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New Member

high cpu/multicast traffic


I have configured VLAN300 through 3 switches:

customer--C2960 - C7600 - C6500--customer

As it can be seen customer is connected via Ethernet(VLAN300).Customers has multicast traffic and on both sides sender and receivers.

I don't have multicast enabled on my switches, and regarding multicast configuration is default on all devices.

Problem starts when I configure Interface VLAN 300 on C7600, then sh proc cpu shows 100%/98% (which indicates that packets goes to CPU).

on C2960 and C6500 CPu utilisation is normal 3%.

I sniffed the traffic and packets are (50Mbps in each direction):

src IP: port UDP 49152 dst IP: port UDP 2001


src IP: port UDP 4915 dst IP: port UDP 1234

TTLs are 64, and also all checksums are correct.

when I put interface VLAN300 to shutdown, CPU utilisation on C7600 goes to 3%.

interface vlan300 doesn't have ip address, I was planning to use it just for policinig (to rate limit traffic on VLAN 300 - mls qos vlan-based)

C7600 has sup720BXL and IOS 12.2(18)SXF.

Why CPU utilisation goes to 100% when I just create interface VLAN, even without IP address?

Thanks in advance,


Hall of Fame Super Bronze

Re: high cpu/multicast traffic

If you configure multicast on a Vlan and you don't have PIM configured on any of the SVI, the IGMP Snooping service on switches won't work as designed.

In order to prevent this problem, you need a device with IGMP Snooping Querier configured on that Vlan and this device must have a SVI with an IP address assigned, in your case it can be the 7600 router.

Your issue is that when enabling the SVI - even without an IP address - unknown multicast/broadcast packets are being punted to the CPU.

For configuring IGMP Snooping Querier, please refer to the documentation:



New Member

Re: high cpu/multicast traffic

Thank you very much Edison.

Thing is that I provided layer 2 connectivity to the customer, and I didn't know what type of traffic he is going to run through this VLAN.

I only noticed that CPU utilisation was high if I created interface VLAN.

So if C7600 detects multicast traffic in i.e. VLAN 10 and has configured interface VLAN 10 this traffic will be punted to CPU.

Can I avoid this behavior on C7600 (somehow to disable multicast on this VLAN)?

Since I don't want to interfere in customer's multicast traffic, I would like to avoid configuring "IGMP Snooping Querier".

Is there some other way or I must remove "interface VLAN 10"?



Hall of Fame Super Bronze

Re: high cpu/multicast traffic

If Vlan 10 has an IP address the punting will be minimal but ideally you need to have a querier on a L3 SVI or just enabling PIM on the interface.



New Member

Re: high cpu/multicast traffic

Hi Edison,

I tried all 3 solutions which you had suggested but CPU utilisation doesn't drop below 80%-90%.

I tried:

1) adding IP address on interface VLAN10

2) adding IP address on Interface VLAN 10 and enabling PIM on that interface

router(config-if)#ip pim sparse-mode
WARNING: "ip multicast-routing" is not configured,
            IP Multicast packets will not be forwarded

3) adding IP address and enabling IP snooping querirer



Hall of Fame Super Bronze

Re: high cpu/multicast traffic

On the 2nd option, did you enable multicast-routing? You need to do so in order for PIM to work.

It seems like you are being affected by a software bug so I recommend opening a TAC case to verify if that's the case.



New Member

Re: high cpu/multicast traffic

I didn't enable multicast globaly. I don't know how would it affect other traffic since

this router is in production.

Thanks for advice Edison.