cisco CDP with indirect link

[Ibrahim Jamil]

Hi Experts

SW1---trunk---Huawei------trunk------SW2

i have the above topology cisco 3560 conneted to Huwawei switch via 802.1q , and the same Huawei switch connect to cisco 3560 SW2 , no direct link between SW1 and SW2 , but when i issue sh cdp neih from SW1 i can see SW2 how that is happen,pls any clear answer?

thanks

jamil

[nkarthikeyan]

Normally if you connect your switches to a hub.... it will behave like this.... since that acts as a repeater.... but an not sure how the huwaie switch functions.... if that does the repeater function to the connected devices.... it will give behave like thaty only.

[John Blakley]

Dells work this way too. The switch passes the traffic, but since your intermediary switch doesn't support cdp you obviously won't see it as a neighbor. To you, it looks like the Cisco is connected directly to your trunk which in reality it's not.

Sent from Cisco Technical Support iPad App

[Peter Paluch]

Hi Jamil,

To add to other friends' answers here, CDP is just a simple protocol encapsulated into multicast Ethernet frames. If a switch does not support the CDP protocol, it simply floods the frame out all ports except the ingress port just like any other broadcast/multicast frame. Because of this, such a switch is essentially invisible and transparent for CDP. That is common for all non-Cisco switches, as the CDP is Cisco-proprietary protocol and very few other vendors support it.

On the other hand, if the switch supports CDP then it specifically listens on the multicast MAC address 01:00:0c:cc:cc:cc towards which the CDP packets are sent. Such frames are not flooded and instead, they are delivered directly to the CPU of the switch, ensuring they are processed by the switch. This can also be seen in that switch's MAC address table:

SW-Dist1#show mac address-table 
          Mac Address Table
-------------------------------------------
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 All    0100.0ccc.cccc    STATIC      CPU

Best regards,

Peter