MTU Issues

[Shahid Ishaq]

Hello,

I have configured a cisco router as the one I have from my provider is not really good and keeps dropping wifi.

Internet is now working fine from the Cisco, the other day when I was playing PS4 using LAN connection and the sync speed was showing as 16Mbps while the game was loading. I then tried the same test using my ISP router and the sync speed was showing as 54Mpbs. Can someone advise where I'm going wrong or missing something.

I used an on-line utility to to determine the MTU size.

Sending 32 bytes to 90.253.204.247 <- not fragmented
Sending 750 bytes to 90.253.204.247 <- not fragmented
Sending 1125 bytes to 90.253.204.247 <- not fragmented
Sending 1313 bytes to 90.253.204.247 <- not fragmented
Sending 1407 bytes to 90.253.204.247 <- not fragmented
Sending 1454 bytes to 90.253.204.247 <- not fragmented
Sending 1478 bytes to 90.253.204.247 <- FRAGMENTED!
Sending 1466 bytes to 90.253.204.247 <- FRAGMENTED!
Sending 1460 bytes to 90.253.204.247 <- not fragmented
Sending 1463 bytes to 90.253.204.247 <- not fragmented
Sending 1465 bytes to 90.253.204.247 <- FRAGMENTED!
Sending 1464 bytes to 90.253.204.247 <- not fragmented
Sending 1465 bytes to 90.253.204.247 <- FRAGMENTED!
Sending 1464 bytes to 90.253.204.247 <- not fragmented

From the tests we did, we can assume that 1464 bytes is the largest unfragmented packet size. The MTU size would be 1492, made up from 1464 payload and 28 ICMP/IP Headers and payload information.

Below are my VLAN and dialer interface config.

interface Vlan200
description LOCAL_LAN
ip address 192.168.200.1 255.255.255.0
ip nat inside
ip virtual-reassembly in
ip tcp adjust-mss 1452
no autostate
!
interface Dialer0
description (WAN PORT)
bandwidth 80000
ip address negotiated
no ip redirects
no ip unreachables
no ip proxy-arp
ip mtu 1492
ip flow ingress
ip nat outside
ip virtual-reassembly in
encapsulation ppp
ip tcp adjust-mss 1452

[Giuseppe Larosa]

Hello @Shahid Ishaq ,

what is the model of your router post a show version.

Rather then MTU issues you may be facing different performance on the Cisco router compared to the ISP provided router.

How did you performed the BW test ?

You may want to issue the commands ip mtu 1492 and ip tcp mss 1452 also on SVI interface vlan 200.

I think it is appropriate to do that.

Hope to help

Giuseppe

[Richard Burts]

With performance issues it is sometimes helpful to look for mismatch of duplex (one side is full while other side is hall).

[Shahid Ishaq]

@Richard Burts 

The router is directly connected to my ISP, I have now hard coded my interfaces as full duplex.

Will test later and update.

[Richard Burts]

It might be helpful if you post the output of show version so that we have information about your router. Also the output of show interface might be helpful. 

[Shahid Ishaq]

@Richard Burts 

So should the BW on VLAN 200 be configured to that of D0?

Cisco887_ISP#show int d0
Dialer0 is up (spoofing), line protocol is up (spoofing)
Hardware is Unknown
Description: (WAN PORT CONNECTED TO ISP)
Internet address will be negotiated using IPCP
MTU 1500 bytes, BW 80000 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, LCP Closed, loopback not set
Keepalive set (10 sec)
DTR is pulsed for 1 seconds on reset
Last input never, output never, output hang never
Last clearing of "show interface" counters 1d07h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/1000/64/0 (size/max total/threshold/drops)
Conversations 0/0/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 60000 kilobits/sec
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes
0 packets output, 0 bytes

Cisco887_ISP#show int vlan 200
Vlan200 is up, line protocol is up
Hardware is EtherSVI, address is fc99.47d8.1d0a (bia fc99.47d8.1d0a)
Description: LOCAL WIFI
Internet address is 192.168.200.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive not supported
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
1 packets output, 60 bytes, 0 underruns
0 output errors, 1 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

[Richard Burts]

Thanks for the output. Both the dialer interface and the vlan interface are virtual interfaces and as such their reported bandwidth does not reflect anything about performance. To really look at performance we would need to look at physical interfaces where the traffic flows. 

Since it it appears that the outbound interface is dialer can you tell us a bit more about the connection to your ISP? And can you post show interface for the physical interfaces used for the traffic?

[Shahid Ishaq]

Hello,

So when trying to add ip mtu under vlan 200 seems like max supported on this router is 1452.

Cisco887_ISP(config-if)#ip mtu ?
<68-1452> MTU (bytes)

The MTU i can set under dialer 0 is as below:

Cisco887_ISP(config-if)#ip mtu ?
<68-1500> MTU (bytes)

The test was performed using http://www.letmecheck.it/mtu-test.php