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pyite
join:2009-12-01
Salt Lake City, UT

pyite

Member

[Connectivity] Comcast, SMC 8014, painfully slow ARP changes

I am having a weird problem, when I move a static IP from one machine to another it doesn't work until I power cycle the cable modem (SMC 8014). It is almost as if the unit keeps the ARP information for hours before letting it change. Strangely, I can ping the address fine from the SMC 8014 administration interface, but it doesn't work over the internet at all.

I am trying to set up VRRP failover to match our data center environment, so this is a huge problem. I called Comcast and the tech said that this is how the equipment is supposed to work.

Is there any way around this problem? I haven't tried any NAT or DMZ settings yet and what they call "bridging" mode doesn't seem like bridging at all - it is just disabling DHCP.

Thanks,
Mark

JDCynical
Always Mount A Scratch Monkey
join:2002-01-09
Beaverton, OR

JDCynical

Member

As far as I can find searching around, there isn't a way to adjust the ARP cache timeout values on the SMC's. I hope someone can prove me wrong on that as I'd love to be able to adjust it myself. it's really annoying to have to power cycle it if I change out the firewall with different hardware.

And FYI, the SMC doesn't have a bridge mode. Comcast provisions business accounts with static addresses using a routed subnet (with the router being the SMC).

SpaethCo
Digital Plumber
MVM
join:2001-04-21
Minneapolis, MN

SpaethCo to pyite

MVM

to pyite
said by pyite:

I am trying to set up VRRP failover to match our data center environment, so this is a huge problem.
The default VRRP configuration calls for a virtual MAC that moves from the active to the standby host during a failover, so the ARP entry would never change.

You can configure VRRP to use the physical or burned-in MAC address, but that process requires that the device taking over the service IP transmit gratuitous ARP packets upon transitioning to active to update the ARP tables of all other devices on the segment.

A properly functioning SMC8014 will honor the gratuitous ARPs.
pyite
join:2009-12-01
Salt Lake City, UT

pyite

Member

Thanks for the reply!

I'm using keepalived which does gratuitous arp by default, but the 8014 isn't responding to it. Neither does my Adtran 916. Weird!

The latest keepalived supports VMAC, so I'll try that out instead.

Mark
pyite

pyite

Member

Well it sure looks like the 8014 is not responding to the gratuitous arp. I am using heartbeat's send_arp which does both an arp and arp-reply (verified with tcpdump) but the 8014 doesn't respond at all.

Weird!