BGP Neighbor Idle on Cisco IOS: show ip bgp summary When TCP 179 Never Starts

Cisco routing · Techclick CLI troubleshooting lab

New eBGP neighbor stays Idle. The WAN link is up. Junior already opened TCP 179 on the firewall — twice.

This is a live-ticket style lesson from Techclick Infosec. You will isolate the fault with CLI first, then prove the fix with one clean packet or log.

What you will be able to do

• Name the first CLI command to run on Cisco routing for this ticket

• Separate the layer that is actually broken from the layer people blame

• Apply one scoped fix instead of a global disable

• Prove the ticket closed with a session, log, or counter

Quick answer

Idle is no TCP. Check update-source, neighbor IP reachability, and whether you are sourcing from the wrong loopback.

Why this ticket shows up

BGP will not even send SYN if the neighbor is not reachable from the configured update-source.

Symptoms operators actually see

• Idle forever

• No session in show tcp brief

• update-source loopback not routed

• Wrong VRF

CLI first — copy this block

Run these on the Cisco routing device or console named in the ticket. Do not skip the first show command — it tells you which later command matters.

show ip bgp summary
show ip bgp neighbors 192.0.2.1
show tcp brief | include 179
ping 192.0.2.1 source loopback0
show ip route 192.0.2.1

How to read that output

No TCP 179 row means L3/source. Active/Idle flipping means SYN is going out and not coming back.

Triage order

1. Ping sourced from the update-source

2. Fix routing to the neighbor

3. Confirm TTL-security / ebgp-multihop if needed

4. Then debug ip bgp

5. Only then look at prefixes

Classic traps

What you see	Real cause	Fix
debug ip bgp updates	There is no session	Fix TCP
Change the remote-as	You cannot ping sourced from Lo0	Fix reachability
Disable the firewall	Packet never left this router	Check VRF/source

Proof the ticket is closed

• State Established

• show tcp brief has 179

• Prefix count as expected

Say this in an interview

I do not read BGP tables in Idle. Idle is reachability and TCP 179.

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