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gozu
Asked: 2012-07-18 07:59:44 +0800 CST

How do I find out which physical port my ethersvi MAC address corresponds to on a Cisco switch?

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I'm doing a layer2 network map (my first) and this is the first virtual mac I've encountered.

From the mac address table of switch1, I have

VLAN 1 6416.8d98.52c0 DYNAMIC Te0/1

and on the other, which I'll call switch0, when I am looking for that mac address, this is what I find:

Vlan1 is up, line protocol is up Hardware is EtherSVI, address is 6416.8d98.52c0 (bia 6416.8d98.52c0) Internet address is 192.168.101.252/24

How do I find out the physical port that corresponds to?

When I do a show vlanon switch0, I get:

VLAN Name Status Ports

1 default active Gi0/1, Gi0/2, Gi0/11, Gi0/12, Gi0/13, Gi0/14, Gi0/15, Gi0/16, Gi0/17, Gi0/18, Gi0/19, Gi0/20, Gi0/21, Gi0/22, Gi0/23, Gi0/24

2 VLAN0002 active Gi0/7, Gi0/8

3 VLAN0003 active Gi0/9, Gi0/10

4 VLAN0004 active Gi0/3, Gi0/4

5 VLAN0005 active Gi0/5, Gi0/6

What command should I use?

Edit: Mike's command worked (thanks mike!), but now that I have had time to pay a little more attention to my mac address table, I am re-confused. My ignorance will shine bright, so try not to be blinded by it:

How come several ports belong to many VLANs at once? here is a sample of the output on one of the switches:

VLAN

1    0014.4f97.bb1f    DYNAMIC     Gi0/11 <-- ??? similar to MACs on a solaris box
1    edgertr  Fa0/0    DYNAMIC     Gi0/1
1    sw00     Te0/1    DYNAMIC     Te0/1
1    6416.8d98.52c0    DYNAMIC     Te0/1  <--virtual MAC from before
2    box2     nxge2    DYNAMIC     Te0/1
2    box1   e1000g2    DYNAMIC     Gi0/7
2    box0   e1000g2    DYNAMIC     Te0/1
2    sw00     Te0/1    DYNAMIC     Te0/1
3    box2     nxge3    DYNAMIC     Te0/1
3    box1   e1000g3    DYNAMIC     Gi0/9
3    box0   e1000g3    DYNAMIC     Te0/1
3    sw00     Te0/1    DYNAMIC     Te0/1
4    box2     nxge0    DYNAMIC     Te0/1
4    box0   e1000g0    DYNAMIC     Gi0/3
4    edgertr  Fa0/0    DYNAMIC     Gi0/1
4    boxx      bge0    DYNAMIC     Gi0/2
4    box0   e1000g0    DYNAMIC     Te0/1
4    sw00     Te0/1    DYNAMIC     Te0/1
5    sw00     Te0/1    DYNAMIC     Te0/1

And here is my current attempt at a layer2 diagram for good measure (incomplete, haven't started on the other switch). Anything I should leave out or add?

enter image description here

networking cisco switch mac-address cisco-catalyst

2 Answers

  1. Best Answer

    Usually, the best way to map a Cisco network is with CDP (as long as it is enabled on switch0 and switch1). Do a show cdp nei Te0/1 on switch1; this will tell you what Cisco device/port number is connected on the other side.

    Vlan1 is up, line protocol is up Hardware is EtherSVI, address is 6416.8d98.52c0 (bia 6416.8d98.52c0) Internet address is 192.168.101.252/24

    How do I find out the physical port that corresponds to?

    Strictly speaking an SVI doesn't correlate a virtual mac-addresses to a single physical port; that is the point of an SVI. An SVI's virtual mac-address is available on any port that belongs to that SVINote 1. You can find what physical port in a given SVI connects to another by using the switch's CAM, LLDP, or CDP tables.

    BTW, your switches are using an SVI on Vlan1; best practices dictate that you don't use Vlan1 for data traffic, but that's getting outside the scope of this question.

    Note 1: As long as that port isn't blocked by spanning-tree

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  2. How come several ports belong to many VLANs at once?

    I'm not well aware of the internals of SVIs, like Mike seems to be, or of how it relates to the virtual LANs your switches are using... However:

    One must not confuse virtual LANs (tag-based differenciated traffic on a single physical LAN) which the show vlan displays info about, and a vlan1 interface, which you configure in conf t > int vlan 1 and which has a MAC address.

    Why is this interface named vlan1 ? Is there a good reason related to the fact that an SVI interface is a ghost that listens to any traffic going through the virtual LAN 1?? Beats me.

    One fact: switches need an interface so they can be configured. No interface, (no IP traffic), no interaction, no terminal, no nothing. Another fact: virtual LAN number 1 is a special one: virtual LANs work thanks to ethernet frames wearing numbered tags. The number 1 virtual LAN is a catch-all for any frame that wouldn't be wearing any tags (or any tag with number 1, as far as I know).

    Without configuration done on the switch (no virtual LANs configured yet) it makes sense that the somewhat-virtual interface your create and use to remotely reach the switch would be related to the virtual LAN number one.

    Damn! Forgot to answer your question... But I'm sure you did that yourself by now. Your bewonderment rooted in the confusion between vlans and vlans. Outrageous. :)

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