ARP Table Timeout and Mac-Address-Table Timeout

Frank Bulk frnkblk at iname.com
Sun Sep 14 22:19:53 CDT 2008


Steven:

This was recently discussed on cisco-nsp:
http://marc.info/?l=cisco-nsp&m=121316151010190&w=2

Frank

-----Original Message-----
From: Steven King [mailto:sking at kingrst.com] 
Sent: Sunday, September 14, 2008 7:27 PM
To: nanog at nanog.org
Subject: ARP Table Timeout and Mac-Address-Table Timeout

I am a network engineer for a large web hosting company. We are having
an issue with our distribution routers flooding traffic in one of our VLANs.

We have a customer with a routed mode ASA 5550. They have their own
private VLAN that is a /23 This VLAN is 145. The outside interface of
the firewall is in VLAN 132. We are routing all traffic for VLAN 145 to
the IP of the outside interface of the firewall in VLAN 132.

VLAN 132 is Layer3 routable and VLAN 145 is only Layer2 switchable.

We have two distribution switches which are redundant with HSRP. Dist1
is the active forwarder in this case. Traffic coming into these two
routers are load balanced between Dist1 and Dist2 with EIGRP routes with
equal cost.

We have found that traffic coming into Dist2 (the standby) is flooding
traffic destined for the firewall outside interface. But Dist1 is not.

We have tracked down the cause of this to the MAC-Address-Table timing
out before the ARP table times out. We leave these values at the Cisco
default. ARP = 4hr MAC = 5 minutes. Since Dist2 is not receiving any
traffic from the firewall going out to the internet, it is not updating
the MAC-Address-Table after it expires. Instead, it waits 4 hours for
the ARP cache to expire for that IP, and then updates everything. But
Dist2 ends up flooding traffic for that 4 hours causing latency.

We have done some research on this problem and have found so far the
best solution to be to make the ARP timeout less than the
MAC-Address-Table aging-timer.We have set the ARP = 1hr and MAC = 2hrs
in this case to correct the problem. So when the ARP entry times out
before the MAC entry, the forced update of the ARP entry before the MAC
timeout causes the MAC entry age to reset. Indeed this does correct the
problem.

Is this the best solution to the problem, or is there another preferred
solution? Has anyone ran into this in their own Enterprise Networks?

Please let me know if I didn't explain anything well enough.

--
Steve King

Network Engineer - Liquid Web, Inc.
Cisco Certified Network Associate
CompTIA Linux+ Certified Professional
CompTIA Network+ Certified Professional
CompTIA A+ Certified Professional







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