What is link layer protocol between a Cisco switch and a Cisco router?

johnifanx98

I know between Cisco routers, it is usually PPP. But between Cisco router and a Cisco switch, I don't recall any specific configuration mentioned. Similar is between a Cisco switch and a Cisco switch.

Zartanasaurus

What type of cable do you use to connect a router to a switch? A switch to a switch?

vinbuck

johnifanx98 wrote: »

I know between Cisco routers, it is usually PPP. But between Cisco router and a Cisco switch, I don't recall any specific configuration mentioned. Similar is between a Cisco switch and a Cisco switch.

There are multiple answers to this question, but the one you are probably looking for is Ethernet. Most Cisco switches are Ethernet based just as many of their routers are (but certainly not all). Other types of Link-Layer aka Layer 2 protocols are ATM, Frame Relay, PPP, and HDLC. The physical medium is usually unimportant as most Layer 2 encapsulations can be used over several Layer 0 mediums - Copper, Fiber and RF to name the three I can think of off of the top of my head.

The reason you don't see an associated configuration is because the physical port is only capable of one type of Link Layer protocol - Ethernet. On the other hand, a Serial interface on a router can support different Link-Layer encapsulations like HDLC or PPP which is why you see the associated configuration commands to select the encapsualtion.

johnifanx98

vinbuck wrote: »

There are multiple answers to this question, but the one you are probably looking for is Ethernet. Most Cisco switches are Ethernet based just as many of their routers are (but certainly not all). Other types of Link-Layer aka Layer 2 protocols are ATM, Frame Relay, PPP, and HDLC. The physical medium is usually unimportant as most Layer 2 encapsulations can be used over several Layer 0 mediums - Copper, Fiber and RF to name the three I can think of off of the top of my head.

The reason you don't see an associated configuration is because the physical port is only capable of one type of Link Layer protocol - Ethernet. On the other hand, a Serial interface on a router can support different Link-Layer encapsulations like HDLC or PPP which is why you see the associated configuration commands to select the encapsualtion.

Thank you for detailed info. The real question in my mind is how the link management is implemented by switch's port. If it's a serial interface, it will be either PPP or Frame Relay, which does contain a module to do link management. But if a switch connects to another switch or router with RJ-45 interface, what link layer protocol is used? It cannot be PPP or Frame Relay, but could be HDLC. If it's HDLC, how the link management is implemented? I don't see HDLC has such a function.

vinbuck

Ethernet is the link-layer protocol that manages the link between the ports if the two ports are Ethernet. Just remember that RJ-45 is only a connector type - it can be used for T-1s, Ethernet and console connections to name a few.

johnifanx98

vinbuck wrote: »

Ethernet is the link-layer protocol that manages the link between the ports if the two ports are Ethernet. Just remember that RJ-45 is only a connector type - it can be used for T-1s, Ethernet and console connections to name a few.

Thanks! How can I forget this .... Then does ethernet use anything like supervisory frame in HDLC to do link management?

vinbuck

You might want to do some reading on Ethernet frames to get an idea of how it works.

Ethernet frame - Wikipedia, the free encyclopedia

JDMurray

The "link management" used for Ethernet communications is down in layer one, such as auto-detection and auto-negotiation of link speed and duplex mode, and CSMA/CD.

johnifanx98

JDMurray wrote: »

The "link management" used for Ethernet communications is down in layer one, such as auto-detection and auto-negotiation of link speed and duplex mode, and CSMA/CD.

I doubt the CSMA/CD n layer can be called as link management. I scanned two pages from a book "Understanding internet protocols through hands-on programming",

https://docs.google.com/leaf?id=0B2K33PWtSIZdNWNhZmU3YzItMGM2ZC00OTM0LTg0ZDctMTZiMjA0MzQ3YzUy&hl=en_US
https://docs.google.com/leaf?id=0B2K33PWtSIZdNDIwMDZlNjUtODc2MC00MjU3LWI1MzEtZTM5NGY4MjAzMTJk&hl=en_US

It describes link initialization using a three-way handshake like TCP. It does not mention which protocol. I guess it's something like HDLC. However, I am unable to find detailed description of such handshake in HDLC though HDLC does define supervisory frames or U-frames, etc..

Compared with HDLC, Ethernet frame spec looks like a amateur link layer protocol. Barely does it have anything to do with link management. Looks like ethernet leaves the responsibility of link/session management to its upper layer proto...

Sigh, so tired of such research.

JDMurray

johnifanx98 wrote: »

I doubt the CSMA/CD n layer can be called as link management.

CSMA/CD is part of the MAC sub-layer in Layer 2. HDLC is in both the MAC and LLC sub-layers of Layer 2.