[c-nsp] TCP Throughput / MTU problem ? with Cisco 7304 and MPLS VPN's

[Peter Rathlev]

I can see I was a little fast there. I didn't read correctly what you
wrote. My bad. :-)

But you still can't (shouldn't) use an interface MTU lower that the MPLS
MTU. Your interface MTU should be larger than the "X" MTU for any "X".

Regards,
Peter

On Sat, 2008-01-12 at 01:15 +0100, Peter Rathlev wrote:
> On Fri, 2008-01-11 at 23:43 +0100, Mikael Abrahamsson wrote:
> > On Fri, 11 Jan 2008, Peter Rathlev wrote:
> > > tried "mtu 1520" and "mpls mtu 1520" as stated in the document? OTOH it
> > > should have complained if you configured an MPLS MTU larger that the
> > > interface MTU.
> > 
> > It works just fine to have mpls mtu higher than interface MTU, in effect 
> > these two do the same thing (with reservation that I might have forgotten 
> > some protocol in example 1):
> > 
> > example 1:
> > int gi0/0
> > mtu 1546
> > ip mtu 1500
> > clns mtu 1500
> > 
> > example 2:
> > int gi0/0
> > mpls mtu 1546
> > 
> > All protocols inherit "mtu", if you do "mpls mtu" then only mpls will have 
> > the higher mtu and all other protocols will inherit the default ethernet 
> > 1500 mtu.
> 
> That's not how it works where I am. Adjusting "mpls mtu" does not change
> interface or IP MTU. Adjusting interface MTU automatically changes the
> IP MTU though (and MTU for other protocols). I could be wrong, but
> that's what I've read, and that's the way our 7200s, 6500s and 7600s
> work.
> 
> You can often use a higher "mpls mtu" with no problems, but that's not
> always the case. It depends on the interface drivers. From 12.4(11)T and
> on you cannot configure an MPLS MTU larger than the interface MTU,
> except for Ethernet interfaces with a maximum MTU below 1580 bytes. (And
> that's not guaranteed to work.)
> 
> Example of adjusting MTU with a 7206VXR NPE-G1 running 12.3(6), using
> the on-board interfaces:
> 
> Router#show interface GigabitEthernet0/2 | incl MTU
>   MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec, 
> Router#show ip interface GigabitEthernet0/2 | incl MTU
>   MTU is 1500 bytes
> Router#configure terminal
> Enter configuration commands, one per line.  End with CNTL/Z.
> Router(config)#interface GigabitEthernet0/2
> Router(config-if)#mpls mtu 1546
> Router(config-if)#^Z
> Router#show interface GigabitEthernet0/2 | incl MTU
>   MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec, 
> Router#show ip interface GigabitEthernet0/2 | incl MTU
>   MTU is 1500 bytes
> Router#configure terminal
> Enter configuration commands, one per line.  End with CNTL/Z.
> Router(config)#interface GigabitEthernet0/2
> Router(config-if)#default mpls mtu
> Router(config-if)#mtu 1546
> Router(config-if)#^Z
> Router#show interface GigabitEthernet0/2 | incl MTU
>   MTU 1546 bytes, BW 1000000 Kbit, DLY 10 usec, 
> Router#show ip interface GigabitEthernet0/2 | incl MTU
>   MTU is 1546 bytes
> Router#show version
> Cisco Internetwork Operating System Software 
> IOS (tm) 7200 Software (C7200-JS-M), Version 12.3(6), RELEASE SOFTWARE
> (fc3)
> Copyright (c) 1986-2004 by cisco Systems, Inc.
> Compiled Wed 11-Feb-04 15:56 by kellythw
> Image text-base: 0x60008AF4, data-base: 0x61FF4000
> 
> ROM: System Bootstrap, Version 12.2(20030826:190624)
> [BLD-npeg1_rommon_r11 102], DEVELOPMENT SOFTWARE
> BOOTLDR: 7200 Software (C7200-KBOOT-M), Version 12.2(15)B, EARLY
> DEPLOYMENT RELEASE SOFTWARE (fc1)
> 
> Router uptime is 1 year, 32 weeks, 13 hours, 1 minute
> System returned to ROM by reload at 13:00:55 cet Thu Jun 1 2006
> System restarted at 13:03:15 cet Thu Jun 1 2006
> System image file is "disk2:c7200-js-mz.123-6.bin"
> Last reload reason: Reload command
> 
> 
> cisco 7206VXR (NPE-G1) processor (revision A) with 491520K/32768K bytes
> of memory.
> Processor board ID 31521866
> SB-1 CPU at 700MHz, Implementation 1, Rev 0.2, 512KB L2 Cache
> 6 slot VXR midplane, Version 2.7
> 
> Last reset from power-on
> Bridging software.
> X.25 software, Version 3.0.0.
> SuperLAT software (copyright 1990 by Meridian Technology Corp).
> TN3270 Emulation software.
> 
> PCI bus mb1 has 0 bandwidth points
> PCI bus mb2 has 0 bandwidth points
> 
> 3 Gigabit Ethernet/IEEE 802.3 interface(s)
> 509K bytes of non-volatile configuration memory.
> 
> 62976K bytes of ATA PCMCIA card at slot 2 (Sector size 512 bytes).
> 16384K bytes of Flash internal SIMM (Sector size 256K).
> Configuration register is 0x2102
> 
> Router#
> 
> Regards,
> Peter
> 
> 
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