Why there are two entries for the Beijing device in the IS-IS database?
Question
-- Exhibit --
user@router> show isis database
IS-IS level 2 link-state database:
LSP ID Sequence Checksum Lifetime Attributes
Tokyo.00-00 0x6 0xb978 1154 L1 L2 host.00-00 0x4 0x140c 1154 L1 L2
Wash-DC.00-00 0x1d7 0x7f37 683 L1 L2
Beijing.00-00 0x1d3 0xe603 1024 L1 L2
Beijing.02-00 0x9 0xea81 1175 L1 L2
Amsterdam.00-00 0x1c8 0x8b1c 677 L1 L2
Cairo.00-00 0x1ca 0x1571 1103 L1 L2
Sydney.00-0 0x1dd 0x178a 779 L1 L2
8 LSPs
The exhibit displays the output of the command "show isis database" on a Junos device. Why are there two entries for the Beijing device?
Answers
Explanations
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A. B. C. D.C
All IS-IS routers enabled for a given level generate an LSP that is present in the LSDB. The pseudo portion is always 00. If a DIS exists, that device will advertise an additional LSP representing 'the network'. with a value other than 00, such as 01 or 02.
The output of the "show isis database" command displays the Intermediate System-to-Intermediate System (IS-IS) link-state database. In this case, the output shows eight Link State PDUs (LSPs) from different devices, including two LSPs from the Beijing device.
The reason there are two entries for the Beijing device is that the device has two IS-IS instances running on it, one for Level 1 (L1) and one for Level 2 (L2). The two LSPs from the Beijing device have different LSP IDs, indicating that they are from different IS-IS instances.
IS-IS is a protocol used for routing IP packets within a single autonomous system (AS) or domain. It is designed to support multiple levels of hierarchy for large networks. In IS-IS, a router can participate in one or more IS-IS instances, each of which can operate at different levels of hierarchy. Each instance has its own separate link-state database, and routers participating in different instances can exchange routing information selectively.
In this case, the Beijing device is running two IS-IS instances, one for L1 and one for L2. The LSPs from the Beijing device represent the information that the device is advertising to its neighbors in both IS-IS instances. The LSP ID is used to uniquely identify an LSP within an IS-IS instance, and in this case, the two LSPs have different IDs, indicating that they belong to different instances.
Therefore, option B, C, and D are all incorrect. The correct answer is A, which is irrelevant to the given information and can be disregarded. The presence of two LSPs from the same device in the IS-IS database is not a problem, but a normal behavior when a router is running multiple IS-IS instances.
