Router CPU Utilisation

[sreeraj.murali]

Hi, I have a Router, on which the GRE Tunnel is running to US. Realtime, it is not passing traffic, as this Router is a backup for the MPLS Connectivity to US. Also, the same Router is acting as the Internet Router, on which the NAT overload is setup. So, at Realtime, local Internet traffic is processed by the Router. I am seeing HIGH CPU Utilisation, very often on the Router. Attaching the "show tech" output as well. Please help me in  checking and let me know, what is causing the high CPU Utilisation.

Thanks & Regards

Sreeraj

[Tagir Temirgaliyev]

what is the router?

what is GRE Tunnel bandwidth?

and what is CPU utilisation?

as I remember GRE is proccess switching. not CEF switching.

your file isnot attached

[sreeraj.murali]

Hi,

Missed to attach the show tech ouput. Attaching the same. The Router is Cisco 2921.

Thanks & rEGARDS

Sreeraj

[Joseph W. Doherty]

I see the history of  "high" CPU, but your CPU process stats don't show any lingering processes with high usage (at least for the last 5 minutes).  So, it's possible your router is doing interrupt processing and is just occasionally being bogged down by a burst of traffic.  Realize although the 2921 has gig interfaces, it's not a gig capable router (in fact, Cisco recommends it for up to 50 Mbps of aggregate traffic - i.e. is not really a FE capable router either).

I also noticed ingress queue drops, and many input errors on g0/1.

[Tagir Temirgaliyev]

check and adjast MTU

ping a.b.c.d df size 1500

ping a.b.c.d df size 1480

ping a.b.c.d df size 1460

ping a.b.c.d df size 1440

so you will see actual MTU

if router needs to fragment packets so CPU will make it and CPU utilisation will increase

in your case 1420 is error. because you have GRE over IPSEC

interface Tunnel1
 
 ip tcp adjust-mss 1420

[Julio E. Moisa]

Hi

I have had experiences when the GRE tunnels increase the CPU utilization, I recommend to configure netflow if it is supported on your device n order to determinate the kind of traffic passing through your interfaces including the GRE tunnel. Also try to adjust your MTU on the tunnels to 1400 to get space for the header.

int tu0  (both ends)
ip mtu 1400 (or 1490 at least)
ip tcp adjust-mss 1360

ip flow-top-talkers
ip flow-export version 9
ip flow-export source loopback 0
cache-timeout 10000
sort-by bytes
top 50

interfaces G x/z
ip flow ingress
ip flow egress (optional)

Hope it is useful.

:-)

[Joseph W. Doherty]

BTW, Julio makes a good suggestion to allow for GRE packet overhead (to avoid fragmenting packets).  GRE overhead is 24 bytes, so IP MTU (for 1500) should be at least 1476 and its corresponding adjust-mss 1436.

Cisco has a great white paper on this: http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html