The main answer to your question is that AS6 can use a routing protocol such as BGP to advertise to its neighboring Autonomous Systems (ASes) that it prefers to receive traffic for the upper half of its address space only via the c-y link. This can be achieved by setting a higher preference or metric for routes received from other links.
Furthermore, AS6 can also configure its border routers to filter incoming traffic from other links for the upper half of its address space, except for the c-y link. This can be done using Access Control Lists (ACLs) or prefix-lists.
In case the c-y link is broken, AS6 can use BGP to advertise to its neighboring ASes that it is no longer preferring traffic via that link. The other links can be configured with lower preference or metric to ensure traffic flows through the c-y link when it is operational again.
In conclusion, AS6 can use BGP and network filtering techniques to prefer receiving traffic for the upper half of its address space only via the c-y link and ensure proper traffic flow in case of link failures.
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Main answer: To inform the world that AS6 prefers receiving traffic for the upper half of its address space only via the C-Y link, it would use BGP (Border Gateway Protocol) with selective route advertisement and path prepending.
Explanation:
1. AS6 would configure its BGP routers to only advertise the upper half of its address space to the C-Y link's neighboring autonomous system.
2. AS6 would prepend its own AS number multiple times to the AS_PATH attribute for the upper half of its address space. This makes the path via C-Y appear longer and therefore more preferable, directing incoming traffic to use this link.
3. If the C-Y link is broken, BGP will automatically detect the failure and stop advertising the route. Traffic will then be rerouted using alternative paths available.
Conclusion: AS6 can control its preferred traffic flow for the upper half of its address space by using BGP route advertisement and path prepending, ensuring that traffic is primarily received via the C-Y link unless it is broken.