How do you read an IS-IS NET such as 49.0001.0000.0000.0001.00?
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The NET is an NSAP address with the last byte, the NSEL, set to 00, meaning the router itself. Read it from the right: 00 is the NSEL, the six bytes before it (0000.0000.0001) are the system ID, unique in the routing domain, and everything before that (49.0001) is the area address. 49 is the AFI for private use, the IS-IS version of RFC 1918. A router has one system ID and can have up to three area addresses by default, which is how you merge or split areas without downtime.
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The pieces
| Part | In the example | Length |
|---|---|---|
| Area address | 49.0001 (AFI 49 plus area 0001) | 1 to 13 bytes |
| System ID | 0000.0000.0001 | Always 6 bytes on Cisco |
| NSEL | 00 | 1 byte, 00 for a NET |
The whole NSAP is 8 to 20 bytes. Routers in the same area must share an area address to form an L1 adjacency.
Turning a loopback into a system ID
A common convention encodes the loopback. For 10.255.0.1:
- Pad each octet to three digits: 010.255.000.001
- Regroup the 12 digits into threes of four: 0102.5500.0001
- NET:
49.0001.0102.5500.0001.00
Configuring it
router isis CORE
net 49.0001.0102.5500.0001.00Junos puts the NET on the loopback instead:
set interfaces lo0 unit 0 family iso address 49.0001.0102.5500.0001.00They will ask next
What happens if two routers share a system ID?
Both generate LSPs with the same LSP ID. Each sees the other's LSP as a copy of its own with different content and floods a newer one to replace it, so the database churns and routing becomes unreliable. System IDs must be unique across the whole routing domain.
Why would a router have two NETs?
To migrate an area. Give routers both the old and new area address, move them over, then remove the old one. While both are present, the area is treated as one.
Traps
- Ending a NET with anything other than 00. A non-zero NSEL addresses a service, not the router.