Examine Link-State Advertisements in Detail

You can obtain important information about the routers in your network by examining LSAs in detail.

To examine OSPF LSAs, follow these steps:

1. Examine a Type 1 Router LSA
2. Examine a Type 3 Summary LSA
3. Examine a Type 4 ASBR Summary LSA
4. Examine a Type 5 AS External LSA
5. Examine Type 7 NSSA External LSA

Examine a Type 1 Router LSA

Purpose

To examine a Type 1 router LSA, enter the following CLI operational mode command:

Action

Sample Output

    OSPF link state database,  area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *10.0.0.1          10.0.0.1         0x8000005a  1180  0x0  0x5828  60
   bits 0x2 ,  link count 3
  id 10.0.0.1, data 255.255.255.255, Type Stub (3)
  TOS count 0, TOS 0 metric 0
  id 10.0.0.2, data 10.1.12.1, Type PointToPoint (1)
  TOS count 0, TOS 0 metric 1
  id 10.1.12.0, data 255.255.255.252, Type Stub (3)
  TOS count 0, TOS 0 metric 1
  Gen timer 00:30:19
  Aging timer 00:40:19
  Installed 00:19:40 ago, expires in 00:40:20, sent 00:19:38 ago
   Ours
Router   10.0.0.2         10.0.0.2         0x8000004b   679  0x0  0xe6c0  48
   bits 0x3, link count 2
  id 10.0.0.1, data 10.1.12.2, Type PointToPoint (1)
  TOS count 0, TOS 0 metric 1
  id 10.1.12.0, data 255.255.255.252, Type Stub (3)
  TOS count 0, TOS 0 metric 1
  Aging timer 00:48:40
  Installed 00:11:16 ago, expires in 00:48:41, sent 3w0d 23:33:12 ago

Meaning

The sample output shows the details of two router LSAs: the first for R1 (*10.0.0.1) and the second for R2 (10.0.0.2). The asterisk (*) indicates that the LSA was generated by R1. You can also determine ownership of the LSA by the last line of the output in this case, ours.

Each time the LSA is updated, the sequence (seq) field increments, indicating that the router has the most recent version of the LSA. Values range from 0x80000001 to 0x7FFFFFFF. If the sequence field is not incrementing, there may be problems with the connection.

The bits field is set to 0x2 in the first LSA and 0x3 in the second LSA. When the bits field is set to 0x2,the originating router (R1) is an ASBR. When the bits field is set to 0x3, the originating router (R2) is both ABR and ASBR.

R1 has three links connected to area 0.0.0.1 shown by the link count field that is set to a value of 3. The Type field shows that R1 has a single point-to-point link to R2 and two links advertised as stub networks.

Each OSPF router generates a single Type 1 LSA to describe the status and cost (metric) of all links on the router. This LSA is flooded to each router in the OSPF area. It is defined as having an area scope, so it is not flooded across an area boundary.

Examine a Type 3 Summary LSA

Purpose

To examine a Type 3 summary LSA, enter the following CLI operational mode command:

Action

Sample Output

     OSPF link state database, area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Summary *10.0.0.1           10.0.0.2          0x80000043    529  0x2  0xfd05  28
  mask 255.255.255.255
  TOS 0x0,  metric 1
  Gen timer 00:34:13
  Aging timer 00:51:10
  Installed 00:08:49 ago, expires in 00:51:11, sent 00:08:47 ago
   Ours , 
[...Output truncated...)
OSPF link state database, area 0.0.0.1
[...Output truncated...]
Summary *10.0.0.5         10.0.0.2         0x80000047  2198  0x0  0xf506  28
  mask 255.255.255.255
  TOS 0x0, metric 2
  Gen timer 00:07:19
  Aging timer 00:23:22
  Installed 00:36:38 ago, expires in 00:23:22, sent 00:36:36 ago
  Ours, 

Meaning

The sample output shows that R2 is an ABR because it contains two databases: one for the backbone area 0.0.0.0 and one for area 0.0.0.1. Within the backbone area, the summary LSA *10.0.0.1 is generated from R2 as indicated by the asterisk (*) next to the link-state ID field, and ours in the last line of the LSA. The cost to transmit data out of the interface is 1, as indicated by the metric field.

Within area 0.0.0.1, the summary LSA *10.0.0.5 is generated by R2 and has a metric of 2, which is the cost to R5 from R2. Before calculating the SPF algorithm, the local router (R2) must add an additional metric of 1 to the existing metric of 1. The additional metric of 1 must be added because there is another router between R2 and R5 (R4).

Each time the LSA is updated, the sequence (seq) field increments, indicating that the router has the most recent version of the LSA. Values range from 0x80000001 to 0x7FFFFFFF. If the sequence field is not incrementing, there may be problems with the connection.

Examine a Type 4 ASBR Summary LSA

Purpose

To examine a Type 4 ASBR summary LSA, enter the following CLI operational mode command:

Action

Sample Output

 user@R3> show ospf database asbrsummary extensive
   OSPF link state database, area 0.0.0.0
[...Output truncated...]
ASBRSum *10.0.0.6         10.0.0.3         0x80000042  1023  0x2  0xb943  28
  mask 0.0.0.0
  TOS 0x0, metric 1
  Gen timer 00:27:57
  Aging timer 00:42:57
  Installed 00:17:03 ago, expires in 00:42:57, sent 00:17:01 ago
  Ours, 
[...Output truncated...]

Meaning

The sample output shows that an LSA within the backbone area, *10.0.0.6, is generated by ASBR R3, as indicated by the asterisk (*) next to the link-state ID field and ours in the last line of the LSA.

Each time the LSA is updated, the sequence (seq) field increments, indicating that the router has the most recent version of the LSA. Values range from 0x80000001 to 0x7FFFFFFF. If the sequence field is not incrementing, there may be problems with the connection.

Because the router ID of all the ASBR summary LSAs is a full 32-bit value, the network mask is not needed and is set to a value of 0.0.0.0. The metric for the LSA within the backbone area is set to 1, which is the cost to the advertising router (R3) from the originating router (R6). The metric is calculated before the SPF algorithm is calculated.

In general, each ABR that must transmit information about an ASBR from one OSPF area into another generates a Type 4 LSA. This LSA is flooded to each router in the OSPF area. A Type 4 LSA is defined as having an area scope so that another ABR does not reflood it across the area boundary.

Examine a Type 5 AS External LSA

Purpose

To examine a Type 5 AS external LSA, enter the following CLI operational mode command:

Action

Sample Output

    OSPF AS SCOPE link state database
 Type       ID               Adv Rtr            Seq       Age  Opt  Cksum  Len 
Extern  *10.0.0.100       10.0.0.2          0x80000047   1377  0x2  0xfd09  36
  mask 255.255.255.255
   Type 2, TOS 0x0,  metric 0,  fwd addr 10.0.0.1, tag 0.0.0.0
  Gen timer 00:21:02
  Aging timer 00:37:02
  Installed 00:22:57 ago, expires in 00:37:03, sent 00:22:55 ago
   Ours, 
[...Output truncated...]

Meaning

The sample output shows one Type 5 external LSA, *10.0.0.100. The status of the router represented by this LSA is indicated by the fwd addr field, which shows that it does not belong to any particular OSPF area. The forwarding address provides the address toward which packets should be sent to reach the external router (10.0.0.1). R1 is the ASBR with the connection to external router A.

The mask field represents the subnet mask associated with the advertised router. It is used with the link-state ID field (10.0.0.100), which encapsulates the network address in a Type 5 LSA. This LSA has a metric value of 0, the default value, indicating that this is a Type 2 external metric. Thus, any local router should use the default metric (0) when performing an SPF algorithm.

Each time the LSA is updated, the sequence (seq) field increments, indicating that the router has the most recent version of the LSA. Values range from 0x80000001 to 0x7FFFFFFF. If the sequence field is not incrementing, there may be problems with the connection.

In general, each ASBR generates a Type 5 LSA to advertise any routers external to the OSPF AS. This LSA is flooded to each non-stub router in the entire AS.

Examine Type 7 NSSA External LSA

Purpose

To examine a Type 7 NSSA external LSA, enter the following CLI operational mode command:

Action

Sample Output

    OSPF link state database,  area 0.0.0.1
 Type       ID               Adv Rtr            Seq      Age  Opt  Cksum  Len 
[...Output truncated...]
NSSA     *10.0.0.100        10.0.0.1           0x8000003b    843  0x8  0xa566  36
  mask 255.255.255.255
   Type 2, TOS 0x0,  metric 0, fwd addr 10.0.0.1, tag 0.0.0.0
  Gen timer 00:35:56
  Aging timer 00:45:56
  Installed 00:14:03 ago, expires in 00:45:57, sent 00:14:01 ago
   Ours

Meaning

The sample output shows that the LSA belongs to a single NSSA, 0.0.0.1, and was generated by R1. This router has a metric value of 0, which is the default, and is listed as a Type 2 external metric. Any local router must use the default metric as the total cost for the route when performing an SPF calculation. The default metric of the route must be added to the cost to reach the advertising ASBR. This value then represents the total cost for the route.

In general, each ASBR within the NSSA generates a Type 7 LSA to advertise any routers external to the OSPF AS. This LSA is flooded to each router within the NSSA (R2). Because the LSA has only an area flooding scope, it is not sent into other adjacent areas. For each Type 7 LSA received, the ABR (R2) translates the information into a Type 5 LSA and sends the information into the backbone. The other backbone routers do not know that the original information came from an NSSA. The Type 5 LSA is then flooded to each non-stub router in the entire AS.