Fault Symptom
On a network, three routers process user Internet services. Each router has two LPUs on which sub-interfaces terminate all user VLAN tags, and all users can normally access the network.
Later, one more router is added for expansion, and each router now has four LPUs. Since then, a lot of users make complaints that they fail to access the Internet. The analysis of the MAC addresses of the modems of those users shows that their modems are of the same brand.
Fault Analysis
1. Run the trace access-user object object-id command on any one of the routers to trace the users failing to log in. The command output shows that the router has received the PPP negotiation request but the negotiation process stopped at the LCP negotiation phase.
2. Get packets head on one of the modems. It is found that the modem sends a PADR packet after receiving the first PADO packet. After the router replies with a PADS packet, the modem does not complete PPP negotiation but directly sends a PADT packet to terminate the negotiation. The session ID of the PADT packet head is 0. It indicates that the modem processes only the PADO packets sent from the routers.
3. Users can access the Internet before the network expansion. The only change on the network after expansion is that the number of BAS interfaces increases. After the modem sends the PADI packet, the number of received PADO packets increases from 6 to 16. This may cause the failure of PPP negotiation.
Then, adjust the number of BAS interfaces that respond to the modem. A test shows that the modem counts the received PADO packets right after sending the PADI packet. If more than 10 PADO packets are received, the modem stops PPP negotiation.
Procedure
1. Reduce the number of BAS interfaces that respond to a user's authentication request through certain network optimization.
Summary
The protocol processing flow may vary with the brands or models of modems. In network planning, try to reduce the number of BAS interfaces that respond to a user's authentication request.
