OSPF Cost issue and CDP low level information

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Jul 17th, 2008
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I am facing issue on Cisco 4506 and 4507 switches that I am running OSPF on both the switches and when I set interface bandwidth to 2048 then it is calculating its cost as 488 instead of 48, where as when I set interface bandwidth to 2048 on other switches and routers it shows correct cost of 48.

My 2nd question is regarding CDP

On point to point links cdp information shows correct IP of neighbouring device interface IP but in case of switches where there are multiple Vlans and multiple IPs and both the switches are connected through trunk. Which IP should be displayed through show cdp command?

e.g. Switch A has 2 VLANs with and and Loopback address of which IP address should be seen on switch B when displaying show cdp neigh detail command.

Which criteria does CDP takes when displaying these information, is it the highest IP address of neighbouring device or some other method.

I am getting on my switch but highest IP is Loopback address IP.

Kindly help me out in these 2 problems.

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Overall Rating: 5 (1 ratings)
lee.reade Thu, 07/17/2008 - 00:55
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You may have some kind of bug regarding the ospf cost value, it should be 48, as you have mentioned.

Try remove the bandwidth statement and re-applying.

Regarding cdp, it will use the first available ip inteface to source its cdp packets from, and this can be changed to any interface you wish as per;




khizar.khan Thu, 07/17/2008 - 02:43
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I dont get why its taking first available IP as, what is the criteria of selcting IP, because it is not the lowest niether higher IP, it sits in the middle.

IPs are, and

Kevin Dorrell Thu, 07/17/2008 - 01:38
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For the OSPF problem, I wonder if you have auto-cost reference-bandwidth 1000 somewhere in your config.

Kevin Dorrell


khizar.khan Thu, 07/17/2008 - 02:41
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Hi Kevin,

Thanks for the information, after removing the auto-cost reference-bandwidth command now its working fine, thanks again.

Could you please also reply for CDP issue. If source interface is not set then what is the criteria for selecting IP is the higest IP or any available IP.

Giuseppe Larosa Fri, 07/18/2008 - 10:48
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Hello Khizar,

OSPF issue has been resolved by Kevin and I rated its answer.

For CDP : CDP is a Layer 2 protocol and will go on the trunk link in the native VLAN.

So I would expect to see IP address associated to the Native VLAN if an SVI (L3 interface) exists for that.

I would be surprised to see a loopback address on the sh cdp neigh, the address is provided to allow a direct telnet session from current device and so it is an address that belongs to one of the vlans carried on trunk (native I would expect)

For using a loopback address some form of routing should be in place and CDP cannot know this.

For example the sh cdp neigh of a device that is out of your control can show an ip address that you cannot reach/telnet to.

Hope to help



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