06-08-2007 12:34 PM - edited 03-05-2019 04:35 PM
Folks, I need some clarification on this.
I have a question regarding architecture and a "simple" implementation.
here is the set up:
I have a router and its fa0/0 interface is connected to a catOS L3 switch's 9/1 port.
Here are the pertinent configs:
switch MSFC:
interface vlan 21
ip address 10.27.21.3 255.255.255.0
switch sup:
set vlan 21 9/1
SO, PORT 9/1 IS IN VLAN 21.
Router:
interface fa0/0
ip address 10.27.21.4 255.255.255.0
OK, so whats the big deal, right?
I want to know if this ethernet connection between the L3 switch and the router is considered a layer 2 or layer 3 connection.
My answer would be that it is a layer 3, routed connection because port 9/1 was placed in vlan 21 and vlan 21's layer 3 (SVI) interface was configured on the switch's MSFC. Right? So, its like creating a point-to-point link in which port 9/1 would have the host address of .3 (vlan inerface address on the MSFC) and the router would be .4 In fact, we could have used a /30 subnet mask instead of the /24.
Here is where I need clarification: Lets say I DID use a /24, instead of the /30, and then I added 3 more ports to vlan 21 and connected those 3 ports -- say, 9/2, 9/3 and 9/4 -- to 3 other routers. Now, in the previous example, we said that port 9/1 would take on the IP address of .3, but now I have added 3 more ports to that vlan and have connected them to 3 different routers. So, what IP addresses would those ports assume??? After all, I only have 1 vlan 21 interface configured on the MSFC (as well I should), so what about ports 9/2-3-and 4? What IP addresses do they assume??
Or could it be that I am looking at it the wrong way? I mean, yes, the links would be considered layer 3, ethernet links between the L3 switch and all the routers, but the switch ports (9/1-4) should not be viewed as having "adopted" any IP address. Does that make any sense? If so, how SHOULD I be looking at those port's IP characteristics?
Thank you for your help
06-08-2007 12:42 PM
Hi,
Since the port is assigned a VLAN and configured with switchport access, then it is a Layer 2 port not a layer 3 port (to be a layer 3 port it should have an IP address and "no switchport"), despite routing is done via the SVI interface, and please do care that Ethernet is not a point-to-point technology rather it is a multiaccess technology.
HTH, please do rate all helpful replies,
Mohammed Mahmoud.
06-08-2007 12:42 PM
Port 9/1 is a layer 2 port.
Interface vlan 21 on the switch is the layer 3 interface.
Interface fa0/0 on the router is layer 3 interface.
With that said you can assign ports 9/2, 9/3 etc. to vlan 21 and the host connected to them can use a valid IP from the vlan 21 IP range and use the switch's vlan 21 IP or the router's IP as their layer 3 gateway.
BTW, it's not a p-t-p link. It's a multiaccess segment. In other words multiple hosts can exist on this segment.
HTH
Sundar
06-08-2007 12:50 PM
Sundar:
You said:
"With that said you can assign ports 9/2, 9/3 etc. to vlan 21 and they can use a valid IP from the vlan 21 IP range and use the switch's vlan 21 IP or the router's IP as their layer 3 gateway."
Clarification: Ports 9/2,3 and 4 are NOT user ports. They are ports, like 9/1, that are connected to other routers. So, you have 9/1 connected to router 1; 9/2 connected to router 3; etc. Thats what I meant by point-to-point-type links.
So, my question is, which IP address would 9/1-4 'adopt" when communicating on layer 3 with their respective routers. Lets say routing updates are going to go out ports 9/1-9/4 to their respective neighbor routers, those routing update datagrams must have a source IP address to place in the IP header. Which IP addresses would they use when you only have ONE SVI configured on the MSFC?
That havin gbeen said,
06-08-2007 01:31 PM
Just because these ports are connected to router interfaces it doesn't become p-t-p links and it's still a multiaccess segment.
The ports themselves do not adopt any IP address and they simply facilitate layer 3 communication between all the devices on the subnet. Routing updates from the switch would use the int vlan 21's IP as the source address.
HTH
Sundar
06-08-2007 02:10 PM
Thanks, Sundar. That last statement really answered my question. Now that makes sense.
As afr as the "point-to-point," I know ethernet is a multiaccess technology. Lets say I used 4 different /30 vlans to make those 4 router connections, although the technology is indeed ethernet, would they be considered point-to-point links?
06-08-2007 02:24 PM
If you use a /30 bit mask then there can be only 2 hosts on the subnet and you can consider that as p-t-p link. But, remember a routing protocol like OSPF would still consider that as broadcast segment unless you manually configure the interface as a p-t-p network type.
HTH
Sundar
06-09-2007 06:50 AM
Sundar, thank you!
I really appreciate the info. You clarified what I suspected already.
Thanks again, buddy.
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