Networking - LLM Comparison

Comparison of LLMs with networking tasks

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Comparison Index

Task (prompt)

I have prplOS clients in the lab connected to Cisco LAC and LNS. I can ping from prplOS to LAC but I can’t ping between prplOS clients neither can I ping from LNS to any client. What could be the root cause?

The prompt was intentionally not overly specific to mimic real world prompts. It also allows neutral start, not prefocusing LLM into a specific reasoning path. With neutral start, it’s interesting to compare abilities of LLMs to encount potential root causes and select or sort them in probability given the described cirmustances. One can see the difference in how individual LLM work with cues and side information given in the prompt.

# Results

LLM Name Score Remarks
Qwen3.6-Max-Preview 3 Missed actual cause. Provided other plausible root causes. Lost points for non-plausible causes.
Qwen3.6-27B 🥈 N/A Found root cause in top 3 suspects. Provided other plausible root causes.
Qwen3-Coder ⛔ 2 Missed actual cause. Lost points for non-plausible root causes.
Gemini 3 Flash Preview 🏆 N/A Found root cause as the 1st suspect. Provided other plausible root causes.
Gemma 4 31B IT 🏆 N/A Found root cause as the 1st suspect. Provided other plausible root causes.
gpt-5.5 🏆 N/A Found root cause as the 1st suspect. Lost points for non-plausible root causes.
gpt-5.3-chat-latest 3 Missed actual cause. Provided other plausible root causes. Lost points for non-plausible causes.
gpt-5.3-codex ⛔ 2 Missed actual cause. Lost points for non-plausible root causes.
gpt-4.1-nano ⛔ 1 Missed actual cause. Lost points for non-plausible root causes.
gpt-5.4-mini ⛔ 2 Missed actual cause. Lost points for non-plausible root causes.
o1-pro ⛔ 0 Missed actual cause. Lost points for non-plausible root causes.
Sonet-4.6 4 Missed actual cause. Provided other plausible root causes.
Opus 4.7 🏆 N/A Found root cause as the 1st suspect. Provided other plausible root causes.
Grok Code Fast 1 🥈 N/A Found root cause in top 3 suspects. Lost points for non-plausible causes.
Gwen3 Coder Next ⛔ 1 Missed actual cause. Lost points for non-plausible root causes.
LLama4 Maverick 17B ⛔ 1 Missed actual cause. Lost points for non-plausible root causes.
Nemotron 3 Super 🥉 3 Identified firewall (late). Lost points for non-plausible root causes.
Mistral 🥉 1 Identified firewall (late). Lost points for non-plausible root causes.

Full LLM responses

Qwen3.6-Max-Preview

This is a classic L2TP/IPsec troubleshooting scenario. The fact that prplOS clients can ping the LNS but cannot ping each other and the LNS cannot ping the clients points to specific routing, firewall, or L2TP configuration issues.

Here’s a breakdown of the most likely root causes, ordered by probability:

1. LNS Firewall / ACL Blocking ICMP Return Traffic

2. Missing or Incorrect Route on LNS for Client Subnet

3. Client-to-Client Communication Blocked by LNS Policy

4. Reverse Path Filtering (rp_filter) on LNS

5. IP Address Pool Overlap or Misconfiguration

6. MTU/MSS Issues

  1. Verify Routing on LNS: 
    ip route show
ip neigh show  # Check if LNS has ARP entries for clients
  2. Check Firewall Rules on LNS: 
    iptables -L -n -v
# Temporarily flush rules to test:
iptables -F
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
  3. Enable Proxy ARP on LNS:
    • For Cisco: ip local-proxy-arp on the virtual-template.
    • For Linux: Add proxyarp to PPP options.
  4. Test from LNS to Client: 
    ping <client_ip>
tcpdump -i l2tp0 icmp  # Capture traffic on tunnel interface
  5. Test Client-to-Client:
    • If proxy ARP is enabled, try pinging between two clients.
    • If still failing, check if the LNS is forwarding traffic between tunnel endpoints.

Most Likely Root Cause

Given your symptoms, the most probable cause is a combination of:

  1. Firewall on LNS blocking inbound ICMP (preventing LNS → client pings).
  2. Proxy ARP not enabled (preventing client ↔ client communication).

Start by checking the LNS firewall and enabling proxy ARP.

Qwen3.6-27B

This is a classic L2TPv3 or IPsec over GRE (common in prplOS/Cisco lab setups) troubleshooting scenario. The fact that you can ping from prplOS to the LNS but not between clients or from LNS to clients points to specific asymmetries or configuration gaps.

Here’s a structured breakdown of likely root causes, ordered by probability:

1. Asymmetric Routing / Missing Return Path

2. Missing Client Subnet Routes on the LNS

3. ACLs/Firewall Blocking Inter-Client Traffic

4. MTU/MSS Issues Causing Fragmentation Drop

5. NAT Hairpinning / Symmetric NAT Issues

6. VRF Isolation

7. L2TPv3 Specific: Pseudowire Not Fully Established

🔍 Recommended Debug Steps

  1. Test Small Pings: 
    ping -s 50 <target_ip>

    If small pings work, it’s an MTU issue.

  2. Check Routes:
    • On prplOS: ip route show table all
    • On Cisco: show ip route vrf <vrf-name> or show ip route
  3. Check ACLs/Firewall:
    • prplOS: iptables -L -n -v or nft list ruleset
    • Cisco: show access-lists
  4. Verify Tunnel Status:
    • prplOS: ip l2tp show
    • Cisco: show l2tp session detail
  5. Trace Route: 
    traceroute <client_ip>

    See where packets drop.

  6. Enable Debugging:
    • Cisco: debug l2tp packet, debug ip icmp
    • prplOS: Enable kernel logging or use tcpdump -i <tunnel_interface>

✅ Most Likely Root Cause

Given your symptoms (ping to LNS works, but not between clients or LNS→clients), the top two suspects are:

  1. Missing routes on the LNS for client subnets.
  2. ACL/firewall blocking inter-client or inbound traffic on prplOS.

Start by verifying routes on the LNS and checking prplOS firewall rules.

Qwen3-Coder

Based on your L2TP (LAC/LNS) setup with prplOS clients, the issue where you can ping from prplOS to LNS but not between clients or from LNS to clients suggests a layer 3 routing/forwarding problem within the L2TP tunnel. Here are the most likely root causes:

1. IP Addressing Issues

2. LNS Routing Configuration

3. Forwarding/Security Restrictions

4. Layer 2 vs Layer 3 Issues

Troubleshooting Steps:

  1. Check IP addressing of all prplOS clients and LNS interfaces
  2. Verify LNS configuration allows client-to-client communication
  3. Examine routing table on LNS for client subnets
  4. Review any ACLs or firewall rules on the LNS
  5. Check L2TP session details to see if multiple clients appear as separate sessions

The fact that you can ping LNS → client but not client ↔ client strongly indicates a routing policy issue rather than a basic connectivity problem.

Gemini 3 Flash Preview

Input tokens: 57
Output tokens: 2,029
Total tokens: 2,086
Cost Estimation *
Input token cost: $0.000029
Output token cost: $0.006087
Total cost: $0.006116
Elapsed time: 19.9s

The failure to ping between prplOS clients or from the LNS to the clients, despite the clients being able to ping the LNS, usually points to routing isolation or firewall restrictions on either the LNS or the prplOS clients.

Here are the most likely root causes and how to troubleshoot them:

1. prplOS (OpenWrt) Firewall Settings (Most Likely)

prplOS is based on OpenWrt. By default, OpenWrt firewalls are configured to REJECT or DROP all incoming traffic on interfaces in the wan zone.

2. Cisco LNS: “Hairpinning” and Inter-Client Routing

For two clients to ping each other, the LNS must receive a packet from one Virtual-Access interface and route it back out through another Virtual-Access interface.

3. Missing Routes on the Clients (Split Tunneling)

If the prplOS clients do not have a route for the L2TP IP pool, they won’t know they should send pings to other clients through the tunnel.

4. LNS Routing to the Client Pool

Even if the LNS has the sessions up, it needs a valid route in its routing table to reach the clients’ IP addresses.

Summary Checklist

  1. On prplOS: Check /etc/config/firewall to allow icmp input on the wan zone.
  2. On Cisco LNS: Verify sessions are in the same VRF: show vpdn session.
  3. On Cisco LNS: Check for ACLs on the Virtual-Template: show run int Virtual-Template X.
  4. Connectivity Test: From the LNS, try to ping the client while running a packet capture on the client: tcpdump -i ppp0 icmp. If the client sees the echo request but doesn’t send an echo reply, it is a firewall issue on the client. If the client sees nothing, it is a routing/ACL issue on the LNS.

Gemma 4 31B IT

Input tokens: 57
Output tokens: 1,628
Total tokens: 1,685
Open source model $0
Elapsed time: 20.7s

Based on the symptoms described, your L2TP tunnel and PPP sessions are established (since the client can ping the LNS), but you are facing issues with unsolicited inbound traffic and inter-client routing.

Here is a breakdown of the root causes, categorized by the two failure scenarios.

1. Why you cannot ping from LNS $\rightarrow$ prplOS

Since the client can ping the LNS, the “path” exists, but the return path or the entry point is being blocked.

2. Why you cannot ping prplOS $\rightarrow$ prplOS

This is a “hairpinning” or “inter-session” routing issue. In an L2TP environment, the LNS acts as the hub.

Summary Checklist for Debugging

Test Path Likely Culprit Action
LNS $\rightarrow$ Client prplOS Firewall iptables -F (Disable firewall)
LNS $\rightarrow$ Client Source IP Mismatch ping <ip> source <VT_IP>
Client $\rightarrow$ Client prplOS Firewall Disable firewall on receiving client
Client $\rightarrow$ Client LNS ACLs Check show run interface Virtual-Template
Client $\rightarrow$ Client LNS Routing show ip route $\rightarrow$ verify Virtual-Access routes

Quickest way to isolate the problem: Log into the prplOS clients and completely disable the firewall. If pings start working, the issue is the prplOS security policy. If they still don’t work, the issue is the Cisco LNS routing/ACL configuration.

gpt-5.5

Input tokens: 61
Output tokens: 3476
Elapsed time: 64s

Most likely root cause: prplOS is dropping unsolicited inbound traffic on its WAN/PPP interface, i.e. firewall/ACL behavior.

Why it matches your symptoms:

What to check:

  1. On prplOS, verify firewall rules for the PPP/WAN interface.
  2. Temporarily disable firewall or allow ICMP echo-request on WAN/PPP.
  3. Run tcpdump on prplOS PPP interface:
tcpdump -ni pppoe-wan icmp

or whichever PPP interface is used.

If you see echo requests arriving but no replies leaving, it is definitely local firewall/filtering.

Also check Cisco side:

show ip route <client-ppp-ip>
show vpdn session
show users

If the LNS has routes to the client PPP IPs, the Cisco side is probably fine.

If you are trying to ping LAN hosts behind the prplOS routers, then you also need routing/NAT/firewall changes and unique LAN subnets. But for pinging the prplOS PPP client addresses, the main suspect is the prplOS WAN firewall blocking inbound ICMP.

## gpt-5.3-chat-latest

Input tokens: 61
Output tokens: 937
Elapsed time: 16s

What you’re describing almost always comes down to the LNS treating each PPP session as isolated point‑to‑point endpoints and not actually routing or permitting traffic between them.

The key clues:

That narrows it to what the LNS is doing with those virtual-access interfaces.

Most likely root causes on Cisco LNS:

1) Missing or incorrect routing on the LNS
Each PPP client should appear as a /32 route via a Virtual-Access interface. If ip routing is off, or those routes aren’t being installed, the LNS won’t know how to send traffic back to clients.

2) ACL or policy on the Virtual-Template
Very common: an inbound/outbound ACL, zone-based firewall, or control-plane policy is blocking traffic between subscribers or from the LNS itself. Check for:

3) Subscriber isolation (intentional or accidental)
Some configs explicitly prevent client-to-client communication (like broadband isolation features). Depending on platform/features, this can be:

4) Source issue when pinging from LNS
When you ping from the LNS, if you don’t specify a source, it may use a loopback or physical interface that the client has no route back to. Try:

5) Proxy ARP or return path issues
If clients are in the same IP pool but the LNS isn’t properly routing between /32s or proxying, inter-client traffic fails.

If I had to bet on one root cause:
An ACL / firewall or subscriber policy on the Virtual-Template is blocking intra-subscriber and LNS-originated traffic. That’s the most common in lab setups that otherwise “look correct.”

Quick checks:

If you paste your Virtual-Template config, I can point to the exact line.

gpt-5.3-codex

Input tokens: 61
Output tokens: 432
Elapsed time: 13.2s

Most likely root cause: PPP/L2TP subscriber isolation on the LNS (or in the Virtual-Template/AAA policy).

Your symptom pattern is classic:

That usually means sessions are up, but forwarding to subscriber routes is blocked or not installed correctly, often due to one of these:

  1. Per-user isolation / split-horizon behavior (common in broadband/BNG style configs)
    Prevents traffic between PPP subscribers on same LNS.

  2. Missing or incorrect return routes on LNS for client-assigned IPs
    LNS can receive packets from client (connected via virtual-access), but cannot properly send back.

  3. Unnumbered/virtual-access template issue
    If virtual-access interfaces are built from a template with restrictive ACL/policy, only control-plane reachability may work.

  4. ACL/uRPF/Firewall on Virtual-Template or loopback
    Could allow client→LNS ICMP but deny transit or LNS-initiated traffic.

  5. Proxy-ARP / IPCP address-pool mismatch
    Client gets address, but LNS forwarding logic for those pools is incomplete.

What to check on Cisco LNS

Quick conclusion

If all clients terminate on same LNS and only LNS itself is reachable, the most probable issue is subscriber isolation / forwarding policy on virtual-access sessions, not basic tunnel establishment.

gpt-4.1-nano

Input tokens: 62
Output tokens: 585
Elapsed time: 4.4s

Based on your description, here’s a summary of the situation:

This indicates that:

  1. The connection from prplOS clients to the network (LNS) is functional.
  2. There may be an issue with intra-client communication (client-to-client).
  3. The LNS cannot reach the clients, possibly indicating a routing, ACL, or subnet issue.

Possible root causes and troubleshooting steps:

1. Layer 2 connectivity or VLAN issues

2. Routing or subnet misconfiguration

3. Firewall or ACL restrictions

4. LNS configuration

5. Client configurations

Specific checks to perform:

Summary

Most likely, the root cause is a VLAN mismatch, routing issue, or ACL blocking traffic between clients. Since the clients can reach the LNS, the link from them to the network is working, but internal segmentation or filtering is preventing client-client communication and LNS connectivity to clients.

Would you be able to provide:

This info could help diagnose more precisely.

gpt-5.4-mini

Input tokens: 61
Output tokens: 2381
Elapsed time: 28.7s

Most likely it’s a routing/forwarding issue on the LNS side, not basic tunnel connectivity.

Typical causes:

Less often:

Since client → LNS ping works, the tunnel is up. Since LNS → client and client ↔ client fail, the problem is usually that the LNS is not routing subscriber traffic between sessions (or it’s being blocked).

If you want, I can help you check the Cisco LNS config/route table to pinpoint it.

o1-pro

Input tokens: 61
Output tokens: 4120
Elapsed time: 57s

A common reason this happens in an LAC/LNS lab setup is that each client’s PPP session is treated as a /32 (or unnumbered) interface from the LNS perspective. Although the clients can send traffic “up” to the LNS (and hence you can ping from prplOS to the LNS), there is no corresponding route (or ARP/bridging) on the LNS that allows it to reach back “down” to each client. Likewise, traffic from one PPP session to another is never switched at Layer 2 and has no Layer 3 routing path unless the LNS is explicitly configured to bridge or route between client subnets.

In other words:

Sonet-4.6

This is a classic L2TP/PPPoE topology issue. Based on your description — one-directional reachability (prplOS → LNS works, but LNS → prplOS and client-to-client fail) — here are the most likely root causes:

1. Proxy ARP / ARP not being forwarded (Most Likely)

The LNS needs to know how to reach the PPP client addresses. In L2TP, the LNS assigns IPs to clients but may not have proper ARP or routing entries back to them.

2. IP Routing Not Configured on LNS for Client Pool

The LNS knows the client IPs (it assigned them), but the return path routing may be missing or pointing to the wrong interface.

3. Client-to-Client Traffic Hairpinning Not Enabled

PPP clients behind the same LNS typically can’t communicate directly because:

4. Reverse Path Forwarding (uRPF) Dropping Return Traffic

If uRPF is enabled on the LNS tunnel or WAN interface:

5. PPP IPCP Negotiation Asymmetry

6. ACL or Firewall on Virtual-Template Blocking Inbound

Quick Diagnostic Checklist

Check Command
Are client IPs in LNS routing table? show ip route (look for /32 hosts)
Is the session fully up? show vpdn session / show l2tp session
Are virtual-access interfaces bound? show ip interface brief | include Virtual
uRPF dropping traffic? show ip interface virt-acc X
ACLs on virtual-template? show run | section virtual-template
ICMP allowed through? debug ip icmp on LNS while pinging

Most probable single cause: missing /32 host routes for client IPs on the LNS, or client-to-client hairpinning not configured. Start there.

Opus 4.7

Token count: 2312 tokens
Cost: $0.059385
Duration: 23.2s

Root Cause Analysis: Asymmetric Reachability over L2TP

The symptom pattern (outbound works, inbound and lateral don’t) is a classic signature of ingress filtering on the PPP/WAN interface combined with the point-to-point nature of L2TP sessions.

Most Likely Root Cause: prplOS Firewall Blocking Ingress on ppp Interface

On prplOS, the PPP interface created by the L2TP client is placed in the wan firewall zone by default. That zone has:

This alone explains all three symptoms you described, because the return ICMP echo-request never reaches the client’s network stack.

How to verify on prplOS

# Check which zone the ppp interface belongs to
fw3 print | grep -i ppp
uci show firewall | grep -i wan

# Watch drops live
logread -f &
iptables -L INPUT -v -n
nft list ruleset | grep -i ppp   # if nftables

# Temporary test (do NOT leave enabled)
iptables -I INPUT  -i ppp+ -p icmp -j ACCEPT
iptables -I FORWARD -i ppp+ -j ACCEPT

If ping from LNS starts working immediately, the firewall is confirmed as the cause.

Second Contributor: Cisco LNS Inter-Session Forwarding

Even after opening the firewall, client-to-client traffic needs the LNS to forward between two separate PPP sessions. Two common misconfigurations:

Issue Symptom Fix on Cisco LNS
Clients in same subnet via local pool, no proxy-ARP Client A can’t ARP/reach Client B ip proxy-arp on Virtual-Template
no ip routing or CEF issue on Virtual-Template No host routes installed per session ip routing + verify show ip route \| inc Virtual-Access
Unicast RPF strict mode Drops forwarded traffic ip verify unicast source reachable-via rx allow-default or remove uRPF
ACL on Virtual-Template Drops inter-session traffic Inspect show access-list

Check on the LNS:

show ip route | include Virtual-Access
show caller ip
show vpdn session
show interface Virtual-Access<n>  | include proxy

You should see a /32 host route per connected client pointing at its own Virtual-Accessx interface. If those routes are missing → routing/PPP config issue. If they exist but traffic still fails → proxy-ARP or ACL.

Third Thing to Check: MTU / MSS (not your current symptom, but will bite you)

L2TP+PPP overhead shrinks effective MTU to ~1460 or less. Ping works (small packets) but TCP sessions hang. Configure:

ppp-mss-clamp 1400   # or similar on the LNS Virtual-Template
  1. Packet capture on the prplOS client’s ppp0 while LNS pings it: 
    tcpdump -ni ppp0 icmp
    • If echo-requests arrive but no reply → firewall INPUT drop (confirmed).
    • If echo-requests don’t arrive → problem is on LNS side (routing/uRPF/ACL).

  2. Repeat for client-to-client: capture on both ppp0 interfaces simultaneously to see where the packet disappears.

  3. Apply the corresponding fix from the tables above.

Summary

Start with the tcpdump on ppp0 — it will tell you in 10 seconds whether the issue is on the prplOS side or the Cisco side.

Grok Code Fast 1

Token count: 4477 tokens
Cost: $0.00673426
Duration: 43.0s

Based on your description, this sounds like a classic L2TP (Layer 2 Tunneling Protocol) VPN setup with multiple prplOS clients connected to a Cisco LAC (L2TP Access Concentrator), which tunnels traffic to a Cisco LNS (L2TP Network Server). I’ll break down the problem, potential root causes, and troubleshooting steps. Keep in mind that prplOS is an open-source OS (often used in IoT devices like routers from the prpl Foundation), so it may have default network/firewall behaviors that affect reachability.

Key Observations from Your Issue

In a standard L2TPv2 setup (common for WAN/connections), each client has its own PPP session over the tunnel, which by default isolates them. Traffic isn’t automatically bridged or routed between peers unless explicitly configured. This explains why inter-client communication fails, and it also ties into the server-to-client ping failure.

Potential Root Causes

Here are the most likely explanations, ranked by probability based on common L2TP issues:

  1. LNS Configuration Issues (Most Likely):
    • Inter-Client Isolation: By default, Cisco LNS (using virtual-access interfaces for PPP sessions) treats each client’s connection as isolated. Each prplOS client gets a unique PPP virtual interface, and the LNS doesn’t forward traffic between them. This prevents routing or bridging between clients in the same subnet unless you explicitly configure it (e.g., by bridging virtual-access interfaces into a bridge group or using proxy routing). Inter-client pings fail because packets aren’t forwarded across the LNS—they stay isolated.
    • No Server-to-Client Routing: The LNS assigns IPs to clients (e.g., via DHCP or IPCP), but for the LNS itself to reach those IPs, there must be a route (e.g., pointing to the LAC or virtual-access interface). If missing, packets from the LNS to a client IP (e.g., 192.168.1.100) are sent out the LNS’s physical interface instead of being encapsulated back into the tunnel. In lab setups, this is often not configured by default.
    • Proxy ARP or Routing Not Enabled: On the LNS, you may need proxy ARP on the shared Ethernet interface (where the LNS IP is) to respond to ARP requests from clients. Without it, the LNS can’t “see” the clients’ IPs properly for incoming traffic.
  2. Firewall/Rules on LNS or prplOS Devices:
    • The LNS (running Cisco IOS) might have ACLs (access control lists) or CBAC (Context-Based Access Control) blocking specific traffic. For example, it could allow tunnel traffic out to external destinations but block intra-tunnel communication or server-initiated pings.
    • prplOS devices often have built-in firewalls (e.g., iptables or similar) that allow outbound ICMP (for ping LNS) but drop inbound ICMP (blocking pings from LNS). If interfaced-based rules on prplOS block replies to unsolicited traffic, LNS pings would fail. Between clients, if both sides block inbound, pings could be one-sided or fail entirely, amplified by the above isolation.
  3. IP Address/Subnetting Problems:
    • Mismatched subnets: If clients are assigned IPs in different subnets (e.g., one client at 192.168.1.0/24, another at 10.0.0.0/24), they can’t directly ping without routing. The LNS might not be set up to route between these subnets.
    • DHCP or IPCP assignment issues on the LNS: If the pool is exhausted or misconfigured, clients might get overlapping IPs or /32 host routes that don’t allow broadcast/proxying.
  4. LAC (Access Concentrator) Configuration or NAT Issues:
    • The LAC might be performing NAT or has ACLs that block forwarding between clients or from LNS to clients. Since it’s aggregating multiple clients, it needs to correctly encapsulate/de-encapsulate traffic. If NAT is applied, it could mask client IPs or break return-path routing.
    • Tunnel peering: If multiple clients are on different LACs or the LAC isn’t configured to allow multi-peer communication, inter-client traffic could be dropped.
  5. Lower-Level Tunnel or MTU Issues (Less Likely for Basic Pings)**:
    • UDP fragmentation/MTU mismatches across the tunnel could drop larger packets, but basic ICMP pings usually succeed unless the path MTU discovery is broken.
    • Authentication or session state: If client sessions are timing out or not fully establishing, intermittent drops could occur, but this doesn’t explain the directional failure.

Other factors like physical cabling (e.g., all clients on the same segment?) or interference aren’t implied here.

Troubleshooting Steps

Use these steps in sequence to isolate the issue. Test incrementally and capture logs (e.g., via debug l2tp on Cisco devices or tcpdump on prplOS/LNX nodes).

  1. Verify Tunnel Status and IPs:
    • On the LAC: Run show l2tp tunnel and show l2tp session to confirm sessions are active and check assigned IPs.
    • On clients: Run ip addr show or equivalent to note assigned IPs and routes (e.g., is the default route pointing to the tunnel’s PPP interface?).
    • Ping tests:
      • From client to LNS: Confirm it works via ping <LNS_IP> with -c 4 (or IPsec if protecting L2TP).
      • From LNS to client: Try ping <client_IP> and check if it succeeds; use traceroute to see the path.
  2. Check Routing on LNS:
    • Run show ip route to see if routes to client IPs exist (e.g., they should point to the virtual-access interface for that session). Add static routes if missing, e.g.: 
      ip route 192.168.1.10 255.255.255.255 Tunnel1  # Example for a client's IP via tunnel/LAC
    • Enable proxy ARP on the LNS’s LAN interface: ip proxy-arp (replaces the need for full routing).
    • For inter-client isolation: Configure a bridge group if needed. Create a bridge (e.g., with bridge-group 1) and assign virtual-access interfaces to it. This allows peers in the same group to communicate.
  3. Examine Firewall Rules:
    • On LNS (Cisco): Check ACLs with show ip access-lists. Temporarily add a rule allowing ICMP (e.g., for ping): 
      access-list 101 permit icmp any any

      Apply it to the interface.

    • On prplOS clients: Inspect firewall (often iptables or nfsvc). Look for rules like -A INPUT -p icmp -j DROP blocking inbound pings. Change to allow: 
      iptables -A INPUT -p icmp --icmp-type echo-request -i <tunnel_interface> -j ACCEPT
    • Test inter-client after changes.
  4. Debug and Packet Capture:
    • On LAC/LNS: Enable debug ip icmp and debug l2tp packet to log ping failures, then reproduce.
    • Use Wireshark or tcpdump on the LNS: 
      tcpdump -i <interface> icmp and host <client_IP>

      Look for encapsulated traffic in the tunnel (UDP port 1701 for L2TP).

  5. Test with Simplified Setup:
    • Disconnect one client and add its IP/manually route it on the LNS to test server-to-client.
    • Use a non-prplOS device as a client (if possible) to rule out prplOS-specific firewall.

Recommendations

Gwen3 Coder Next

Token count: 842 tokens
Cost: $0.0011224
Duration: 4.6s

Based on your symptoms:

This pattern strongly suggests a routing or forwarding issue on the LNS side, specifically that return traffic from LNS to prplOS clients is not properly routed. Since LNS can receive packets from prplOS clients (reverse path works), but cannot send packets to them (forward path broken), here are the most likely causes:

🔧 Most Likely Causes

  1. Missing reverse route on LNS
    The LNS may not have routes back to the prplOS client IP subnets.
    • When prplOS initiates ping, it sends ICMP Echo Request to LNS → works
    • But when LNS tries to send Echo Reply back to prplOS, it has no route → packet drops

  2. NoGRE/IP-in-IP decapsulation / tunnel interface misconfiguration
    prplOS likely uses a tunnel interface (e.g., GRETAP, IP-over-IP) — if the LNS doesn’t properly configure the tunnel source/destination or doesn’t assign IP addresses correctly on tunnel interfaces, LNS won’t be able to forward traffic to the client.

  3. LNS tunnel interface not in same subnet as prplOS clients
    Common mistake: prplOS clients get IPs from pool X.X.X.0/24, but LNS tunnel interface is configured with Y.Y.Y.Y/30 — so LNS can’t reach client IPs.

  4. Firewall/NAT blocking return traffic on LNS
    LNS may have:
    • iptables rules blocking ICMP replies
    • rp_filter (reverse path filter) too strict, dropping return packets
    • NAT rules rewriting source addresses inconsistently
  5. Missing ip route or routing protocol adjacency
    If using dynamic routing (OSPF/BGP), LNS may not be advertising client subnets back to upstream routers or itself.

🛠️ Diagnostic Steps

Run these on the LNS:

# 1. Check routes for client IP ranges
ip route show | grep -E "client_subnet|prpl"

# 2. Verify tunnel interface status and IP assignment
ip link show | grep -A2 tunnel
ip addr show dev <tunnel_iface>

# 3. Test LNS → client ping (with debug)
ping -I <tunnel_iface_ip> <client_ip>

# 4. Check if ICMP replies are being sent
tcpdump -ni any host <client_ip> and icmp

# 5. Check reverse path filtering
cat /proc/sys/net/ipv4/conf/all/rp_filter

Also check:

📝 Common Fix Examples

Option A: Add static route on LNS

ip route add <client_subnet>/24 dev <tunnel_iface>

Option B: Disable reverse path filtering (if policy allows)

sysctl -w net.ipv4.conf.all.rp_filter=0
sysctl -w net.ipv4.conf.default.rp_filter=0

Option C: Ensure tunnel interface IP is in same subnet as clients

ip addr add <LNS_tunnel_ip>/<prefix_len> dev <tunnel_iface>

Would you like to share your LNS tunnel configuration (e.g., ip addr, ip route, interface details) for a more precise diagnosis?

LLama4 Maverick 17B Instruct

To troubleshoot the issue where you can ping from prplOS clients to the LNS (L2TP Network Server) but cannot ping between prplOS clients or from the LNS to any prplOS client, let’s consider the key components and their roles in an L2TP (Layer 2 Tunneling Protocol) setup, particularly when involving an LAC (L2TP Access Concentrator).

  1. L2TP Basics: L2TP is used to create VPNs. The LAC is the device that initially connects to the client’s PPP (Point-to-Point Protocol) session and tunnels it to the LNS. The LNS is the endpoint of the L2TP tunnel and is where the PPP session is terminated.

  2. LNS Configuration and Routing: For ping to work from a prplOS client to the LNS, the LNS is correctly configured to accept L2TP connections, and routing is properly set up on the client side. However, the inability to ping between prplOS clients or from the LNS to clients indicates a potential issue with how the LNS handles or routes traffic between clients or back to them.

Here are several potential root causes for the issue:

1. LNS Not Routing Between Clients

2. Missing or Incorrect Routes on LNS

3. Firewall or ACL Blocking Traffic

4. Client IP Addressing and Subnetting

5. PPP Configuration and Authentication

Troubleshooting Steps

  1. Verify LNS Configuration: Check the LNS configuration for L2TP, PPP, and IP address assignment.
  2. Check Client IP Addresses: Ensure that clients are being assigned the correct IP addresses and are in the expected subnet.
  3. Inspect Routing and Firewall Rules: On the LNS and relevant network devices, inspect routing tables and any firewall or ACL rules that might block the traffic.
  4. Test with Different Traffic: Try pinging with different options (e.g., source interface) or testing with different types of traffic to isolate if it’s an ICMP-specific issue.

By systematically checking these potential causes and performing the corresponding troubleshooting steps, you should be able to identify and resolve the issue preventing communication between prplOS clients and from the LNS to clients.

Nemotron 3 Super

This is a classic symptom of a missing or misconfigured routing or ARP resolution path in a L2TP (Layer 2 Tunneling Protocol) network — which is exactly what Cisco LAC (L2TP Access Concentrator) and LNS (L2TP Network Server) implement. Your observation:

…strongly suggests that the L2TP tunnel is established and IP-layer connectivity from client to LNS is functional, but the LNS is not correctly forwarding or routing traffic between clients — or the clients are not on the same IP subnet, or ARP resolution is failing within the tunnel.

🔍 Root Cause Analysis

✅ What’s Working:

❌ What’s Not Working:

🚨 Most Likely Root Causes

🔴 1. Clients are not on the same IP subnet as seen by the LNS

✅ Check: On the LNS, run show ip route or show ip interface brief and verify that the client IPs are in a subnet directly connected to the LNS (e.g., via a virtual-access interface or loopback).

🔴 2. LNS is not proxy-ARPing for client IPs

✅ Check: On Cisco LNS, verify proxy-ARP is enabled:

show running-config | include proxy-arp

If not, enable it on the relevant interface (e.g., virtual-template or loopback used for client termination):

interface Virtual-Template1
  ip proxy-arp

🔴 3. No routing or NAT between clients

✅ Check: Are all clients getting IPs from the same subnet? If not, you need either:

  • A single IP pool for all clients (recommended for direct client-client comms), OR
  • Static routes on the LNS pointing to each client subnet via the appropriate virtual-access interface.

🔴 4. ACL or firewall blocking ICMP or IP traffic on LNS

✅ Check:

show access-lists
show ip interface Virtual-Template1

Look for ip access-group in/out and verify it’s not blocking intra-subnet traffic.

🔴 5. prplOS client firewall or stack blocking ICMP

✅ Check: On prplOS client, run:

tcpdump -i any icmp

Then ping from LNS → client.
If you see no packets arriving at the client → LNS is not sending them (routing/ARP issue).
If you see packets arriving but no reply → client firewall or stack is dropping reply.

🔴 6. L2TP session not properly bound to client IP

✅ Check: On LNS:

show vpdn session
show vpdn tunnel
show ip interface brief | include Virtual-Access

Look for the client’s IP under show ip interface Virtual-AccessX → should be listed as Internet address is .../...

✅ Diagnostic Steps to Confirm

Step Command Expected Outcome if Healthy
1 On LNS: show ip route Client IPs appear in directly connected subnets (e.g., C 10.0.0.0/24 is directly connected, Virtual-Access1)
2 On LNS: show arp Client IPs have MAC addresses learned (not Incomplete)
3 On LNS: ping <client-ip> Should succeed (you said this fails → so this is your key test)
4 On LNS: debug vpdn event + debug vpdn packet See if L2TP packets are being decapsulated and routed correctly
5 On prplOS client: tcpdump -i any icmp Ping from LNS → client: see ICMP request arrive? If not → LNS not sending. If yes but no reply → client firewall.
6 On LNS: show running-config | include proxy-arp Must be enabled on interface terminating clients
7 On LNS: show ip interface Virtual-Template1 Check for ip proxy-arp, ip address, and no ip redirects (sometimes redirects break things)

💡 Recommended Fix

Assuming you’re using standard L2TP over PPP (most common with Cisco LNS):

  1. Ensure all clients get IPs from the **same subnet (e.g., 10.0.0.0/24)**
    → Configure LNS to assign from a single IP pool via ip local pool or DHCP.

  2. Enable proxy-ARP on the LNS interface (critical!): 
    interface Virtual-Template1
  ip proxy-arp

  3. Verify no ACL is blocking ICMP or IP between clients
    → Remove or adjust ACLs if present.

  4. Check that client IPs appear in show ip route and show arp on LNS
    → If not, the IPCP negotiation is failing — check PPP debugs.

  5. (Optional) Test with TCP
    → Try telnet or http between clients. If TCP works but ICMP doesn’t, it’s likely an ACL blocking ICMP specifically.

📌 Summary: Most Probable Cause

✅ The LNS does not have proxy-ARP enabled, and/or clients are not in the same IP subnet as seen by the LNS.

This causes:

🔧 Quick Fix to Try Now

On the Cisco LNS:

configure terminal
interface Virtual-Template1   ! or whatever template you're using
 ip proxy-arp
end
write memory

Then:

If it works → proxy-ARP was the issue.

If not → check IP pool assignment and routing.

Let me know your LNS config snippet (virtual-template, ip local pool, vpdn group) if you need more targeted help!

Mistral

When prplOS clients can ping the LNS (L2TP Network Server) but cannot ping each other or be pinged from the LNS, the issue is likely related to routing, NAT, or L2TP configuration on the Cisco LAC (L2TP Access Concentrator) or LNS. Here are the most common root causes and troubleshooting steps:

1. L2TP Configuration Issues

2. Routing Issues

3. NAT or Firewall Blocking ICMP

4. L2TP Session Isolation

5. MTU Issues

6. Debugging Steps

7. Cisco-Specific Commands

8. prplOS Configuration

Summary Table

Symptom Likely Cause Solution
Can ping LNS but not other clients Missing inter-client routing Add static routes or enable inter-client comms
Can’t ping from LNS to clients Missing reverse route or NAT issue Add route on LNS, check NAT/firewall
All pings fail L2TP session not established Check L2TP auth, IP pool, and session limits

Next Steps: