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Recently, my company bought a Cisco 881. Although I am able to get it up and working by copying some configuration on the Internet, I still do not understand how does the layer 2 (LAN) and layer 3 (WAN) interface interact within the same router?
I tried to google but I still couldn't find a good explanation on how it works.
It is much like a layer-3 switch. The switch interfaces are layer-2 interfaces, and they are configured for VLANs. The default VLAN is VLAN 1. For each VLAN you use, you can create and SVI (Switched Virtual Interface). You place the layer-3 VLAN configuration on the SVI, and the layer-3 address of the SVI is the gateway address for the VLAN. The SVI is like another router interface.
Something like:
interface Vlan1
ip address 10.1.0.1 255.255.255.0
no shutdown
!
You can create multiple SVIs and assign different switch interfaces to different VLANs.
The router will automatically route between directly connected networks, so it will route between your virtual and physical interfaces.
Think of it as a router with an Ethernet switch linecard attached. If you aren't very familiar with configuring Cisco equipment, have a look at AutoSecure.
The AutoSecure feature secures a router by using a single CLI command to disable common IP services that can be exploited for network attacks, enable IP services and features that can aid in the defense of a network when under attack, and simplify and harden the security configuration of the router.
