Query regarding IPV6 multipath (ECMP) routes in Linux

[Preethi Ramachandra]

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                 * nexthops have been replaced by first new, the rest should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