Description

 

This article explains a scenario wherein the device itself becomes unreachable when the primary Routing Engine goes down or is rebooting. The workaround in this case would be to resolve the device name in the DNS registry to the loopback address of the device.

 

Symptoms

 

Device cannot be accessed via SSH/telnet when the primary RE has gone down or is rebooting.

 

Solution

 

On MX platforms with redundant Routing Engines, there may be instances when the primary RE (referred to as RE0 here) goes down or is rebooting and the device too becomes unreachable. Note that the device may be functional with the previous backup RE (referred to as RE1) taking mastership.

This is because the device name has been resolved to the management IP address of RE0 in the DNS registry that is used in the customer network. 

show interfaces terse em*

Interface               Admin Link Proto    Local                 Remote
em0                     up    up
em0.0                   up    up   inet     10.0.0.4/8      
                                            128.0.0.1/2     
                                            128.0.0.4/2     
                                   inet6    fe80::200:ff:fe00:4/64
                                            fec0::a:0:0:4/64
                                   tnp      0x4             
em1                     up    up
em1.0                   up    up   inet     10.0.0.4/8      
                                            128.0.0.1/2     
                                            128.0.0.4/2     
                                   inet6    fe80::200:1ff:fe00:4/64
                                            fec0::a:0:0:4/64
                                   tnp      0x4

If you run the above output on your device, you can find the IP addresses configured on the management "em" interfaces. If the corresponding RE goes down, the em interface goes down as well.

 

 

Ideally with systems running redundant REs, the DNS registry should resolve the device hostname to an IP address that is always up, as long as the device is up, such as the loopback address.

Therefore, the solution to this problem would be to resolve the device name in the DNS registry to the loopback address of the device. Thus, whichever RE has the primary role, the device will always be accessible.

You can check the address assigned to the loopback address by using the following command: 

show interfaces terse lo0  
Interface               Admin Link Proto    Local                 Remote
lo0                     up    up
lo0.0                   up    up   inet     1.1.1.21            --> 0/0
lo0.16384               up    up   inet     127.0.0.1           --> 0/0
lo0.16385               up    up   inet