Consider the network shown below. Suppose AS$1,$ AS$2,$ AS$3,$ and AS$4$ are running OSPF for their intra$-$AS routing protocol. Suppose eBGP and iBGP are used for the inter$-$AS routing protocol. Initially suppose there is no link between AS$2$ and AS$4.$ Indicate the protocol by which a router learns about the network prefix x$,$ which is attached to AS$4.$