How can I solve the problem of ARP Cache in Cisco Catalyst 6506 Core Switch?

Unanswered Question
Apr 11th, 2010

We have problem of ARP cache on Core sw 6506 . Topology for this is as per attachment.

Problem: Pcs which are turned off for 3- 4 hour can not communicated. from this PCs some of Devices and some of Servers also can not pinging . Some of PCs have an error : Reply from x.x.x.x(Gateway):TTL expires in transit.

Now if i ping this effected PC from MSFC than its works well. Before it i have checked sh arp | i x.x.x.x ( PC Ip address ) than I can not find it. But After pinging from MSFC i can found in ARP list. So I suspect it is due to ARP cache.

I had tried to clear all arp but the problem is still exist.

This problem is in all PCs with static IPs as in PCs with DHCP - Continuously communicated so not isolated.

I have this problem too.
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Giuseppe Larosa Sun, 04/11/2010 - 23:20

Hello Advait,

there is no attachment file.

Also you should add more details:

what supervisor model is in the chassis and what operating system is running, or are running?

6506 can range from sup1A/MSFC2 in hybrid mode to Sup 720 3CXL in Native IOS so it is quite different.

Have you configured any feature like Dynamic ARP inspection that could interfere with ARP protocol?

or for example Port security or DHCP snooping or IP Source Guard ?

provide more details in order to get better help.

Parts of configuration file related to affected vlans would be useful.

Hope to help


advait.sharma Sun, 04/11/2010 - 23:35

There are 2 cisco6506 with  2 MSFC WS-SUP720-3B

HSRP has been configured on both Core.

There are total 65 Vlans.

Half Vlan are active on MSFC1 & half on MSFC2

I am facing above mentioned problem in 4 particular Vlans.

And yes port security has been applied on around 2000 Desktops.

I have added Dhcp server IP address as helper IP address XXX.XXX.XXX.XX on all vlans

Configuration of vlans is as follows:


interface Vlan14
description ***** OTC Segment VLAN *****
ip address alt ip address
ip helper-address
ip helper-address
no ip redirects
standby 14 priority 115 alt standby 14 priority 120 preempt
standby 14 ip alt standby 14 ip
hold-queue 125 in


interface Vlan14
description ***** OTC Segment VLAN *****
ip address alt ip address
ip helper-address
ip helper-address
no ip redirects
standby 14 priority 110 alt standby 14 priority 105
standby 14 ip alt standby 14 ip
hold-queue 125 in

Giuseppe Larosa Mon, 04/12/2010 - 01:13

Hello Advait,

you have sup720 3B, but from your configuration I guess you have two supervisors and two MSFC on each chassis.

Also from the fact you are using the alternate keyword I understand you are using dual router mode DRM.

Dual routing mode means when both MSFCs in same chassis are active and have different IP addresses in each vlan.

This was the preferred mode in the past.

Be aware that single routing mode is recommended SRM in modern C6500.

there is little sense in having 4 MSFCs

and keeping two shut down on the second device from a redundancy point of view

May you confirm you have two MSFC in each chassis?

post a sh module

to see this

I will look for how to detect the routing mode and how to move to single routing mode later if you have two MSFCs in each chassis

Hope to help


advait.sharma Mon, 04/12/2010 - 01:54

Thats right, we have 2 MSFC in 1core. i.e total 4 MSFC in 2 Cores.

At at time only 1 MSFC is active in 1 chassis and another is in standby mode.

Inter-vlan static route is configured on both MSFCs:

Now what happened when I shutted Vlan14 from MSFC 1. It automatically got active on MSFC2 But problem is Desktop PCs stopped communicating in inter-vlan aswell as in Outer Vlan. PCs starts communicating only when I ping the PCS manually from MSFC2.

So I thought that problem is with ARP.

Giuseppe Larosa Mon, 04/12/2010 - 04:55

Hello Advait,

let's call the MSFCs in the following way:

MSFC1_1 first MSFC in chassis 1

MSFC1_2 second MSFC in chassis 1

MSFC2_1 first MSFC in chassis 2

MSFC2_2 second MSFC in chassis 2

given this  terminology what you have shut down?

SVI vlan 14 in MSFC1_1 and you would expect to see MSFC2_1 to take over ?

what makes you think that MSFC1_2 and MSFC2_2 are not active?

Hope to help


marcosaner Thu, 09/02/2010 - 06:45

Hello Advait

You have ajust the ARP timer of you 65XX to 300 sec.




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