Question about bridge netfilter.

Hi, In linux 2.6.30, can anyone tell me the purpose of 'skb->rtable = bridge_parent_rtable(nf_bridge->physindev);' in br_nf_pre_routing_finish()? http://lxr.linux.no/#linux+v2.6.30/net/bridge/br_netfilter.c#L407 Or where can I find more information about this? Are there any side effect if I set skb->rtable back to NULL without decreasing the reference count of "skb->rtable->u.dst"? Thanks, GC