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I'm trying to understand networking and when looking at the different routers, firewalls and switches, I came across Cisco ASA which many use for firewall and routing abilities, so if you use an ASA you don't necessarily need a router or l3 switch?

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It is good to use devices for what they are designed for.

Routers are good at routing protocols and using one where you connect to the ISP (and perhaps run BGP) is correct.

Switches are good and cost effective for providing large numbers of access ports for your users.

A stateful firewall is generally required in the middle to protect against attacks. ASA’s can route or bridge traffic but their purpose is to firewall, NAT, and (sometimes) site-to-site VPN. The only reason they route or bridge is to get the packets through the firewall logic.

One missing piece is: what device is going to act as the DHCP forwarder and default gateway for all those internal switchports? If the switch was a L3 switch, it could do it. In a small network the ASA could do it. A medium enterprise would add a layer of hierarchy: an L3 switch for internal routing with a bunch of L2 switches for cheap access ports.

While a cisco device can act as DHCP server, it is recommended to have the Cisco forward DHCP to a dedicated server.

You also need to provide DNS. Having your own DNS resolver with malware-domain filtering is good for security.

For redundancy: double all the devices. But understand that every access port only connects to one access switch. The complexity of adding redundancy causes human-configuration outages, but they are generally shorter because you have the hardware to recover on site. Hardware-caused outages are rare but you don’t want to be down for a day waiting for TAC to ship you something. It’s also nice to be able to reboot one device at a time without causing outages (note the switchport exception).

One other point about using ASA’s as routers: stateful firewalls deny “asymmetric” traffic. So you have to use them at chokepoints where you enforce that traffic goes through them in both directions. That’s also why redundant ASA’s are deployed in “clusters” where two physical boxes act as one logical box in the chokepoint.

Edit: it’s also important to consider the “financial layer” of the OSI model:

(Price per 10-gig port)

 Router capable of doing BGP with full internet routes: expensive
 Router capable of doing BGP with small number of routes: moderately expensive
 ASA: very expensive
 L3 switch: moderately expensive
 L2 switch: inexpensive

You don’t buy an expensive ASA where a moderately priced L3 switch will do.

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Basically firewall is security device where incoming and outgoing traffic is controlled , restricted , and inspected and monitored. Firewall operates on Layer3 , layer4 and layer7 of OSI model . It's has routing capabilities too ..

It's totally depends upon business requirements what all devices need to be used in setup.

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