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Let us say, I have configured the managed router with a loopback addresses -

Interface                            IP-Address        OK?    Method     Status              Protocol

GigabitEthernet0/0          10.232.92.102    YES    NVRAM       up                    up    

GigabitEthernet0/1          10.232.92.106    YES    NVRAM       up                    up    

GigabitEthernet0/2          unassigned       YES    NVRAM       down                down  

Loopback0                   10.123.11.145     YES    NVRAM       up                    up     

Loopback1                   10.123.11.200    YES    NVRAM       up                    up  

Loopback2                   10.123.11.201    YES    NVRAM       up                    up

I want to understand what happens -

  1. When I send a packet FROM another router (router2) targeted to the IP address 10.123.11.200 (Loopback1)? Which physical interface of this device will be used to receive the packets intended for loopback1 interface?
  2. When I want to send a packet TO another router (router2) with source IP address as 10.123.11.200 (Loopback1)? Which physical interface of this device will be used to send out the packets?
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  1. It depends on routing table of router2 and other devices in your network (if you have any between router1 and router2). Router1 does not influence on it, it can be do something only when packet arrives to the its interfaces.
  2. It depends on routing table of router1, if you don't use any policy-based routing. So if router2 has address 10.232.92.101, for example, a packet to this address will be sent from gig 0/0 (assuming, that gig0/0 has 10.232.92.102/30).

Each router decides which interface will be used to send packets basing on it own routing table. Input packet is send to loopback, when it arrived to router by physical interfaces.

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