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Router on a Stick performance issue


Hi all,

I have a router-on-a-stick scenario where I have 7-8 subnets configured as sub-interface on a gigabit interface on a Cisco 3825 router. Only 3 VLANs are active now.

Say vlan 10=pc data, vlan11=voice, vlan1=apple MAC server.

When an apple PC/laptop connect to vlan10 and need access to vlan1, the performace is horrible, but when apple pc/laptop is moved to the same vlan, performance is way faster.

Gigabit interface on the router shows only 100M utilized.

However I am getting alot of throttles on input queue of the router interface, lots of input queue drops and flushes, and lots of encapulation errors/fragmentaton when I did "show ip traffic".

Seems like a Router-on-a-stick issue but I am not convinced because first of all, the Gigabit is only used 100M, 2nd, maybe the apple pc/laptop is generating a bigger packets than 1500bytes.

Is that because I have too many sub-interfaces? Is there any max number of sub-interface I can have? Is this because of extra Dot1Q tagging which cause a lots of fragmentation/encasulation errors?

I understand that all sub-interface share the same bandwidth, but if I am not even hitting the high water mark, how can I prove that it is a bottle neck?

4 Replies 4

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Suidong,

if you suspect an mtu problem you can try to increase it on the main gigabit ethernet interface and on subinterfaces.

To verify the MTU you can use an extended ping with size 1500 and DF bit set: see if you can send these packets between different Vlans.

About performance, every packet is received on Vlan X and sent out Vlan Y so in some way it counts double.

The key parameter is pps(packets per second) more then the aggregate traffic volume.

I agree that you shouldn't be facing a performance problem.

Hope to help


VIP Alumni
VIP Alumni

Check to see what kind of utilization you are getting on that link . Any traffic that has to be routed between vlans travels up that link to be routed and then back down that same link to the other vlan along with any traffic going off the router to the wan etc.. With the errors you sseeing you probably seeing at least bursts that may be tieing up that 100 meg link . Check nic settings on the devices.

Joseph W. Doherty
Hall of Fame
Hall of Fame

One simple issue you might be bumping up against, a 3825 is not a high performance LAN router. I don't recall whether 3825 interfaces support jumbo Ethernet, but if they don't, MTU fragmentation can also drag down a router's performance. What does the router's CPU usage look like when it's busy?

Performance with the same VLAN being better is likely due to the router being bypassed.

If the router is overstressed, you might consider obtaining a L3 switch, the 8 port 3560 might be ideal for you.


perform "show idb". (Interface descriptor blocks). The 3825 series has a maximum of 1200 (sub)interfaces with 12.3 IOS. So I dont think that is your problem


Also check the interface queue, whether FIFO or fari-queue. may want to increase input queue.

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