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To enable anycast for DNS queries, you configure two or more DNS servers within the AS routing domain with the same anycast address. When you select OSPF as the routing protocol, the upstream router determines the nearest server within the group of servers configured with that anycast address. (The "nearest" DNS server may not necessarily be the geographically closest DNS server; it is the DNS server with the lowest cost associated with its reachability from the current node. This is calculated through the OSPF routing algorithm, a discussion of which is far beyond the scope of this manual.) The nearest DNS server configured with the correct anycast address then responds to the DNS query. In the case where the nearest server becomes unavailable, the next nearest server responds to the query. OSPF anycast provides a dynamically routed failover to ensure that DNS can always resolve client requests within the AS. From the client perspective, anycasting is transparent and the group of DNS servers with the anycast address appears to be a single DNS server.
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Anycast and BGP4
BGP4 (also referred to as BGP) is designed to distribute routing information among ASs, exchange routing, and reachability information with other BGP systems using a destination-based forwarding paradigm. Unlike OSPF, which calculates routes within a single AS, BGP is a vector routing protocol that distributes routing information among different ASs. A unique ASN (autonomous system number) is allocated to each AS to identify the individual network in BGP routing. A BGP session between two BGP peers is an eBGP (external BGP) session if the BGP peers are in different ASs. A BGP session between two BGP peers is an iBGP (internal BGP) session if the BGP peers are in the same AS.
BGP configuration enables large enterprises using BGP as the internetworking protocol, to provide resilient DNS services using the Infoblox solution. While BGP is mostly used by ISPs, it is also used in larger enterprise environments that must interconnect networks, that span geographical and administrative boundaries. In these environments, it is required to use BGP to advertise anycast routes. Using BGP allows the appliance to advertise DNS anycast addresses to neighboring routers across multiple ASs, that also use BGP as their routing protocols.
To enable anycast for DNS queries among three different networks that span different geographical regions, you can configure two DNS servers with the same DNS anycast addresses in the AS 65497 network. Since other network routers in AS 65498 and AS 65499 also use BGP as the routing protocol, the DNS anycast addresses can be advertised across these networks.
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You can configure anycast addressing on the DNS servers and select BGP as the protocol to advertise the anycast addresses to Router 1 and 2 in AS 65499. For more information, see Configuring Anycast Addresses. Once you have configured the DNS servers, the appliances automatically add filters on the advertising interfaces to limit the advertisements to the configured anycast IP addresses. Similarly, BGP filters are applied to ensure that the DNS servers only receive default route advertisements from the neighboring routers.
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For additional information on Anycast, see the following:
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