cisco pvlan layer 3 processing?

Posted on 2011-04-22
Last Modified: 2012-05-11
As I understand it, in order for different pvlan types to communicate, they have to cross at layer 3.

Does this communication use the router processor? Does it not? How does this transaction take place?

Please provide documentation if possible.
Question by:c-h-r-i-s-t-o-p-h
    LVL 17

    Expert Comment

    Yes, vlan need a L3 device to communicate. How do you communicate between 2 networks? It's via router. Vlan splits network and hence to communicate vlans need a router to route traffic from one network(vlan) to another(vlan).

    L3 switches like cisco6500 has got L3 card that does routing. It's called the route processor or MSFC card. Several terminologies exist but zest is either you need a router or a routing card in L3 switch.

    Type "what is a vlan" in google and pick any document out of 1000's available.

    Author Comment

    According to documentation, the pvlan secondary vlans can communicate via proxy arp. Is this a layer 3 function?
    LVL 17

    Accepted Solution

    It's not a L3 function. It is is a L2 function since it works on the MAC address of the hosts.

    Well, vlans actually never communicates. It's the hosts in the vlans that will communicate. Proxy arp is only one mean to reach destination. Please do not confuse it with routing. It's not a replacement of routing for VLAN communication. You can only install some proxy arp server and make 2 hosts talk in 2 different subnets.

    Proxy ARP (Address Resolution Protocol) is a technique by which a device on a given network answers the ARP queries for a network address that is not on that network. The ARP Proxy is aware of the location of the traffic's destination, and offers its own MAC address in reply, effectively saying, "send it to me, and I'll get it to where it needs to go." Serving as an ARP Proxy for another host effectively directs LAN traffic to the Proxy. The "captured" traffic is then typically routed by the Proxy to the intended destination via another interface or via a tunnel.

    LVL 17

    Expert Comment

    Proxy arp can be used to communicate, but in real life scenario it has got very limited use. You can enable "local proxy arp" feature in cisco devices or rather most networking routers/switches.


    For example, suppose a host, say A, wants to contact another host B, where B is on a different subnet/broadcast domain than A. For this, host A will send an ARP request with a Destination IP address of B in its ARP packet. The multi-homed router which is connected to both the subnets, responds to host A's request with its MAC address instead of host B's actual MAC address, thus proxying for host B. In the due course of time, when host A sends a packet to the router which is actually destined to host B, the router just forwards the packet to host B. The communication between host A and B is totally unaware of the router proxying for each other

    You can consider both A and B to be in separate VLAN's.

    LVL 17

    Assisted Solution

    I would say that proxy arp is neither a layer 2 or layer 3 function. It works inbetween the two associating mac addresses to ip addresses.

    Lets say you have two networks. Network A has a subnet of and network B has a subnet of routing is configured on the router that connects the two subnets.

    In this scenario Network A believes it has access to the whole network and therefore beleives it is directly connected to (which is not the case). to get these two to communicate you can configure proxy arp on the router interface connected to network A. Now Hosts on network A will associate IP addresses on the network to the router's interface mac address.

    All traffic to the network will have a destination mac address of the router and a destination ip address of the network. The router will just forward the traffic and neither network will be any wiser as to what is going on.

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