netdev - Re: [PATCH net v3 1/2] net: fib: restore ECMP balance from loopback

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Message-ID: <willemdebruijn.kernel.7a5aa1f215dd@gmail.com>
Date: Mon, 22 Dec 2025 09:05:28 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, 
 "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>
Cc: Shuah Khan <shuah@...nel.org>, 
 Ido Schimmel <idosch@...dia.com>, 
 netdev@...r.kernel.org, 
 Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Subject: Re: [PATCH net v3 1/2] net: fib: restore ECMP balance from loopback

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@...ux.dev>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>