+ Reji
On 15/04/19, 3:02 PM, "Preethi Ramachandra" <[email protected]> wrote:
Hi,
I want to understand the IPV4 and IPV6 multipath(ECMP) routes behaviour in
Linux.
I tried configuring IPV4 and IPV6 multipath routes on Linux using ip route
add command. Linux versions used in this testing are
1. Linux Kernel: 3.14.52-rt50-WR7.0.0.9_ovp
2. Linux Kernel: 4.8.28-WR2.2.1_standard
IPV4 multipath route addition and display:
ip route add 1.1.1.0/24 nexthop via 192.168.1.254 nexthop via 192.168.0.254
ip route show
1.1.1.0/24
nexthop via 192.168.1.254 dev virbr0 weight 1
nexthop via 192.168.0.254 dev eth0 weight 1
IPV6 multipath route addition and display:
ip route add 2001:d98:0:e101::1/128 nexthop via
3100:db8:c70:a20:c2bf:a7ff:fed2:1111 nexthop via 2102:db8:c70:a20::2
ip -6 route show
2001:d98:0:e101::1 via 3100:db8:c70:a20:c2bf:a7ff:fed2:1111 dev eth0
metric 1024
2001:d98:0:e101::1 via 2102:db8:c70:a20::2 dev eth0 metric 1024
In IPV4 a single route is added with multipath nexthops. However, in IPV6 ,
multipath routes are added as separate single routes with each multipath NH. I
wanted to understand why is this behaviour change in Linux for IPV6 multipath
routes. Why wasn’t same IPV4 approach followed in this case?
The code path to add IPV6 multipath routes is
inet6_rtm_newroute()
rtm_to_fib6_config()
ip6_route_multipath_add()
list_for_each_entry(nh, &rt6_nh_list, next) {
err = __ip6_ins_rt(nh->rt6_info, &cfg->fc_nlinfo, &nh->mxc);
/* nh->rt6_info is used or freed at this point, reset to NULL*/
nh->rt6_info = NULL;
if (err) {
if (replace && nhn)
ip6_print_replace_route_err(&rt6_nh_list);
err_nh = nh;
goto add_errout;
}
/* Because each route is added like a single route we remove
* these flags after the first nexthop: if there is a collision,
* we have already failed to add the first nexthop:
* fib6_add_rt2node() has rejected it; when replacing, old
* nexthops have been replaced by first new, the rest should
* be added to it.
*/
cfg->fc_nlinfo.nlh->nlmsg_flags &= ~(NLM_F_EXCL |
<<<< This flag is unset here to allow duplicate routes in
fib6_add_rt2node() function.
NLM_F_REPLACE);
nhn++;
}
Thanks,
Preethi
