Some ADSL users connot browse web page, but they can ping through any Internet IP address even URL. At the same time, the other ADSL users who access Internet using same path can browse web page normally.For these user group, we deploy VPLS between the router with BRAS and the router with DSLAM, and we deploy VLAN stacking in DSLAM(MA5300), the L2VPN bear the datagram with 2 layer VLAN tag.
And do PING from BRAS to DSLAM, only 1450Bytes packet can be transmitted successfully.
we do some testing base on increasing the MTU prameters in transmission, and make an extension PING with 1500Bytes packets from BRAS to DSLAM(also through L2 MPLS VPN, before only can througe 1450B packets maxmum), it's successful.
Then we switch back the service traffic, everything become to OK.
In the interface which is binding L2VC, the received packet cannot be fragmented to the interface MTU value, so the size of the packet depend on upstream MTU parameter (in this case, it's modem, different kind of modem has different default MTU parameter). The router interface sometimes receive big size packet, and add two layer MPLS lable, then send it out to transmission, it results in the size of packet beyond the MTU parameter of transmission when it's up to transmission interface, so the packet will be dropped.