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[204.195.96.226]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6fafec3c10bsm23431696d6.107.2025.06.05.08.32.12 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Jun 2025 08:32:12 -0700 (PDT) Date: Thu, 5 Jun 2025 08:32:09 -0700 From: Stephen Hemminger To: netdev@vger.kernel.org Subject: Fw: [Bug 220195] New: [Issue] Linux Not Sending ARP to Cisco C8000v Virtual Router (IOS XE 17.09.01a) Message-ID: <20250605083209.73808675@hermes.local> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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@kernel.org To: stephen@networkplumber.org Subject: [Bug 220195] New: [Issue] Linux Not Sending ARP to Cisco C8000v Vi= rtual Router (IOS XE 17.09.01a) https://bugzilla.kernel.org/show_bug.cgi?id=3D220195 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@networkplumber.org Reporter: rvhdrywesy48@gmail.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=20 Hello everyone, We=E2=80=99re 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-cat= alyst-8000v-edge-platform-ios-xe-amsterdam-17-09-01a-software-download-link/ Setup Overview: =E2=80=A2 Linux Host: Ubuntu 22.04 (Kernel 5.15.x) =E2=80=A2 Virtual Router: Cisco Catalyst 8000v (C8000v) runni= ng IOS XE Amsterdam 17.09.01a =E2=80=A2 Connection: Linux <=E2=80=93> vNIC <=E2=80=93> Cisc= o 8000v =E2=80=A2 Virtualization: KVM/QEMU Problem: Despite the interfaces being up on both the Linux side (ip link shows UP) a= nd 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=E2=80=99ve confirmed the following: =E2=80=A2 Static IPs configured on both ends =E2=80=A2 Interface eth0 is up and has no MAC address conflict =E2=80=A2 No firewall (ufw disabled, iptables -F) =E2=80=A2 tcpdump on Linux shows no ARP at all =E2=80=94 not = even when doing a manual ping =E2=80=A2 C8000v side shows no ARP entries either What we suspect / tried: =E2=80=A2 Verified vNIC model is virtio-net-pci, tried switch= ing to e1000 =E2=80=94 same behavior =E2=80=A2 Recompiled kernel with CONFIG_ARP=3Dy, just in case= =E2=80=94 no change =E2=80=A2 Changed C8000v interface to bridge and virtio modes= =E2=80=94 issue persists =E2=80=A2 Added a static ARP entry on Linux =E2=80=94 ping st= ill doesn=E2=80=99t work What=E2=80=99s strange: =E2=80=A2 On other routers (e.g., open-source or FRRouting), = Linux sends ARP normally under identical QEMU network configuration =E2=80=A2 On Wireshark, it=E2=80=99s like Linux decides not t= o even try sending ARP to the C8000v MAC/IP =E2=B8=BB Question: Is there a known kernel-level quirk where Linux might suppress ARP probing = to a VM MAC/interface type it =E2=80=9Cdistrusts=E2=80=9D or doesn=E2=80=99t rec= ognize as reachable? Any suggestions on forcing or debugging ARP emission on Linux (e.g., netlink-level tracing or ARP stack debug)? --=20 You may reply to this email to add a comment. You are receiving this mail because: You are the assignee for the bug.