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Re: flow/packet loss through L3 C3560, pings OK

 
 
stephen
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      05-01-2008
"Phil Begriffenfeldt" <(E-Mail Removed)> wrote in
message news:MoWdncFmX8mt04TVnZ2dnUVZ_gSdnZ2d@visi...
> I have a Catalyst 3560G that is doing L3 routing. I tried to use it as
> default gateway for a web cluster, which was doing about 120mbps of
> traffic, 5kpps each in and out. However, users noticed slow page loads,
> broken inline images, etc.
>
> I was able to ping all the servers from outside the 3560G with zero
> packet loss in tens of thousands of 1500-byte pings. I moved the web
> cluster to a C6509 (same interface config) and the issue disappeared.


we had some issues with the 10/100 versions with buffer tuning where we had
problems with traffic bursts overwhelming the buffers, esp when you turn QoS
on as you effectively reduce the buffer pool for any 1 QoS type by 75%.

If you have several GigE connected servers contending for a congested or
rate limited port this could be an issue.

there are some commands to look at the buffers - something like
show platform port-asic statistics..... you want the drop stats for any
overloaded outbound ports.
>
> Web client experience was noticably impacted, so if it were simple
> packet loss, I think I would have seen it with ping. It seemed as
> though the issue was related either to the type of traffic (plain http)
> or flow (lots of flows).
>
> The 3560 has a pretty vanilla config; the web cluster traffic was being
> routed between a "no switchport" interface and a Vlan interface. I did
> notice that the "no switchport" interface had "ip route-cache
> same-interface" configured, and I'm not sure why. Also, the 3560 is
> carrying about 7k external routes, but I monitor it to make sure it
> doesn't hit the limit. I didn't see any clues in syslog.


you need "sdm prefer routing" in the config to handle lots of IP routes - if
not they overflow the hardware forwarding table and get dealt with in
software.
>
> Phil

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stephen
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      05-02-2008
"Phil Begriffenfeldt" <(E-Mail Removed)> wrote in
message news:eaidnQwIL7Vqi4bVnZ2dnUVZ_rSrnZ2d@visi...
> stephen wrote:
> > you need "sdm prefer routing" in the config to handle lots of IP routes

>
> Thanks for reminding me. I did set that last May (it's logged), and
> then power-cycled the switch, but I do not appear to have verified "show
> sdm" after the power cycle. Now I see that the switch is using
> default/desktop, which could be the source of my trouble. Weird.


yes - hardware forwarding tables will fill with 1 to 2k routes.

everything that arrives after the tables fill goes in software forwarding -
so whether it is an irritation or a disaster depends on the order the routes
arrive.

Not a fun thing to trouble shoot, but it does log an "out of space"
message - shame Cisco couldnt make it obvious what it is an error about....
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