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reducing router latency

I have a multi-VLAN LAN built from 4506's with a 3508G core. The 4506's are layer-2 only. I have a 3660 with 1000BaseT interface connected to one of the ports on the core as a one-armed router (it also has WAN interfaces, but those aren't relevent here). The main user VLAN is the native VLAN for the trunks connecting the access switches & router to the core. There are other VLANs as 802.1q subinterfaces on that same GB router interface. Router is configured for CEF switching.

Some users are complaining of latencies between the main (native) VLAN and one of the subinterface VLANs that are 2-3X as long as latencies within a single VLAN (either the subif'ed one or the native one) and that this translates into proportionally slow performance of their application. Ping times when not passing thru the router are ~120-150 usec vs ~270-350 usec when passing thru the router.

Another possibly contributing factor is that when passing thru the router, one of the systems is also connected thru an IP phone (on the native VLAN).

Interface utilizations are very small (<1%). Proc util is small. No queues building anywhere. Might it help to move the native VLAN to a subif on the router?

Are these differences reasonable and any suggestions to reduce the routed latency?

1 REPLY
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Re: reducing router latency

The first thing to check is to make certain that these packets are being cef switched--I'm not certain if you mean "native" vlan in terms of the vlan coming in to its own port, or if you are switching into and out of the same interface on this native vlan, rather than between two subinterfaces. If you are switching into and out of the same logical interface, I'd probably move the native vlan to a subinterface to see if that increases your switching speed.

Other than that, take a look at show cef not, and see if the traffic is being cef switched at all.

Russ.W

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