[c-nsp] l2tpv3 config - MTU question

[Paul Stewart]

Hi folks.

 

I've setup a pair of 1841's back to back for testing l2tpv3 deployment for a
client..

 

FastE0/0 from each 1841 is connected to one another at 10.0.0.0/24 - each
router has a loopback of 192.168.254.1 and .2  - OSPF is running and am able
to successfully ping each other's loopback with redistributed subnets etc..

 

Configured each router to look like this:

 

pseudowire-class test

 encapsulation l2tpv3

 sequencing both

 ip local interface Loopback0

 

interface FastEthernet0/0

 ip address 10.0.0.2 255.255.255.0

 duplex auto

 speed auto

 

interface FastEthernet0/1

 no ip address

 duplex auto

 speed auto

 no cdp enable

 xconnect 192.168.254.2 1234 pw-class test

 

Have a notebook hooked up to each FastE0/1 port and assigned 172.16.0.1 and
.2 on them.  I can ping back and forth proving connectivity etc.

 

My problem/question is how to get a packet of 1500 bytes to transverse the
link - obviously fragmented but that's ok.    In the real-world deployment
of this setup we are limited to 1500 MTU in most situations and will presume
no mini-jumbo support anywhere (from a config perspective at least).

 

In my first config I had Path MTU discovery enabled and could only ping up
to 1440 bytes.  With that disabled I can now ping to 1472 but not beyond.  

 

With Path MTU turned on it looked like this:

 

site2#sh l2tun session all

 

%No active L2F tunnels

 

L2TP Session Information Total tunnels 1 sessions 1

 

Session id 53211 is up, tunnel id 32076

Call serial number is 1293300000

Remote tunnel name is site1

  Internet address is 192.168.254.1

  Session is L2TP signalled

  Session state is established, time since change 00:26:44

    114 Packets sent, 116 received

    30446 Bytes sent, 29032 received

  Last clearing of "show vpdn" counters never

    Receive packets dropped:

      out-of-order:             0

      total:                    0

    Send packets dropped:

      exceeded session MTU:     1

      total:                    1

  Session vcid is 1234

  Session Layer 2 circuit, type is Ethernet, name is FastEthernet0/1

  Circuit state is UP

    Remote session id is 22201, remote tunnel id 12358

  Session PMTU enabled, path MTU is 1500 bytes

  DF bit on, ToS reflect disabled, ToS value 0, TTL value 255

  No session cookie information available

  UDP checksums are disabled

  SSS switching enabled

  Sequencing is on

    Ns 114, Nr 116, 0 out of order packets received

  Unique ID is 1

 

%No active PPTP tunnels

 

 

Upon looking further I could see the DF bit on which I believe would explain
the 1440 byte limit I hit.  But with that disabled I am puzzled or missing
something as to why I cannot fragment packets up to full 1500?   What I am
missing here?  Do I need to make MTU adjustments towards the FastE0/1
interface to force fragmentation before the l2tpv3 tunnel?

 

Thanks in advance,

 

Paul