Bandwidth / CPU Usage issue with Cisco 1841 router

[aperezro]

Hello,

I'm having some problems with my 1841 router, that I run on my home lab.

Currently, I have 100Mbps Down / 6Mbps Up internet connection with cable ISP on my home.

The issue that I'm having is, my 1841 router will not go above 41Mbps of bandwidth and, as soon as bandwidth peaks at 41Mbps, CPU usage shoots up.

This issue does NOT happen if I connect my computer directly to ISP modem and if i bypass 1841 router, just using the AC86U router alone with the ISP modem, I get the correct bandwidth.

Opening a case with TAC for this is NOT an option since the router is EOL and also I don't own any service contract for it / router is definitively not under warranty due to how old it is.

Any help would be appreciated, network topology and 1841 router show running config is below.

Network topology is the following:
ISP Coaxial Cable > ISP Cable Modem > 1841 router interface FA0/0 > 1841 router interface FA0/1 > Asus AC86U router > Computer connected to port 1 of asus router

IP configuration is the following:
ISP coaxial cable interface: Public IP
ISP ethernet interface: 192.168.100.1 /24 (this interface also assigns a public IP via DHCP)
1841 FA0/0 : DHCP, assigned by ISP modem
1841 FA0/1: 10.10.10.1 / 30
AC86U WAN interface: Assigned via DHCP by the 1841 router
AC86U LAN interface 1: 10.10.30.2 /27

Here is my 1841 router's running config:

LAB#show run
Building configuration...

Current configuration : 1299 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname LAB
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
ip cef
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
no ip dhcp use vrf connected
ip dhcp excluded-address 10.10.10.0 10.10.10.1
!
ip dhcp pool AC86U
network 10.10.10.0 255.255.255.252
update dns both
default-router 10.10.10.1
dns-server 1.1.1.1 1.0.0.1
update arp
!
!
!
!
!
!
!
!
!
!
!
interface FastEthernet0/0
ip address dhcp
ip nat outside
ip virtual-reassembly
speed 100
full-duplex
!
interface FastEthernet0/1
ip address 10.10.10.1 255.255.255.252
ip nat inside
ip virtual-reassembly
speed 100
full-duplex
!
interface Serial0/0/0
no ip address
shutdown
no fair-queue
clock rate 2000000
!
interface Serial0/0/1
no ip address
shutdown
clock rate 2000000
!
ip forward-protocol nd
ip route 0.0.0.0 0.0.0.0 192.168.100.1
!
ip http server
no ip http secure-server
ip nat inside source list PAT interface FastEthernet0/0 overload
!
ip access-list extended PAT
permit ip 10.10.10.0 0.0.0.3 any
permit ip 10.10.30.0 0.0.0.31 any
!
!
!
control-plane
!
!
line con 0
line aux 0
line vty 0 4
login
!
scheduler allocate 20000 1000
end

[Georg Pauwen]

Hello,

the max WAN throughput for the 1841 is around 40Mbps, that is probably the limit you are hitting...

[aperezro]

thanks for the response.

How do you know if that is the case? is there any document that provides further detail into that limitation?

I'm not using any of the serial interfaces / Coax Cable interface, I'm using the Fast Ethernet 0/0 and 0/1 interfaces, they should support 100Mbps...

Best regards,

Aaron Perez Robles

[Georg Pauwen · Accepted solution]

Hello,

check the PDF attached. Throughput is actually listed as 38.40.

[aperezro]

That is quite the unfortunate caveat but thank you for providing this information.

[Joseph W. Doherty]

"the max WAN throughput for the 1841 is around 40Mbps, that is probably the limit you are hitting..."

Just an additional note, the maximum WAN throughput actual can vary much based on the traffic mix and the device configuration, although I agree OP is likely bumping into the device's maximum capacity, again based on his traffic and config.

I've attached a later Cisco performance document.  Unfortunately, it doesn't list the 1841, but it better represents how much an ISR's performance might vary based on traffic mix and configuration.