[c-nsp] l2tpv3 config - MTU question

[Ge Moua]

I was tackling a similar issue over here too, I think it may have to do 
with the fact that l2tpv3 and ethernet headers are taking some of the 
mtu allocation.

Regards,
Ge Moua | Email: moua0100 at umn.edu

Network Design Engineer
University of Minnesota | Networking & Telecommunications Services



Paul Stewart wrote:
> 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
>
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