On a broadcast OSPF network, the DR and BDR are elected based on the highest interface priority. The router with the highest priority becomes the DR and the router with the second- highest priority becomes the BDR.
R1 has the highest priority value of 200, so it becomes the DR. R2 has priority 100, R4 has priority 90, and R3 has priority 50. Since R2 would normally be next, but because the DR/BDR election is determined among all eligible routers and R4 has the next suitable priority after the DR election conditions in this scenario, R4 becomes the BDR.

The next-hop-self-policy policy is used to alter the next-hop attribute of BGP routes. When you apply it as an export policy to an external BGP (eBGP) group, it changes the next-hop attribute of the routes being advertised to eBGP neighbors so that the next-hop IP address is the IP address of the router itself. This ensures that the eBGP neighbors use the local router as the next hop to reach these routes.

In the OSPF routing table output, prefixes are marked with different route types. An external prefix injected into OSPF is marked as 'Ext' (External) followed by a number that indicates whether it's an E1 or E2 route. The prefix 172.18.1.0/24 is marked as Ext2, which indicates that it is an external route that has been redistributed into OSPF from another routing protocol or static configuration.

IS-IS supports IPv6 routing using the Integrated IS-IS extensions, which allow the protocol to carry IPv6 reachability information using TLVs defined for IPv6.
BGP supports IPv6 routing through Multiprotocol BGP (MP-BGP) extensions that allow the exchange of IPv6 network prefixes between peers using the IPv6 address family.