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Due to Improper Configuration, the Network Is Inaccessible When One Uplink Is Down In the Dual-Uplink ME60 Network

Publication Date:  2013-09-30 Views:  39 Downloads:  0
Issue Description

Version: ME60 V600R002C02SPC700
Networking: An ME60 is dual-homed to the remote router
Symptom: The ME60 is configured with two uplinks: g5/0/0 and g6/0/0. When the g5/0/0 is down, subscribers cannot access the network. The normal process is that the traffic goes out through g6/0/0, but no traffic is checked on the g6/0/0. After the g5/0/0 recovers, the g6/0/0 can also forward traffic and services recover too.
The route entry for the dual-uplink is shown as follows:

<ME60>display ip routing-table

Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
        Destinations : 3791     Routes : 3935

Destination/Mask    Proto  Pre  Cost       Flags NextHop         Interface

        0.0.0.0/0   ISIS   160  3000         D   60.160.219.225  GigabitEthernet6/0/0
                    ISIS   160  3000         D   60.160.219.221  GigabitEthernet5/0/0

No other alarms is found in the ME60 logs except for that the g5/0/0 interface is Down.
Handling Process

Possible cause:

1. The transmission problem impacts on the two links.
2. The peer end device is abnormal.
3. Improper configuration (This is the cause to the problem in this case)

Handling process:

1. Check the transmission and no problem is found. In addition, the status of g6/0/0 ISIS neighbor is normal.
2. Check the device at the peer end and no problem is found.
3. Check the configuration of the ME60. Both the g5/0/0 and g6/0/0 interface are configured with "ip urpf loose" but the "allow-default" parameter is not configured. Therefore, for packets that match the default route, they cannot pass the URPF.
For the ME60 (V600R002), when there are multiple egress load balancing, route 0.0.0.0 in common route. However, when there is only one egress port, route 0.0.0.0 is a default route and it cannot pass the URPF detection. Therefore, the network cannot be connected after the g5/0/0 is Down.
4. The problem is resolved when the interface configuration is changed to "ip urpf loose allow-default".
Root Cause
The "ip urpf loose" parameter is configured but "allow-default" is not configured, resulting in URPF detection failure of the packets.
Solution
Change the interface configuration to "ip urpf loose allow-default."
Suggestions
The default route is used when users attached to the BRAS connect to the network. When configuring the URPF for an ME60 (V600R002), remember to configure the allow-default parameter.

END