Cisco VLAN – How to Prevent VLAN Traffic Flooding on All Interfaces

ciscoswitchvlan

This is very odd problem so place stay with me because it's very strange issue.

We have C3850 L3 core switch and we have created VLAN interface on it and we are using it for L3 routing.

I have other switches connected to C3850 core switches, so issue is if i send any traffic to any other server this switch forwarding that traffic to all its interface, just like SPAN port. ( We don't have any mirror session running on this switch)

Does anybody see this kind of issue in network? look like L3 flooding.

Look like this is happening Unicast Flooding: https://en.wikipedia.org/wiki/Unicast_flood

any idea what could be the issue of it?

Update:

After googling i found people saying reduce ARP timeout and increase MAC timeout, (Arp < Mac)

Cisco default Arp timeout is 4 hour, and Mac timeout is 300s (5min).

ARP timeout:

C3850#sh int port-channel 23 | in Timeout
  ARP type: ARPA, ARP Timeout 04:00:00

MAC timeout:

C3850#show mac address-table aging-time
Vlan    Aging Time  Configured Aging Time
----    ----------  ---------------------
Global Vlan Admin Age: 300

Should i change these values? what is the best practice?

Best Answer

It was HSRP asymmetric routing issue. We has HSRP and i found few VLANs are active on SW1 and few VLANs were active on SW2 and that was creating ARP flooding. as soon as i move all VLANs (HSRP) on single switch flood is gone.

here is the cisco doc: http://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/10583-62.html#t8

Related Topic