CDP help

Hello,

Just want to ask something regarding CDP, what if I have a cisco router 2600 connected to a 1900 series sw and on this sw another router is connected like :

2600

1900

2500

It was mentioned on the book that cdp can't pass to switch only directed device, so if I'm in the console of 2600 I can only see the 1900 sw or both I can see?

TY

Find more posts tagged with

SAVE $250 on Your CISCO Boot Camp — Use Code "EXAM26"

Exclusively for TechExams members for Infosec Boot Camps starting before April 30, 2026

View CISCO Boot Camps

Comments

EricO

The 2600 and the 2500 are connected together if they are on the same vlan. If they are then you can see both....

chinaman

wait, I cannot understand what you mean

darkuser

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a00801f0a3e.html

chinaman

Thanks for the help, so on my above example 2600 discover the 1900
and on 1900 sw it will discover the 2500 series router

mikej412

chinaman wrote:

Thanks for the help, so on my above example 2600 discover the 1900
and on 1900 sw it will discover the 2500 series router

I think you've got it....

The 2600 can see the 1900 with cdp

The 1900 can see both the 2600 and 2500 with cdp

The 2500 can see the 1900 with cdp

I think this was the most "technical part" of the CCDA -- maping a network. If you started from the 2600, you'd find the 1900. Then you'd telnet to the 1900 and from there you would find the 2500.

Remember this only works when cdp is running and only for Cisco equipment.

ciscos

the whole thing is that witn cdp you can see only neighborhood cisco devices.

dublin_101

chinaman wrote:

Hello,

Just want to ask something regarding CDP, what if I have a cisco router 2600 connected to a 1900 series sw and on this sw another router is connected like :

2600
1900
2500

It was mentioned on the book that cdp can't pass to switch only directed device, so if I'm in the console of 2600 I can only see the 1900 sw or both I can see?

TY

just remember the basics of cdp.......:

- operates at layer 2
- works only on directly connected devices
- it is protocol independent
- of course works on all cisco devices!

Yankee

It's not exactly true to say only directly connected devices will be seen. If there is a non-cisco, non-cdp like supporting device inbetween Cisco devices, the CDP packet will be passed thru that device to the next device. More and more vendors are adding a CDP like protocol (Foundry and HP come to mind) so those devices will not pass the packet, but many older devices will pass it. What you may end up seeing is a router being flooded with CDP duplex mismatch errors when a Cisco IP phone is set to 10/half and the router is connected to an old 3COM switch at 100/ full. The 3COM passes the CDP packet to the phone, so the phone and the router believe there is a mismatch when it is actually not true.

Yankee