anycast load balancing issue

Måns Nilsson mansaxel at
Wed Jan 4 12:02:55 UTC 2012


I'm in the process of deploying an anycast DNS service internally. We're
on a pretty provider-like network, where we run MPLS to provide several
network overlays for different services. iBGP is used to distribute
routing information, and ISIS is used as IGP. In one of the VRFen we
would like to place name servers using a common IP address. To get speedy
network updates when outages occur we'll be using OSPF on the name servers
to inject the routes into the IGP. The P/E router then redistributes the
route into the right VRF. (the name server OSPF process is not aware of
MPLS; it just talks to a router.)

So far so good. This works. 

Trouble is, we find that (untweaked) cost and metric are such that all
nodes are equal. The last resort (peer router ID) gets invoked and all
traffic goes to one single instance. Of course, when that instance falls
off the net recalculation takes place and another node steps in, but
I'd like true path lengths (IGP hop count) to influence more than iBGP
(route-reflector-style) selection.

Any clues? 

Oh, all-cisco, all ASR1000 series. All links GE. ~90 routers in IGP. 

Måns Nilsson     primary/secondary/besserwisser/machina
MN-1334-RIPE                             +46 705 989668
... this must be what it's like to be a COLLEGE GRADUATE!!
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