Explanation of some basic IOS commands please?

[Mottok]

Hi - hope this is the right place to post this. Basically looking to understand 4 snippets of code from our Cisco 7706 Nexus. I am very new to networking but as our resident Cisco expert has left I have to figure this out, so apologies for the newbie-ness of the questions.

I need to know (in as plain English as possible!) what each of the following means and how they work, also trying to understand which direction to think about things e.g. is the interface used to inform traffic coming in, going out or both?

Many thanks, very much appreciated.

1.

interface Vlan2
  no shutdown
  ip address 10.1.2.150/22

2.
interface Vlan50
  no shutdown
  ip address 192.168.250.2/24
  ip router ospf 10 area 0.0.0.0

3.
interface port-channel90
  switchport mode trunk
  switchport trunk native vlan 99

 4.

interface Ethernet1/3
  switchport mode trunk
  switchport trunk native vlan 99
  switchport trunk allowed vlan 2,13,35,50
  channel-group 35 mode active
  no shutdown

[ebenav11]

Hi @Mottok,

1. This is the manner of SVI configuration, the NX-OS use the bar notation "/" for the mask address

2. Idem to the previous item, here you can to announce the network segment in the OSPF process 10 in the respective area. 

3. Is the configuration for the Port-Channel layer 2 interfaces, with the native vlan 99 (default is vlan 1).
 

 4.Is the configuration for a Trunk interface that is part of the Port-channel 35, the configuration is replicated from the logical interface to the physical interface, my advice is to validate the configuration between the logical and physical interfaces.

I hope I've helped

Kind regards 

[Mottok]

Hi, very much appreciate your quick reply.

I'm sorry to be a pain but I'm after REALLY basic info here. Maybe I would be best asking direct questions of each.

No1: I understand vlan2 is a virtual interface on the Nexus but what is the IP address actually telling me?

No2: What does "ip router ospf 10 area 0.0.0.0" actually mean and why would code No1 not use it?

No 3 & 4 I've just worked out with a colleague of mine :-)

Thanks very much.

[Joseph W. Doherty · Accepted solution]

"No1: I understand vlan2 is a virtual interface on the Nexus but what is the IP address actually telling me?"

It's the IP address of the SVI. It's a entry/exit point for that VLAN using the IP and network provided. I.e. if you want to get to/from hosts in the range of 10.1.0.1 - 10.1.2.149, 10.1.2.151 - 10.1.3.254, the SVI if often a way to accomplish that on a L3 switch for a VLAN.

"No2: What does "ip router ospf 10 area 0.0.0.0" actually mean and why would code No1 not use it?"

It means place 192.168.250.0/24 in OSPF process 10's area zero and activate OSPF on that interface.

Why number 1 doesn't use it, possibly to not include it in OSPF, however it might be included with a network statement contained within an OSPF process on the device. (I.e. there are different ways to get networks/interfaces into OSPF, so the fact that statement is not present doesn't guarantee the interface is not included. If you do a show ip ospf interface, that will list all the interfaces defined to OSPF.)

[Mottok]

Thanks you very much, that's the kind of level I needed! I will read through this a couple of hundred times and let it sink in.

Many thanks to all.

[Mottok]

Hi - I'm stuck on one bit and just can't figure it out, any plain English explanations?

Say I have configured the vlan2 SVI on the Nexus core as below and I am trying to get to a server (say 10.1.2.166) on vlan2 from my PC (say vlan11). How/where does it do the routing to get from the SVI at 10.1.2.150 to the IP address on vlan2 (10.1.2.166)?

'interface Vlan2
no shutdown
ip address 10.1.2.150/22'

I just can't figure out how this happens and how to show it on the Nexus, it's driving me mad.

Many thanks.

[Joseph W. Doherty]

Normally, to jump between different networks, you need to route.

A host will "know" what network it's on. If it's sending to an IP not on the same/own network, it generally sends the packet to a defined gateway IP (on the same network). The host with the gateway IP is generally a routing device that "knows" how to forward traffic to other networks.

The first hop routing device sends the IP packet toward the destination network. The routing device that hosts the destination network sends it to the host on that network. (The latter is often the inverse of the first gateway, i.e. the last hop device often uses its gateway IP to send to the destination host.

For example:

host A at 192.168.1.50/24 wants to send to host 192.168.9.100
it sends to a gateway IP of 192.168.1.1/24 (a routing device)
that device forwards the packet to the next IP hop, hopefully closer to the destination host's 192.168.9.100
eventually the last hop routing device, with an IP of 192.168.9.1/24, sends to host 192.168.9.100.

The first hop and last hop might be on the same physical routing device, or there may be many routing devices in between the first and last hop.

VLANs often host a single network. From a routing perspective, routing doesn't "see" VLANs.

A SVI on a L3 switch allows the VLAN and a L3 interface to be on the same device. Without a L3 switch, you might have a L2 switch (with or without VLANs) that connects to an actual router (or even a L3 switch with a "routed interface").

[Mottok]

That's excellent Joseph, thanks very much for that detailed explanation.