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Does anycast addressing, in itself, add any additional latency to network connections? Specifically, if you had two servers located in the same data center that had extremely similar routes/number of hops in route, up until the actual destination, one using unicast and the other using anycast, would there be any difference in latency between the two, noticeable or not?

It seems like having to look up the shortest route using BGP would have some sort of effect on latency, versus simply being able to use the one unicast address. I've been unable to find anything that mentions any additional overheard of using anycast addressing.

I also understand that if there was any additional overheard using anycast, it would probably be negligible in real world scenarios and that the benefits of anycast over unicast would still remain true.

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Does anycast addressing, in itself, add any additional latency to network connections?

No.

one using unicast and the other using anycast

Anycast is unicast. It is just that the same network is in two different places, and the routing protocol chooses the closest network to which it send traffic destined for that network.

It seems like having to look up the shortest route using BGP would have some sort of effect on latency

That is exactly what a router does. A routing protocol can receive multiple paths to a network, with or without anycast, but the router chooses one to place in the routing table, and it uses that one for traffic destined to that network. If the current route to that network in the routing table is lost, the next best path is automatically placed in the routing table and used.

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  • 1
    Thank you, that makes a lot of sense now! Jun 22 '19 at 2:24
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    Or, worded differently, the best route selection is done at the time of routing table updates, not while routing the packet itself.
    – jcaron
    Jun 24 '19 at 10:30
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Anycast is a scheme that terminates a specific route/IP address/IP subnet in multiple locations on the Internet. That way, a client connects to the closest location (the path with the lowest metric).

By looking at a packet, anycast is no different from unicast, so it doesn't introduce any additional latency or such.

Each router has to look up the route for each packet from its local routing table anyway. Updating that table is done in intervals or when changes happen, depending on the routing protocol, and doesn't impact the routing function itself.

Note that a router doesn't ask its neighbors for the route for a specific packet at hand but needs to know all required routes at all times. The routing table can range from a simple default route (for a single upstream gateway) to a full Internet routing table (some 400,000 entries).

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  • The a anycast routes might contribute to the number of routes. This could in theory put some more strain on the router resources, however it is pretty much a solved problem for IPv4 (especially if the anycast happens in lower routing infrastructure at the aggregation level like it does for some CDN and Video Caching I can imagine)
    – eckes
    Jun 23 '19 at 18:34
  • The question was about anycast itself adding delay which it doesn't. Hypothetically, the larger routing tables could produce more overhead and delay, but then again, routers actually handling these large routing tables are predominantly hardware-driven devices using TCAM tables. With those, the table size doesn't matter unless it exceeds their capacity.
    – Zac67
    Jun 24 '19 at 9:31
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Anycast , Broadcast, Multicast are ways the traffic flow from source to destination . This way of flow has nothing to do with latency in network .

They will add latency in network when there is limited band width is available . But I can say only Anyycast traffic won't create much impact on latency of network .

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