How does a Level 1 IS-IS router reach destinations outside its area?
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Through a default route it builds itself. An L1/L2 router that is connected to the L2 backbone sets the attached (ATT) bit in its Level 1 LSP. L1 routers see that bit and install a default route toward the nearest L1/L2 router with ATT set. No L2 prefixes enter the area unless you configure route leaking, so by default an L1 area behaves like an OSPF totally stubby area.
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What you see on the L1 router
R1# show ip route isis | include 0.0.0.0
i*L1 0.0.0.0/0 [115/10] via 10.1.12.2, 00:06:12, GigabitEthernet0/0/0And in the database, the ATT column for R2's LSP is 1:
R1# show isis database
Tag CORE:
IS-IS Level-1 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL
R1.00-00 * 0x00000004 0x5D1E 1061/* 0/0/0
R2.00-00 0x00000005 0x8A42 1102/1199 1/0/0The catch: nearest exit
With two L1/L2 routers in the area, every L1 router uses whichever is closest by IS-IS metric, even if the other one is much nearer to the actual destination. Traffic can cross the backbone the long way. Route leaking fixes this by giving L1 routers specific prefixes to compare.
Losing the exit
If an L1/L2 router loses all its L2 adjacencies, it clears ATT, and L1 routers stop using it as an exit. That is correct behaviour, and it is why a single L1/L2 router is a single point of failure for the area.
They will ask next
Does the L1 router need any configuration to install that default?
No. Installing a default toward the nearest router with ATT set is standard L1 behaviour. Some platforms let you ignore the ATT bit, for example when the area has its own default from elsewhere.
Traps
- Saying the L1/L2 router sends a 0.0.0.0/0 prefix into L1. By default it only sets the ATT bit; the L1 routers create the default themselves.