With IPHost Network Monitor you can run simple snmp requests against a Nortel device in your network.
vrPpScMtu
Virtual Router Passport Sc Mtu
This attribute indicates the Layer 2 MTU (that is, CPCS-SDU for ATM) of this connection.
Back to Nortel-Magellan-Passport-ShortcutConnectionMIB MIB page.
IPHost Network monitor allows you to monitor vrPpScMtu on Allied Telesis device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Nortel multiplexers right now.