailed process of database exchange over that subnet
Routers elect a DR, and a backup DR (BDR), based on information in the OSPF Hello. The
Hello message lists each router’s RID and a priority value. When no DR exists at the time,
routers use the following election rules when neither a DR nor BDR yet exists:
■ Choose the router with the highest priority (default 1, max 255, set with ip ospf priority
value interface subcommand).
■ If tied on priority, choose the router with highest RID.
■ Choose a BDR, based on next-best priority, or if a tie, next-best (highest) RID.
Although the preceding describes the election when no DR currently exists, the rules differ
a bit when a DR and BDR already exist. After a DR and BDR are elected, no election is
held until either the DR or BDR fails. If the DR fails, the BDR becomes the DR–—regardless
of whether a higher priority router has joined the subnet—and a new election is held
to choose a new BDR. If the BDR fails, a new election is held for BDR, and the DR remains
unchanged.
On LANs, the choice of DR matters little from a design perspective, but does matter from
an operational perspective. Throughout this chapter, note the cases in which output of
show commands identify the DR and its role. Now, back to the topic of Type 2 LSAs.
Note: On Frame Relay WAN links, the choice of DR may impact whether OSPF functions
at all. This topic is covered in Chapter 8, “OSPF Virtual Links and Frame Relay
Operations.”
a DR will be elected. (OSPF elects a DR on LANs when at least two routers pass the neighbor
requirements and can become neighbors.) If both R3 and R4 default to use priority 1,
then R4 wins the election, due to its 4.4.4.4 RID (versus R3’s 3.3.3.3). So, R4 creates the
Type 2 LSA for that subnet and floods the LSA. Figure 6-5 depicts the area 34 topology,
and Example 6-4 shows the related LSDB entries.
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Chapter 6: OSPF Topology, Routes, and Convergence 189
R5#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 3)
Router Link States (Area 34)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1061 0x80000002 0x00EA7A 4
2.2.2.2 2.2.2.2 1067 0x80000001 0x0061D2 4
3.3.3.3 3.3.3.3 1066 0x80000003 0x00E2E8 5
4.4.4.4 4.4.4.4 1067 0x80000003 0x007D3F 5
Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
10.10.34.4 4.4.4.4 1104 0x80000001 0x00AB28
Summary Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
10.10.5.0 1.1.1.1 1023 0x80000001 0x000BF2
10.10.5.0 2.2.2.2 1022 0x80000001 0x00EC0D
! lines omitted for brevity
R3#show ip ospf database router 4.4.4.4
OSPF Router with ID (3.3.3.3) (Process ID 3)
Router Link States (Area 34)
LS age: 1078
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 4.4.4.4
Advertising Router: 4.4.4.4
LS Seq Number: 80000003
Checksum: 0x7D3F
Length: 84
Example 6-4 Area 34 LSAs for R4, Network 10.10.34.0/24
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Number of Links: 5
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 10.10.24.4
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.24.0
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 1.1.1.1
(Link Data) Router Interface address: 10.10.14.4
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.14.0
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 64
Link connected to: a Transit Network
(Link ID) Designated Router address: 10.10.34.4
(Link Data) Router Interface address: 10.10.34.4
Number of TOS metrics: 0
TOS 0 Metrics: 1
R3#show ip ospf database network 10.10.34.4
OSPF Router with ID (3.3.3.3) (Process ID 3)
Net Link States (Area 34)
Routing Bit Set on this LSA
LS age: 1161
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 10.10.34.4 (address of Designated Router)
Advertising Router: 4.4.4.4
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Chapter 6: OSPF Topology, Routes, and Convergence 191
LS Seq Number: 80000001
Checksum: 0xAB28
Length: 32
Network Mask: /24
Attached Router: 4.4.4.4
Attached Router: 3.3.3.3
The show ip ospf database command lists a single line for each LSA. Note that the (highlighted)
heading for network LSAs lists one entry, with LSID 10.10.34.4, which is R4’s
Fa0/0 IP address. The LSID for Type 2 Network LSAs is the interface IP address of the DR
that creates the LSA.
The show ip ospf database router 4.4.4.4 command shows the new style of entry for the
reference to a Transit Network, which again refers to a connection to a Type 2 LSA. The
output lists a LSID of 10.10.34.4, which again is the LSID of the Type 2 LSA.
Finally, the show ip ospf database network 10.10.34.4 command shows the details of
the Type 2 LSA, based on its LSID of 10.10.34.4. Near the bottom, the output lists the attached
routers, based on RID. The SPF process can then use the cross-referenced information,
as shown in Figure 6-5, to determine which routers connect to this transit
network (pseudonode). The SPF process has information in both the Type 1 LSAs that refer
to the transit network link to a Type 2 LSA, and the Type 2 LSA has a list of RIDs of
Type 1 LSAs that connect to the Type 2 LSA, making the process of modeling the network
possible.
OSPF can model all the topology inside a single area using Type 1 and 2 LSAs. When a
router uses its SPF process to build a model of the topology, it can then calculate the
best (lowest cost) route for each subnet in the area. The next topic completes the LSA
picture for internal OSPF routes by looking at Type 3 LSAs, which are used to model interarea
routes.
LSA Type 3: Summary LSA
OSPF areas exist in part so that engineers can reduce the consumption of memory and
compute resources in routers. Instead of having all routers, regardless of area, know all
Type 1 and Type 2 LSAs inside an OSPF domain, 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.
However, even though ABRs do not flood Type 1 and Type 2 LSAs into other areas,
routers still need to learn about subnets in other areas. OSPF advertises these interarea
routes using the Type 3 summary LSA. ABRs generate a Type 3 LSA for each subnet in
one area, and advertises each Type 3 LSA into the other areas.
For example, if subnet A exists in area 3, then the routers in area 3 learn of that subnet as
part of Type 1 and Type 2 LSAs. However, an ABR connected to area 3 will not forward
The ABR creates and floods each Type 3 LSA into the next area. The ABR assigns an
LSID of the subnet number being advertised. It also adds its own RID to the LSA as well,
so that routers know which ABR advertised the route. It also includes the subnet mask.
The correlation between the advertising router’s RID and the LSID (subnet number) allows
the OSPF processes to create the part of the topology as shown with Type 3 LSAs at the
bottom of Figure 6-6.
Example 6-5 focuses on the Type 3 LSAs in Area 34 of the network shown in Figure 6-1.
Ten subnets exist outside area 34. As ABRs, both R1 and R2 create and flood a Type 3
LSA for each of these 10 subnets, resulting in 20 Type 3 LSAs listed in the output of the
show ip ospf database command inside area 34. Then, the example focuses specifically
on the Type 3 LSA for subnet 10.10.99.0/24.
Example 6-5 Type 3 LSAs in Area 34
R3#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 3)
Router Link States (Area 34)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 943 0x80000003 0x00E87B 4
2.2.2.2 2.2.2.2 991 0x80000002 0x005FD3 4
3.3.3.3 3.3.3.3 966 0x80000004 0x00E0E9 5
4.4.4.4 4.4.4.4 977 0x80000004 0x007B40 5
Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
10.10.34.4 4.4.4.4 977 0x80000002 0x00A929
Summary Net Link States (Area 34)
Link ID ADV Router Age Seq# Checksum
10.10.5.0 1.1.1.1 943 0x80000002 0x0009F3
10.10.5.0 2.2.2.2 991 0x80000002 0x00EA0E
10.10.12.0 1.1.1.1 943 0x80000002 0x00F323
10.10.12.0 2.2.2.2 991 0x80000002 0x00D53D
10.10.15.0 1.1.1.1 943 0x80000002 0x0021BA
10.10.15.0 2.2.2.2 993 0x80000003 0x008313
10.10.17.0 1.1.1.1 946 0x80000002 0x00BC55
10.10.17.0 2.2.2.2 993 0x80000002 0x00A864
10.10.18.0 1.1.1.1 946 0x80000002 0x00B15F
10.10.18.0 2.2.2.2 994 0x80000002 0x009D6E
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10.10.25.0 1.1.1.1 946 0x80000002 0x00355C
10.10.25.0 2.2.2.2 993 0x80000002 0x009439
10.10.27.0 1.1.1.1 946 0x80000002 0x0058AE
10.10.27.0 2.2.2.2 993 0x80000002 0x0030D3
10.10.28.0 1.1.1.1 947 0x80000002 0x004DB8
10.10.28.0 2.2.2.2 993 0x80000002 0x0025DD
10.10.98.0 1.1.1.1 946 0x80000002 0x004877
10.10.98.0 2.2.2.2 993 0x80000002 0x002A91
10.10.99.0 1.1.1.1 946 0x80000002 0x003D81
10.10.99.0 2.2.2.2 993 0x80000002 0x001F9B
R3#show ip ospf database summary 10.10.99.0
OSPF Router with ID (3.3.3.3) (Process ID 3)
Summary Net Link States (Area 34)
Routing Bit Set on this LSA
LS age: 1062
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.10.99.0 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000002
Checksum: 0x3D81
Length: 28
Network Mask: /24
TOS: 0 Metric: 2
Routing Bit Set on this LSA
LS age: 1109
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.10.99.0 (summary Network Number)
Advertising Router: 2.2.2.2
LS Seq Number: 80000002
Checksum: 0x1F9B
Length: 28
Network Mask: /24
TOS: 0 Metric: 2
Note: The Type 3 Summary LSA is not used for the purpose of route summarization.
OSPF does support route summarization, and Type 3 LSAs may indeed advertise such a
summary, but the Type 3 LSA does not inherently represent a summary route. The term
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Chapter 6: OSPF Topology, Routes, and Convergence 195
Key
Topic
Table 6-3 Facts about LSA Types 1, 2, and 3
LSA
Type
(Number)
LSA
Type
(Name)
This Type
Represents
Display Using show ip
ospf database
keyword...
LSID Is
Equal To
Created By
1 Router A router router RID of
router
Each router
creates its
own
summary reflects the idea that the information is sparse compared to the detail inside Type
1 and Type 2 LSAs.
The upcoming section “Calculating the Cost of Inter-area Routes” discusses how a router
determines the available routes to reach subnets listed in a Type 3 LSA and how a router
chooses which route is best.
Limiting the Number of LSAs
By default, Cisco IOS does not limit the number of LSAs a router can learn. However, it
may be useful to protect a router from learning too many LSAs to protect router memory.
Also, with a large number of LSAs, the router may be unable to process the LSDB with
SPF well enough to converge in a reasonable amount of time.
The maximum number of LSAs learned from other routers can be limited by a router using
the max-lsa number OSPF subcommand. When configured, if the router learns more
than the configured number of LSAs from other routers (ignoring those created by the
router itself), the router reacts. The first reaction is to issue log messages. The router ignores
the event for a time period, after which the router repeats the warning message. This
ignore-and-wait strategy can proceed through several iterations, ending when the router
closes all neighborships, discards its LSDB, and then starts adding neighbors again. (The
ignore time, and the number of times to ignore the event, can be configured with the
max-lsa command.)
Summary of Internal LSA Types
OSPF uses Type 1, 2, and 3 LSAs to calculate the best routes for all routes inside the
OSPF routing domain. Later, Chapter 9 explains Types 4, 5, and 7, which OSPF uses to
calculate routes for external routes–routes redistributed into OSPF.
Table 6-3 summarizes some of the key points regarding OSPF Type 1, 2, and 3 LSAs. In
particular for the ROUTE exam, the ability to sift through the output of various show ip
ospf database commands can be important. Knowing what the OSPF LSID represents can
help you interpret the output, and knowing the keywords used with the show ip ospf
database lsa-type lsid commands can also be very useful. Table 6-3 summarizes these details.
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196 CCNP ROUTE 642-902 Official Certification Guide
Table 6-3 Facts about LSA Types 1, 2, and 3
LSA
Type
(Number)
LSA
Type
(Name)
This Type
Represents
Display Using show ip
ospf database
keyword...
LSID Is
Equal To
Created By
2 Network A subnet in
which a DR
exists
network DR’s IP address
in the
subnet
The DR in
that subnet
3 Summary Subnet in another
area
summary Subnet
number
An ABR