[c-nsp] l2tp xconnect problem with packets > 1430

[Antonio Querubin]

I have two cisco 2821 routers running IOS 12.4(25e) configured with a l2tp 
xconnect.  The relevant portion of the configs are below.  Both routers 
can ping each other's loopback interface with large packets (> 1500).  But 
hosts off opposite ends of the xconnect cannot consistently ping each 
other with anything larger than 1430 bytes.  However, throughput between 
hosts across the tunnel running iperf is good (> 400 Mbps).

However, if I remove 'ip pmtu' and 'ip dfbit set' from the 
pseudowire-class, large pings (9000 byte packets) start to work but 
the throughput across the tunnel drops to less than 50 Mbps.

The switch interfaces attached to the xconnect endpoints are configured 
with MTUs of 1500 and so are all the hosts on the switches.


Router 1
========
ip cef
!
pseudowire-class pwubfc
  encapsulation l2tpv3
  ip local interface Loopback0
  ip pmtu
  ip dfbit set
!
interface Loopback0
  ip address 10.0.13.1 255.255.255.255
!
interface GigabitEthernet0/1
  no ip address
  duplex auto
  speed auto
  no cdp enable
  xconnect 10.0.13.2 1 pw-class pwubfc

Router 2
========
ip cef
!
pseudowire-class pwubfc
  encapsulation l2tpv3
  ip local interface Loopback0
  ip pmtu
  ip dfbit set
!
interface Loopback0
  ip address 10.0.13.2 255.255.255.255
!
interface GigabitEthernet0/1
  no ip address
  duplex auto
  speed auto
  no cdp enable
  xconnect 10.0.13.1 1 pw-class pwubfc


-- 
Antonio Querubin
e-mail:  tony at lavanauts.org
xmpp:  antonioquerubin at gmail.com