Can I configure a switch to use a specific SVI or does the switch chooses it automatically?

[Jacob Gwangwa]

I have a 6509 VSS. This switch is configured with a Management VLAN called SWITCH_MANAGEMENT with this IP address 192.168.25.250 which i expect it to display in all neighbour switches when i do show cdp neighbour details.

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

5    SWITCH_MANAGEMENT                active    Gi1/2/26, Gi1/2/27, Gi1/5/1, Po21, Po22, Po24, Po25, Po26 - not complete.

However when i'm on a neighbour switch and do show cdp neighbour details it displays the below results which is not what i'm expecting;

Entry address(es):

IP address: 172.16.2.2

Platform: cisco WS-C6509-V-E,  Capabilities: Router Switch IGMP

Interface: GigabitEthernet2/3,  Port ID (outgoing port): GigabitEthernet2/2/2

Holdtime : 176 sec

Why cant this neighbour switch show this IP 192.168.25.250 as its purely configured for management? Instead it shows the IP of VLAN 2 see below config:

This IP 172.16.2.2 is configure for SVI VLAN 2;

interface Vlan2
description SERVER_VLAN_2
ip address 172.16.2.2 255.255.255.0

Any help in understanding this would be appreciated.

Thanks,

Jacob Gwangwa

[Edison Ortiz · Accepted solution]

By default, CDP will use the IP address of the first available interface with an IP.

You can use the 'cdp source-interface' command to force which IP address is presented to the neighbors.

[Reza Sharifi]

Edison,

Do you have reference to this command?

I can't seem to find this command in IOS 15 running on a stack of 3750s.

There is hodltime, advertise-v2, timers, etc... but no source command.

Reza

[Edison Ortiz]

Not supported under that platform. Only under 7600, 6500 and Cisco Routers.

[Reza Sharifi]

Ok, thanks

[Edison Ortiz]

Ok, I must retract my previous statement - my apologies.

This command is not available with the 6500 platform hence no use to the original poster.

The documentation I read was wrong.

I tried the command on a lab box and it was not available.

It is available on IOS routers, though.

This new documentation reflects the supported platforms:

http://www.cisco.com/en/US/docs/ios-xml/ios/cdp/command/cdp-cr-a1.html#GUID-BF50AA69-6AFE-4153-B2DE-1B9843B5A4B9

[Reza Sharifi]

I was just about to go and run this command on a 6500.

Thanks for the update.

[Jacob Gwangwa]

Thanks to all. You've really helped me figure out what I need to do. Probably I'll change the VLAN ID to be the least one for instance VLAN 5 (192.168.25.250-preferred) to VLAN 2 (172.16.2.2 - Not Prefered) & test the results.

This will be done when I return from my leave next January 20, 2013.

Thanks once again!

[Reza Sharifi]

Hi Jacob,

If the answers and discussion was helpful to you, please rate and mark it as answered, so others can benefit from it.

Thanks

[Jacob Gwangwa]

Hi Reza

The correct answer is this one by Edison where he said "

By default, CDP will use the IP address of the first available interface with an IP.

You can use the 'cdp source-interface' command to force which IP address is presented to the neighbors."

I did a simple test since i'm home using packet tracer specifically just for this matter i've raised.

I configure vlan 2 and gave it an IP 172.16.2.2 and also configured vlan 5 with IP 192.168.25.250 on a 3560 switch. I then connected 5 switches - 2960 x2 and 2950 x 3 and their ports are in auto. I configured 1 to 5 interface ports on the 3560 switch  to be trunk links and connected all 5 switches. On the neighobour switches when i do show cdp neighbour details. i can see the IP of VLAN 2 which is 172.16.2.2. I then deleted the vlan and their IPs and started afresh by giving VLAN 2 IP 192.168.25.250 and VLAN 5 172.16.2.2. I did the show cdp neighbour details on the neighbour switches and the IP  i can see is 192.168.25.250.

So the VLAN id number determines the IP the neighbour is going to use.

Thanks to all for your insightful knowledge.