[j-nsp] MTU issue on Martini Tunnel

Junaid junaid.x86 at gmail.com
Wed Jul 23 02:59:16 EDT 2008


I am trying to test a Martini Tunnel between an ERX1440 and M10i
router. Physical topology is as follows:

Host1 --e-- ADSL CPE1 ---a--- ATM Cloud ---a--- ERX(PE1) ---e---
M-series(P) ---e--- M10i(PE2) ---a--- ATM Cloud ---a--- ADSL CPE2
--e-- Host2

--e-- : Ethernet connectivity
--a-- : ATM connectivity

ERX and M10i has IP connectivity via another M-series router in
between. I am trying to connect CPE1 and CPE2 via a Martini tunnel so
that they are connected via an end-to-end PVC and the two LANs (shown
as Host1 and Host2) are connected to each other on the same L2
broadcast domain. Layer 2 protocol is Ethernet (VLANs exist on core)
except around the ATM cloud where it is ATM. ADSL CPEs are in bridge
mode. I am using LDP for signaling. The L2 circuit comes up and the
two hosts are able to ping each other but only with a maximum IP
packet size of 1468. If I break the Martini and terminate the two PVCs
on the two PEs directly creating a sub-interface and assigning IPs, I
am able to ping between hosts with a maximum IP packet size of 1500
bytes. Why is the MTU reduced by 32 bytes when I connect the hosts via
a Martini tunnel.

I have set an interface MTU of 1684 and IP mtu of 1500 on core links;
between P and PEs.

Config on ERX and M10i are as follows:

interface atm 13/0.10101
 atm pvc 10101 0 62 aal5all 0 0 0
 mpls-relay <Loopback IP address of M10i> 786

#show configuration interfaces at-0/2/1
mtu 9192;
clocking external;
encapsulation atm-pvc;
atm-options {
    pic-type atm2;
    promiscuous-mode {
        vpi 1;

#show configuration interfaces at-0/2/1.786
encapsulation atm-ccc-vc-mux;
vci 1.101;

#show configuration protocols l2circuit
neighbor <Loopback IP address of ERX> {
    interface at-0/2/1.786 {
        virtual-circuit-id 786;

Any help/pointers?


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