pppd's mtu setup

Sorry I copy these from gitweb trunk:
when lcp is up, it will call netif_set_mtu to set stack's ppp interface mtu with MIN(MIN(mtu, mru), ao->mru), but why it is followed by ppp_send_config with mtu only?
the 2nd setting could be different with first setting.
I can see sys-linux didn't do anything with 2nd mtu setting so it is correct, but why there is 2nd setting and with different mtu?

    mtu = ho->neg_mru? ho->mru: PPP_MRU;

1908 mru = go->neg_mru? MAX(wo->mru, go->mru): PPP_MRU;

1909 #ifdef HAVE_MULTILINK

1910 if (!(multilink && go->neg_mrru && ho->neg_mrru))

1911 #endif /* HAVE_MULTILINK */

1912 netif_set_mtu(f->unit, MIN(MIN(mtu, mru), ao->mru));

1913 ppp_send_config(f->unit, mtu,

1914 (ho->neg_asyncmap? ho->asyncmap: 0xffffffff),

1915 ho->neg_pcompression, ho->neg_accompression);

thanks,
xiaozhao

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