How cdp works?

[CATYO]

Hi there.

I just wondering that how cdp works in cisco switch and not cisco device.

cdp is propagated multicast 01:00:0c:cc:cc:cc(not sure) on all interface that supports SNAP.

[diagram]

SW1------SW2------SW3

(cisco)   (cisco)    (cisco)

in this case, sw 1 sends cdp multicast to sw2 and sw2 receive this cdp. i think that may igmp snooping is enabled by default in all interface so sw2 receives this multicast and response that "i'm your neighbor" by some mechanism (i dont no) .and doesn't flood this mcast. so it doesn't cross two hops.

sw1 ------------sw2--------sw3

(cisco)     (not cisco)     (cisco)

and some case that sw2 is not cisco device and doesn't work with igmp snooping, it simply flood all interface. so i can see sw3 on sw1.

if i was correct, what if that sw2(not cisco) enables igmp sooping all links?

Can i still see sw3 on sw1??

Best regards.

[Reza Sharifi]

Hi,

CDP is a Cisco layer-2 proprietary protocol and does not work with other vendor's products. Some vendors like Juniper use LLDP instead of CDP to see their directly connected neighbors.  If you have a multi-vendor environment and your device support Standard LLDP, you can try enabling LLDP on Cisco devices to see their none-Cisco neighbors.

Cisco Discovery Protocol is a Layer 2, media-independent, and network-independent protocol that networking applications use to learn about nearby, directly connected devices. Cisco Discovery Protocol is enabled by default. Each device configured for Cisco Discovery Protocol advertises at least one address at which the device can receive messages and sends periodic advertisements (messages) to the well-known multicast address 01:00:0C:CC:CC:CC. Devices discover each other by listening at that address. They also listen to messages to learn when interfaces on other devices are up or go down.

http://www.cisco.com/en/US/docs/ios-xml/ios/cdp/configuration/15-mt/nm-cdp-discover.html#GUID-F729EBA3-2945-456F-9C1F-6CDD993CABEC

HTH

[Rolf Fischer · Accepted solution]

Hi,

if i was correct, what if that sw2(not cisco) enables igmp sooping all links?

IGMP snooping restricts only traffic in IANA-assigned MAC mulitcast groups (01-00-5E-xx-xx-xx).

The multicast MAC-address 01-00-0C-CC-CC-CC (Cisco CDP,DTP,VTP,PAgP,UDLD) will be treated as normal layer-2 multicast by most non-Cisco switches (flooded on all interfaces with the exception of the interface that received the frame).

The same applies to Cisco (per-VLAN) shared spanning-tree BPDUS, they use the multicast destination MAC 01-00-0C-CC-CC-CD.

HTH

Rolf

[CATYO]

Many thanks !!