netdev - Fw: [Bug 220195] New: [Issue] Linux Not Sending ARP to Cisco C8000v Virtual Router (IOS XE 17.09.01a)

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Message-ID: <20250605083209.73808675@hermes.local>
Date: Thu, 5 Jun 2025 08:32:09 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: netdev@...r.kernel.org
Subject: Fw: [Bug 220195] New: [Issue] Linux Not Sending ARP to Cisco C8000v
 Virtual Router (IOS XE 17.09.01a)

Not likely a kernel bug, but someone on list probably has more insight here.

Begin forwarded message:

Date: Thu, 05 Jun 2025 08:18:47 +0000
From: bugzilla-daemon@...nel.org
To: stephen@...workplumber.org
Subject: [Bug 220195] New: [Issue] Linux Not Sending ARP to Cisco C8000v Virtual Router (IOS XE 17.09.01a)


https://bugzilla.kernel.org/show_bug.cgi?id=220195

            Bug ID: 220195
           Summary: [Issue] Linux Not Sending ARP to Cisco C8000v Virtual
                    Router (IOS XE 17.09.01a)
           Product: Networking
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: IPV4
          Assignee: stephen@...workplumber.org
          Reporter: rvhdrywesy48@...il.com
        Regression: No

We are connecting the linux to a virtual router
c8000be-universalk9.17.09.01a.SPA.bin download from   ,but quite strange no arp
sent 

Hello everyone,

We’re currently testing the connectivity between a Linux host and a virtual
Cisco router using the image:

 c8000be-universalk9.17.09.01a.SPA.bin
Downloaded
from:https://www.ioshub.net/c8000be-universalk9-17-09-01a-spa-bin-cisco-catalyst-8000v-edge-platform-ios-xe-amsterdam-17-09-01a-software-download-link/

Setup Overview:
        •       Linux Host: Ubuntu 22.04 (Kernel 5.15.x)
        •       Virtual Router: Cisco Catalyst 8000v (C8000v) running IOS XE
Amsterdam 17.09.01a
        •       Connection: Linux <–> vNIC <–> Cisco 8000v
        •       Virtualization: KVM/QEMU

Problem:

Despite the interfaces being up on both the Linux side (ip link shows UP) and
the C8000v router (GigabitEthernet interface shows up/up), no ARP request is
being sent from the Linux host when trying to ping the virtual router.

We’ve confirmed the following:
        •       Static IPs configured on both ends
        •       Interface eth0 is up and has no MAC address conflict
        •       No firewall (ufw disabled, iptables -F)
        •       tcpdump on Linux shows no ARP at all — not even when doing a
manual ping
        •       C8000v side shows no ARP entries either

What we suspect / tried:
        •       Verified vNIC model is virtio-net-pci, tried switching to e1000
— same behavior
        •       Recompiled kernel with CONFIG_ARP=y, just in case — no change
        •       Changed C8000v interface to bridge and virtio modes — issue
persists
        •       Added a static ARP entry on Linux — ping still doesn’t work

What’s strange:
        •       On other routers (e.g., open-source or FRRouting), Linux sends
ARP normally under identical QEMU network configuration
        •       On Wireshark, it’s like Linux decides not to even try sending
ARP to the C8000v MAC/IP

⸻

Question:
Is there a known kernel-level quirk where Linux might suppress ARP probing to a
VM MAC/interface type it “distrusts” or doesn’t recognize as reachable?

Any suggestions on forcing or debugging ARP emission on Linux (e.g.,
netlink-level tracing or ARP stack debug)?

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