One of the most useful things we can use to troubleshoot or
just to understand the topology of routing protocols is to have the ability to
look into their databases or tables and to understand what the routing protocol
see and learn and which information he is holding.
OSPF protocol exchange information with his adjacent neighbors
using LSA, which stands for Link-State Advertisement. While exchanging LSA’s OSPF
is building an LSDB – Link-State Database, which holds all the relevant information
which allow OSPF to take routing decisions based on this information.
Each router, in the OSPF AS, is holding an LSDB which relevant
and organized from his point of view and the SFP algorithm is applied on the
LSDB to choose the shortest path available according to this database.
So following the network topology, in my last post, let’s
run some show commands:
R3#show ip ospf database
OSPF Router with ID (3.3.3.3)
(Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 R2 1197 0x80000035 0x00C1C0 2
3.3.3.3 R3 867 0x80000035 0x0030CC 3
4.4.4.4 R4 530 0x8000003A 0x00CC15 3
5.5.5.5 R5 2 (DNA) 0x80000003 0x00C8E4 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.23.3 R3 867 0x80000032 0x0003C0
10.1.34.3 R3 1364 0x8000002E 0x00F5BE
Summary Net Link States (Area
0)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 R2 1197 0x80000032 0x002BC9
5.5.5.5 R4 530 0x8000002A 0x00469E
5.5.5.5 R5 22 (DNA) 0x80000001 0x0016FD
6.6.6.6 R5 22 (DNA) 0x80000001 0x004CB9
10.1.12.0 R2 1198 0x80000032 0x003CA6
10.1.45.0 R4 1775 0x8000002F 0x009923
10.1.45.0 R5 22 (DNA) 0x80000001 0x00D70F
10.1.56.0 R5 22 (DNA) 0x80000001 0x005E7D
192.168.0.0 R2 1198 0x8000002A 0x00D5F7
Summary ASB Link States (Area
0)
Link ID ADV Router Age Seq# Checksum
5.5.5.5 R4 532 0x8000002A 0x002EB6
6.6.6.6 R5 22 (DNA) 0x80000001 0x0034D1
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.0.0 R6
1436 0x80000029 0x00CFEB 999
172.16.4.0 R6 1436 0x80000032 0x001F77 0
172.25.0.0 R5 396 0x80000029 0x00EBB9 0
Displays a quick summary of the LSDB for OSPF router with ID
3.3.3.3 (which is R3 in this case).
The first section, Router
Link States (Area 0), refer to LSA type 1 – self-originated links which represent
all interfaces in this area, we can also notice the virtual-link from R5 with a
DNA (Do Not Age) notification.
The second section, Net
Link States (Area 0, LSA type 2 refer
to interfaces advertised by the DR in this segment.
The third section, Summary
Net Link States (Area 0), LSA type 3 - refer to subnets and interfaces,
from other areas, advertised by the ABR into that area.
Forth section, Summary
ASB Link States (Area 0), LSA type 4 - point a route to reach an ASBR
The fifth section, Type-5
AS External Link States, refer to LSA type 5, created by an ASBR, for
external routes injected into the OSPF domain.
Running the same command, sh
ip ospf database, on R6 will result in the following output:
R6#sh ip ospf database
OSPF Router with ID (6.6.6.6)
(Process ID 1)
Router Link States (Area 6)
Link ID ADV Router Age Seq# Checksum Link count
5.5.5.5 R5 6 0x8000003E 0x0065C4 1
6.6.6.6 R6 5 0x80000038 0x00AB53 2
Net Link States (Area 6)
Link ID ADV Router Age Seq# Checksum
10.1.56.6 R6 0 0x80000034 0x00BCB6
Summary Net Link States (Area
6)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 R5 12 0x8000002B 0x00B11A
2.2.2.2 R5 12 0x8000002B 0x001FB2
3.3.3.3 R5 12 0x8000002B 0x008C4B
4.4.4.4 R5 12 0x8000002B 0x00F9E3
5.5.5.5 R5 12 0x8000002B 0x00677C
10.1.12.0 R5 12 0x8000002B 0x00C2F6
10.1.23.0 R5 12 0x8000002B 0x00E4D3
10.1.34.0 R5 14 0x8000002B 0x0007B0
10.1.45.0 R5 14 0x8000002B 0x00298D
192.168.0.0 R5 14 0x8000002C 0x004A51
Type-7 AS External Link States (Area 6)
Link ID ADV Router Age Seq# Checksum Tag
172.16.0.0 R6
337 0x80000001 0x00B809 999
172.16.4.0 R6 337 0x80000001 0x001A8B 0
172.25.0.0 R5 13 0x80000001 0x0020AB 0
Note that no LSA type 4 or 5 are exist due to the fact that
area 6 has been configured, on R5 and R6, as NSSA area which allows only LSA
type 1, 2,3 and 7.
LSA type 7 are much like LSA type 5 (injected external
routes) with a difference that they are injected into an NSSA area, which doesn’t
allow LSA type 5 so they are converted to type 7.
R5#show
ip ospf database adv-router 4.4.4.4 allows us to see all LSA’s
advertised by router R4 (router-id 4.4.4.4):
5#show ip ospf database adv-router 4.4.4.4
OSPF Router
with ID (5.5.5.5) (Process ID 1)
Router
Link States (Area 0)
Link ID ADV
Router Age Seq# Checksum Link count
4.4.4.4 R4 1 (DNA) 0x80000011 0x001FEB 3
Summary
Net Link States (Area 0)
Link ID ADV
Router Age Seq# Checksum
5.5.5.5 R4 216 (DNA) 0x80000001 0x009875
10.1.45.0 R4 1052 (DNA) 0x80000006 0x00EBF9
Summary
ASB Link States (Area 0)
Link ID ADV
Router Age Seq# Checksum
5.5.5.5 R4 216 (DNA) 0x80000001 0x00808D
Router
Link States (Area 5)
Link ID ADV Router Age Seq# Checksum Link count
4.4.4.4 R4 719 0x8000003C 0x000150 1
Net Link
States (Area 5)
Link ID ADV
Router Age Seq# Checksum
10.1.45.4 R4 719 0x8000002B 0x00AEF0
<OUTPUT OMMITTED>
Show ip ospf database network allows us to see all LSA type
2 (advertised by DR’s), for all areas connected to this router, with much more
details such the DR identity, the attached routers to this link, the subnet
mask and the advertising router.
R5#show ip
ospf database network
OSPF Router
with ID (5.5.5.5) (Process ID 1)
Net Link
States (Area 0)
Routing Bit Set on this
LSA
LS age: 919 (DoNotAge)
Options: (No
TOS-capability, DC)
LS Type: Network Links
Link State ID: 10.1.23.3 (address of Designated Router)
Advertising Router: R3
LS Seq Number: 80000009
Checksum: 0x5597
Length: 32
Network Mask: /24
Attached Router: 3.3.3.3
Attached Router: 2.2.2.2
Routing Bit Set on this
LSA
LS age: 1175 (DoNotAge)
Options: (No
TOS-capability, DC)
LS Type: Network Links
Link State ID:
10.1.34.3 (address of Designated Router)
Advertising Router: R3
LS Seq Number: 80000005
Checksum: 0x4895
Length: 32
Network Mask: /24
Attached Router:
3.3.3.3
Attached Router:
4.4.4.4
Net Link
States (Area 5)
Routing Bit Set on this
LSA
LS age: 858
Options: (No
TOS-capability, DC)
LS Type: Network Links
Link State ID:
10.1.45.4 (address of Designated Router)
Advertising Router: R4
LS Seq Number: 8000002B
Checksum: 0xAEF0
Length: 32
Network Mask: /24
Attached Router:
4.4.4.4
Attached Router:
5.5.5.5
Net Link
States (Area 6)
Routing Bit Set on this
LSA
LS age: 479
Options: (No
TOS-capability, DC)
LS Type: Network Links
Link State ID:
10.1.56.6 (address of Designated Router)
Advertising Router: R6
LS Seq Number: 80000034
Checksum: 0xBCB6
Length: 32
Network Mask: /24
Attached Router:
6.6.6.6
Attached Router:
5.5.5.5
Show ip ospf database external will displays all external routes
with their relevant information. Note summary prefix 172.16.0.0/22, which has
been tagged by R6 in the previous post, is also appear here.
R4#sh ip
ospf database external
OSPF Router
with ID (4.4.4.4) (Process ID 1)
Type-5 AS
External Link States
Routing Bit Set on this
LSA
LS age: 930
Options: (No
TOS-capability, DC)
LS Type: AS External
Link
Link State ID:
172.16.0.0 (External Network Number )
Advertising Router: R5
LS Seq Number: 80000001
Checksum: 0x6B64
Length: 36
Network Mask: /22
Metric Type: 2
(Larger than any link state path)
TOS: 0
Metric: 20
Forward Address:
6.6.6.6
External Route Tag: 999
Routing Bit Set on this
LSA
LS age: 930
Options: (No
TOS-capability, DC)
LS Type: AS External
Link
Link State ID:
172.16.4.0 (External Network Number )
Advertising Router: R5
LS Seq Number: 80000001
Checksum: 0xCCE6
Length: 36
Network Mask: /24
Metric Type: 2
(Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 6.6.6.6
External Route
Tag: 0
Routing Bit Set on this
LSA
LS age: 955
Options: (No
TOS-capability, DC)
LS Type: AS External
Link
Link State ID:
172.25.0.0 (External Network Number )
Advertising Router: R5
LS Seq Number: 8000002A
Checksum: 0xE9BA
Length: 36
Network Mask: /21
Metric Type: 2
(Larger than any link state path)
TOS: 0
Metric: 20
Forward Address:
0.0.0.0
External Route
Tag: 0
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