On Tue, Aug 14, 2012 at 1:00 PM, Tobias Heister <li...@tobias-heister.de> wrote: > Hi > > Am 14.08.2012 15:12, schrieb Markus: >> Isn't that weird? Where did that arp entry come from and why was it saved on >> the Juniper for so long, and only got removed after I removed the static >> routing of that /24? > > We saw a similar thing a short time ago on an MX480 running 10.4R9 > In our case it was a bgp route pointing to a no longer existing ip address as > the next-hop. The arp entry for this ip address stayed active as long as > there was an active route for it. > Even clearing the arp cache witch clear arp hostname x.x.x.x did not do the > trick. The next-hop ip was gone for several weeks and the arp entry had low > timeout values left but never expired. > After clearing the route the arp entry vanished as expected. > > I guess something keeps the arp entry from being deleted as long as there are > or were forwarding entries in the fib for it at any time.
Probably because the underlying information ARP is learning is used to build the next-hop in the forwarding table (which needs to know what Ethernet address to put in the destination MAC). However, I would think that the route should become unreachable or pruned if ARP is failing. What if the remote router died for some reason? If the ARP entry and next-hop were kept into place, the path would not work, but the route would stay active. A dynamic routing protocol and BFD would be see this right away and move traffic, but this would break any static routes that rely on any dynamism with ARP and next-hops. Moral of the story, as I see it: avoid static routing. --j _______________________________________________ juniper-nsp mailing list juniper-nsp@puck.nether.net https://puck.nether.net/mailman/listinfo/juniper-nsp
