Nothing tests a network engineer like a "Red Circle" next to an IPSec tunnel. Before you panic, follow this systematic approach to identify the root cause in minutes.
1. GUI & CLI checks.
GUI: Network > IPsec Tunnels. Green is good, red means it’s time to work.
CLI: # show vpn ike-sa (Phase 1) and > show vpn ipsec-sa (Phase 2). No output? Negotiation isn't even starting.
2. Common Error Decoders (The "Cheat Sheet")
Check Monitor > System Logs (filter for subtype eq vpn).
NO_PROPOSAL_CHOSEN: Mismatch in encryption, authentication, or DH Group. Double-check your Crypto Profiles!
AUTHENTICATION_FAILED: Pre-shared key (PSK) mismatch. Re-enter it on both ends—it's case-sensitive.
TS_UNACCEPTABLE: Proxy-ID/Traffic Selector mismatch. Ensure your local and remote subnets are exact mirrors of the peer.
3. Advanced CLI Deep Dives
When logs aren't enough, go deeper:
Live Log Tail: > tail follow yes mp-log ikemgr.log (Essential for catching IKEv2 negotiation errors in real-time).
Packet Capture: Filter for UDP 500 (IKE) or UDP 4500 (NAT-T) to see if the firewall is even receiving the initiation packets.
Force Negotiation: > test vpn ike-sa gateway <name> to manually kickstart the tunnel.
4. Check the following ...
UDP 500/4500: Is an upstream ISP or firewall blocking these ports?
Routing: Is there a route pointing to the tunnel interface for the remote subnet?
NAT-T: If either side is behind a NAT device, ensure NAT Traversal is enabled.
#Always verify that IKEv2 is the default where possible for better security and stability.
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