Layer 2 Protocols on Serial Links

Earlier today I was tasked with getting two serial interfaces to talk with each other. Frame-relay is far from my strong suit, so I question-marked my way through the FR commands, making up DLCIs, adding map statements, etc. After several minutes with no luck I looked at the solution. As it turns out, Frame-relay was entirely unnecessary! And so began my Layer 2 serial interface education.

Under a serial interface on a Cisco router there are quite a few Layer 2 protocols to use:

There are only three that are common today, so let’s talk about those: HDLC, PPP, and Frame-Relay.

HDLC is the default protocol on Cisco serial interfaces. You can read about the details here, but consider this the most basic of the common L2 WAN protocols. Keep in mind that the Cisco version of HDLC includes a “next protocol” field, similar to EtherType in Ethernet or Protocol in IP. This is what allows you to connect two Cisco routers over a serial link and get them to talk with nothing more than an IP address. The next protocol field tells the other side to expect an IP packet, and the rest of the HDLC header provides the framing over the physical link.

PPP uses HDLC as a stepping stone and adds all sorts of features. Again, Wikipedia explains them all, but with a PPP link you can use authentication, compression, error detection, and multilinks. PPP can be much more difficult to configure and use, simply because there are so many more options, but a standard PPP connection might look like this on both sides:

Lastly, Frame Relay reminds me of an MPLS VPN that operates at Layer 2 and is lacking in sex appeal. Generally, a single FR circuit is purchased from the service provider and permanent virtual circuits (PVCs) are created between sites. This allows the customers to control routing as they wish. DLCIs come into play in non-point-to-point links and when Inverse Arp is disabled. DLCIs map IP addresses to numbers the SP’s Frame Relay switch can understand.

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