From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Vadim Fedorenko <vadim.fedorenko@linux.dev>,
"David S. Miller" <davem@davemloft.net>,
David Ahern <dsahern@kernel.org>,
Eric Dumazet <edumazet@google.com>,
Paolo Abeni <pabeni@redhat.com>,
Simon Horman <horms@kernel.org>,
Willem de Bruijn <willemb@google.com>,
Jakub Kicinski <kuba@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>, Ido Schimmel <idosch@nvidia.com>,
netdev@vger.kernel.org,
Vadim Fedorenko <vadim.fedorenko@linux.dev>
Subject: Re: [PATCH net v3 1/2] net: fib: restore ECMP balance from loopback
Date: Mon, 22 Dec 2025 09:05:28 -0500 [thread overview]
Message-ID: <willemdebruijn.kernel.7a5aa1f215dd@gmail.com> (raw)
In-Reply-To: <20251221192639.3911901-1-vadim.fedorenko@linux.dev>
Vadim Fedorenko wrote:
> Preference of nexthop with source address broke ECMP for packets with
> source addresses which are not in the broadcast domain, but rather added
> to loopback/dummy interfaces. Original behaviour was to balance over
> nexthops while now it uses the latest nexthop from the group. To fix the
> issue introduce next hop scoring system where next hops with source
> address equal to requested will always have higher priority.
>
> For the case with 198.51.100.1/32 assigned to dummy0 and routed using
> 192.0.2.0/24 and 203.0.113.0/24 networks:
>
> 2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
> link/ether d6:54:8a:ff:78:f5 brd ff:ff:ff:ff:ff:ff
> inet 198.51.100.1/32 scope global dummy0
> valid_lft forever preferred_lft forever
> 7: veth1@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
> link/ether 06:ed:98:87:6d:8a brd ff:ff:ff:ff:ff:ff link-netnsid 0
> inet 192.0.2.2/24 scope global veth1
> valid_lft forever preferred_lft forever
> inet6 fe80::4ed:98ff:fe87:6d8a/64 scope link proto kernel_ll
> valid_lft forever preferred_lft forever
> 9: veth3@if8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
> link/ether ae:75:23:38:a0:d2 brd ff:ff:ff:ff:ff:ff link-netnsid 0
> inet 203.0.113.2/24 scope global veth3
> valid_lft forever preferred_lft forever
> inet6 fe80::ac75:23ff:fe38:a0d2/64 scope link proto kernel_ll
> valid_lft forever preferred_lft forever
>
> ~ ip ro list:
> default
> nexthop via 192.0.2.1 dev veth1 weight 1
> nexthop via 203.0.113.1 dev veth3 weight 1
> 192.0.2.0/24 dev veth1 proto kernel scope link src 192.0.2.2
> 203.0.113.0/24 dev veth3 proto kernel scope link src 203.0.113.2
>
> before:
> for i in {1..255} ; do ip ro get 10.0.0.$i; done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
> 255 veth3
>
> after:
> for i in {1..255} ; do ip ro get 10.0.0.$i; done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
> 122 veth1
> 133 veth3
>
> Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches source address")
> Signed-off-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Willem de Bruijn <willemb@google.com>
next prev parent reply other threads:[~2025-12-22 14:05 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-21 19:26 [PATCH net v3 1/2] net: fib: restore ECMP balance from loopback Vadim Fedorenko
2025-12-21 19:26 ` [PATCH net v3 2/2] selftests: fib_test: Add test case for ipv4 multi nexthops Vadim Fedorenko
2025-12-22 8:18 ` [PATCH net v3 1/2] net: fib: restore ECMP balance from loopback Ido Schimmel
2025-12-22 14:05 ` Willem de Bruijn [this message]
2025-12-30 10:10 ` patchwork-bot+netdevbpf
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