Charles, The 6509 switch needs some configuration in the background to create a "virtual router". This "virtual router" has virtual interfaces that would mimic the default gateways IP addresses of the physical 2500 router and therefore pass traffic between the virtual interfaces. The two subnets that you list would be on different VLANs on the switch. It would look like the packets are only going from one interface on the switch to the other and back again, but in reality they are passing through the "virtual router" created on a route processing piece of software. This would be on a multi-layer switch module or similar. Layer 2 = MAC addresses, layer 3 = IP addresses. To get between IP subnets you need a routing function which is either provided by a physical router or a "virtual router" which routes between "virtual LANs" created by software. This is not the definitive answer but hopefully it clears away some of the mud.
Cheers, Steve Wilson Network Engineer -----Original Message----- From: DeVoe, Charles (PKI) [mailto:[EMAIL PROTECTED] Sent: 26 February 2003 12:45 To: [EMAIL PROTECTED] Subject: RE: L3 Switching Huh???? [7:63728] OK, let me try this again. I am trying to figure out the difference between conventional layer 3 routing and layer 3 switching. A little background. I am currently working towards my CCNA (have been for about 3 years). At any rate, everything I read and look at says that switching/bridging is a layer 2 function, routing is a layer 3 function. Either I don't have a good grasp of the OSI model, switching, routing, VLANs or all of the above. The network: Host A 10.1.1.2 MAC 00.AA Host B 10.1.2.2 MAC 00.BB | 10.1.1.1 MAC 01.AA 10.1.2.1 MAC 02.BB | switch A-----------------------Router-------------------------switch B 10.1.1.0/24 10.1.2.0/24 This is an ethernet network. Both segments are connected by a traditional router say a 2500. In this instance the router interfaces are subnet A 10.1.1.1, and subnet B 10.1.2.1 For simplicity, assume ARP cache is empty. Host A wishes to ping Host B End user on Host A enters - ping 10.1.2.2 The IP packet places the source address 10.1.1.2 and the destination address 10.1.2.2 into the packet. The IP protocol examines the IP address and based on the IP address determines this is in another subnet. An ARP request goes out for 10.1.1.1 (default gateway) and the MAC address is found. The DLL then places the source MAC address 00.AA and the destination MAC 01.AA into the frame. The frame then goes out the wire to the destination MAC. The router interface sees this frame as destined for itself. It de-encapsulates the frame removing the MAC addresses. The router then examines the IP address, based on the routing table it knows the destination port. The router leaves the same IP source (10.1.1.2) and destination (10.1.2.2) in the packet. The frame is rebuilt with the new MAC address of source 02.BB and destination 00.BB Host B grabs this packet and does it's thing. Now, if I replace the router with a 6509 switch, with routing, how does the process change? Said 6509 would be equipped with a 10/100 card so that the hosts are now directly connected. The router interface is now a virtual interface, there is no physical interface. Which is another question. How does the 6509 determine this virtual address? Am I correct? Inter VLAN communication cannot occur without a router. Switching is based on MAC address. Routing is based on IP address. I believe the term "layer 3 routing" is a marketing term, not scientific or engineering in nature. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=63871&t=63728 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
