210-060 · Question #146
On the PSTN gateway, a network engineer runs sh isdn status and finds the Layer 2 state of each interface to be "TEI_ASSIGNED". What is the status of PSTN connectivity from this gateway?
The correct answer is C. The T1 interfaces are down. A Layer 2 ISDN state of TEI_ASSIGNED means a Terminal Endpoint Identifier was assigned but the LAPD Layer 2 link was never established, confirming the T1 interfaces are down with no active PSTN connectivity.
Question
On the PSTN gateway, a network engineer runs sh isdn status and finds the Layer 2 state of each interface to be "TEI_ASSIGNED". What is the status of PSTN connectivity from this gateway?
Options
- AThe T1 interfaces are established and operational.
- BThis information does not indicate the status of PSTN connectivity from the gateway.
- CThe T1 interfaces are down.
- DThe T1 interfaces are being tested by the carrier.
How the community answered
(48 responses)- A29% (14)
- B19% (9)
- C44% (21)
- D8% (4)
Why each option
A Layer 2 ISDN state of TEI_ASSIGNED means a Terminal Endpoint Identifier was assigned but the LAPD Layer 2 link was never established, confirming the T1 interfaces are down with no active PSTN connectivity.
An established and operational T1 would show a Layer 2 state of MULTIPLE_FRAME_ESTABLISHED, not TEI_ASSIGNED.
The 'sh isdn status' output directly reflects ISDN Layer 2 state, and TEI_ASSIGNED definitively indicates Layer 2 is not established, so PSTN connectivity is absent - not ambiguous.
TEI_ASSIGNED is an intermediate ISDN Layer 2 state indicating the interface has obtained a TEI but has not completed the SABME/UA exchange required to reach MULTIPLE_FRAME_ESTABLISHED, meaning the T1 link is not operational and no calls can be placed to the PSTN.
There is no carrier-testing state in the ISDN Layer 2 state machine that corresponds to TEI_ASSIGNED; the state simply reflects that Layer 2 negotiation has not progressed past TEI assignment.
Concept tested: ISDN PRI Layer 2 state interpretation with sh isdn status
Source: https://www.cisco.com/c/en/us/support/docs/voice/isdn/14506-show-isdn-status.html
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