Examine a Link-State PDU Header

Purpose

By using the key word extensive, you can examine each header field of a link-state PDU and gather important details about the network. In this step, you examine the IS-IS database for Router R2.

Action

To examine an link-state PDU header, enter the following CLI command:

Sample Output

IS-IS  level 1 link-state database:
[...Output truncated...]
   Header: LSP ID: R2.00-00 , Length: 139 bytes
    Allocated length: 1492 bytes, Router ID: 10.0.0.2
    Remaining lifetime: 1071 secs,  Level: 1 ,Interface: 0
    Estimated free bytes: 1353, Actual free bytes: 1353
    Aging timer expires in: 1071 secs
    Protocols: IP, IPv6
  Packet: LSP ID: R2.00-00, Length: 139 bytes, Lifetime : 1198 secs
    Checksum: 0xbb8d, Sequence: 0x57,  Attributes: 0xb <L1 L2 Attached>
    NLPID: 0x83, Fixed length: 27 bytes, Version: 1, Sysid length: 0 bytes
     Packet type: 18 , Packet version: 1, Max area: 0
[...Output truncated...]
   Header: LSP ID: R3.00-00 , Length: 139 bytes
    Allocated length: 284 bytes, Router ID: 10.0.0.3
    Remaining lifetime: 823 secs,  Level: 1 ,Interface: 68
    Estimated free bytes: 145, Actual free bytes: 145
    Aging timer expires in: 823 secs
    Protocols: IP, IPv6
  Packet: LSP ID: R3.00-00, Length: 139 bytes, Lifetime : 1198 secs
    Checksum: 0xaaa2, Sequence: 0x59,  Attributes: 0x1 <L1>
    NLPID: 0x83, Fixed length: 27 bytes, Version: 1, Sysid length: 0 bytes
    Packet type: 18, Packet version: 1, Max area: 0
[...Output truncated...]
IS-IS  level 2 link-state database:
[...Output truncated...]
Header: LSP ID: R1.00-00,  Length: 194 bytes
    Allocated length: 284 bytes, Router ID: 10.0.0.1
    Remaining lifetime: 398 secs,  Level: 2 ,Interface: 67
    Estimated free bytes: 145, Actual free bytes: 90
    Aging timer expires in: 398 secs
    Protocols: IP, IPv6
  Packet: LSP ID: R1.00-00, Length: 194 bytes,  Lifetime : 1196 secs
    Checksum: 0x3e10, Sequence: 0x57,  Attributes: 0x3 <L1 L2>
    NLPID: 0x83, Fixed length: 27 bytes, Version: 1, Sysid length: 0 bytes
     Packet type: 20 , Packet version: 1, Max area: 0
[...Output truncated...]
Header: LSP ID: R2.00-00 , Length: 236 bytes
    Allocated length: 1492 bytes, Router ID: 10.0.0.2
    Remaining lifetime: 677 secs,  Level: 2 ,Interface: 0
    Estimated free bytes: 1256, Actual free bytes: 1256
    Aging timer expires in: 677 secs
     Protocols: IP, IPv6
  Packet: LSP ID: R2.00-00, Length: 236 bytes, Lifetime : 1198 secs
    Checksum: 0x9790, Sequence: 0x5e, Attributes: 0x3  <L1 L2>
    NLPID: 0x83, Fixed length: 27 bytes, Version: 1, Sysid length: 0 bytes
    Packet type: 20, Packet version: 1, Max area: 0
[...Output truncated...]

Meaning

The sample output is from Router R2 and shows extensive information about the Level 1 and Level 2 link-state databases, which are identical across all Level 2 routers. Level 1 routers only include information for the Level 1 link-state database, which is identical to the Level 1 information in the output for a Level 2 router. The sample output shows the details of four link-state PDUs. Level 1 and Level 2 link-state PDUs include identical types of information except for the packet type. A Level 2 link-state PDU has a packet type of 20, and a Level 1 link-state PDU has a packet type of 18.

The first three examples illustrate different values in the Attributes field:

• <L1 L2> and the key word Attached in the first example, R2.00-00, indicate that Router R2 is a Level1/Level 2 router acting as a gateway for Level 1 routers.
• <L1> in the second example, R3.00-00, indicates that R3 is a Level 1 router.
• <L1 L2> in the third example, R1.00-00, indicates that R1 is a Level 2 router (the fourth example is also for a Level 2 router).

The fourth example, R2.00-00, is included to show that a Level 1/Level 2 router appears in both the Level 1 and the Level 2 link-state databases. Note that in the Level 2 link-state database, the key word Attached is not included in the Attributes field.

It is useful to examine one link-state PDU in greater detail. The third link-state PDU, R1.00-00, was originated by R1 as indicated by the LSP ID field. R1 is the hostname of the router. The first 00 indicates that the link-state PDU is for the router itself, and the final 00 denotes that the link-state PDU is not fragmented. Both IPv4 and IPv6 are supported by this router, as indicated in the Protocols field. The Attributes field shows that the router is capable of both Level 1 and Level 2 routing, is not connected to Level 1 routers in other areas, and is not overloaded. The key words Attached and Overloaded would appear in the Attributes field if this were the case. The remaining lifetime of the link-state PDU is 1196 seconds. IS-IS lifetimes start at a configured age (1200 seconds by default) and count down to zero.