06-10-2008 09:45 AM
CiscoWorks DFM 2.0.10 generates high queue drop rate on Tunnel interface. Is this a bogus notification? There are no errors on the physical interface.
ROUTER#sh int tu0
Tunnel0 is up, line protocol is up
Hardware is Tunnel
Internet address is 192.168.61.2/30
MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel source 192.168.59.1 (Loopback0), destination 192.168.60.1
Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
Checksumming of packets disabled, fast tunneling enabled
Last input 3w3d, output 4w5d, output hang never
Last clearing of "show interface" counters 1d03h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 12 <==
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
85 packets input, 21672 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
84 packets output, 21019 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
06-10-2008 09:55 AM
This is not bogus. Given the low amount of traffic on this interface, 12 output drops is significant. The queue drop rate is ~ 15%. If this is acceptable, then you can increase the default threshold under DFM > Configuration > Polling and Thresholds > Managing Thresholds.
06-10-2008 10:10 AM
Tunnel interface is logical and there is no real buffer, isn't it?
06-10-2008 10:50 AM
Certainly no hardware buffer, but there is buffer memory carved out. Typically, output drops on tunnel interfaces occur when a packet is queued with the DF bit set, but the packet is larger than the tunnel's IP MTU, so they are dropped. Additionally, if the packet plus the GRE header is larger than the tunnel can handle (and the DF bit is set) the packet will be dropped.
06-10-2008 11:11 AM
So, can I adjust MTU on the Tunnel interface?
06-10-2008 11:12 AM
You might try:
ip mtu 1400
ip tcp adjust-mss 1380
As a start. But this might be a better question for the VPN or one of the routing forums.
06-10-2008 11:15 AM
Unfortunately, I have 12.2(24a) on the router. "ip tcp adjust-mss 1380" is not supported.
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