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CDP advertised IP address from 4500

nsysblh
Level 1
Level 1

What controls the IP address CDP puts out in it's advertisements when there are multiple SVI interfaces defined? By default, it appears to be the ip of the lowest numbered SVI interface. I tried the 'snmp-server trap-source VlanXXX' command, but nothing changed in the advertisements.

Most of the functions allow a source interface to be specified, so CDP should probably use one of them. I just haven't found which one.

For some reason, the 3750 appears to have a different algorithm to figure it out...not sure what it is, though.

5 Replies 5

Prashanth Krishnappa
Cisco Employee
Cisco Employee

You are correct. CDP uses the IP address of the lowest numbered SVI interface. If you have Interface VLAN 1 configured and if it is up/up it should use it. If it is down or not configured, CDP would use the next lowest VLAN number(2,3.. and so on).

I tried in lab 3750's and noticed the same behavior. What different behavior are you noticing?

For example, if I had mgmt vlan as 990 and a data vlan at 989...

The Cat4507r would move the cdp advertisement address to the address of Vlan989 as soon as it was up. and as soon as it was down, the advertisement would go back to the ip for Vlan990.

On the Cat3750, The ip for Vlan990 seems to remain in the CDP advertisements no matter if Vlan989 is up or not. Looks like maybe it chooses one at boot time and sticks with it, while the 4507 goes with whatever is lower.

I'm just going to work around this and have my mgmt interface as the lowest numbered SVI on the box. Turns out that will work out better anyway.

I noticed this behavior a long time ago. Normally, it isn't that big a deal, but we now have some automated software traversing the network based on 'sh cdp ent *', so having usable addresses is nice...and the ip on the low SVI number interface I brought up to test wasn't routable from the mgmt station...

If you are consistently seeing this, it smells to me like a bug. I did a quick search and did not find any.

I would try a newer IOS and if you see the same behavior, open a TAC case and have a bug filed. My lab switches were running 12.2(20)SE3 and I did not see this problem.

C3750-24TS-B>sh ver | inc IOS

IOS (tm) C3750 Software (C3750-I5-M), Version 12.2(20)SE3, RELEASE SOFTWARE (fc1)

C3750-24TS-B>

3750s are doing what I kinda wanted in the first place, so I'm not concerned. However, I'm running:

Cisco IOS Software, C3750 Software (C3750-I5K91-M), Version 12.2(25)SE, RELEASE SOFTWARE (fc)

SO, what you are saying is that:

1) The management interface should always be the lowest numbered SVI, as there is no way to specify which SVI IP will be included in the CDP announcements any other way

2) The 3750 should work just like the 4507 with respect to the CDP packets should always be sourced from the lowest numbered SVI.

3) Dynamically bringing up a lower numbered SVI on a 4507 or 3750 should switch the IP in the CDP announcements

1)There is no command to specify the SVI IP for CDP.

2)I would expect CDP to behave same across platforms.

3)Correct.