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by bogomipz 3563 days ago
Anycast and ECMP work together in the context of load balancing. ECMP without Anycasted destination IPs would be pointless for horizontally scaling your LB tier.

What Anycast means is just that multiple hosts share the same IP address - as opposed to unicast. When all the nodes sharing the same IP are on the same subnet "nearest" is kind of irrelevant. So the implication is different.
1 comments
jssjr 3563 days ago
Sure. Feel free to call it anycast then. I usually hear anycast routing used in the context of achieving failover or routing flows to the closest server/POP, but there is probably a more formal definition in an RFC that I'll be pointed to shortly. =)

We are using BGP to advertise prefixes for GLB inside the data center to route flows to the directors. In our case all of the nodes are not on the same subnet (or at least not guaranteed to be) which is one of the reasons why we chose to avoid solutions requiring multicast. I expect Joe and Theo will get into more details about that in a future post though.

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bogomipz 3563 days ago
Are you running Quagga or Bird on the director instances then? I'm looking forward to reading more about it.
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logicalstack 3562 days ago
We use Quagga.
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jerkstate 3562 days ago
This is really cool work, I worked with a team that implemented an ECMP hashing scheme using a set of IPs kept alive by VRRP in a previous lifetime, so I have a bit of familiarity with the space and a few questions.

The article says the L4 layer uses ECMP with consistent/rendezvous hashing. is this vendor implemented or implemented by you using openflow or something similar? How does graceful removal at the director layer work? I know you would have to start directing incoming SYNs to another group, but how do you differentiate non-SYN packets that started on the draining group vs. ones that started on the new group?

If you are using L4 fields in the hash, how do you handle ICMP? This approach could break PMTU discovery because a icmp fragmentation needed packet sent in response to a message sent to one of your DSR boxes might hash to a different box, unless considerations have been made.

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