What is the link layer protocol of serial cable connection between routers?

johnifanx98johnifanx98 Posts: 329Member
for the ethernet or token ring, it's 802.3/802.4/802.5. Is there any such IEEE protocol for serial cable connection link?

Comments

  • Forsaken_GAForsaken_GA Posts: 4,024Member
    It largely depends on the type of serial link as to what the layer 2 protocol is. For example, a frame relay link will be using frame relay (the standard for frame relay covers both, the layer 1 and layer 2 specification). If you're connecting two routers back to back with serial links, you might be using HDLC. If you're using a dialup connection, it could be PPP or SLIP.

    Serial links are alot more open, they're not bound to a specific layer 2 type because of their layer 1 type like most modern protocols.
  • tengutengu Posts: 13Member ■□□□□□□□□□
    What Forsaken_GA said plus I cannot find any IEEE standards on hdlc, ppp, or frame relay. Bunch of RFC's though.
  • johnifanx98johnifanx98 Posts: 329Member
    It largely depends on the type of serial link as to what the layer 2 protocol is. For example, a frame relay link will be using frame relay (the standard for frame relay covers both, the layer 1 and layer 2 specification). If you're connecting two routers back to back with serial links, you might be using HDLC. If you're using a dialup connection, it could be PPP or SLIP.

    Serial links are alot more open, they're not bound to a specific layer 2 type because of their layer 1 type like most modern protocols.

    Thanks a lot for shedding the light. Now I understand I'm asking about HDLC/PPP. Frame relay looks like a broad topic. Is it a prevalent way of connecting routers in WAN in reality? Looks like Todd thinks leased T1 is expensive.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    Not anymore. Frame Relay used to be the choice for interconnecting sites, but it's being replaced with MPLS, and there are some carriers that are refusing to renew Frame Relay circuits because they're phasing the technology out. And these days, T1's are cheap!
  • johnifanx98johnifanx98 Posts: 329Member
    tengu wrote: »
    What Forsaken_GA said plus I cannot find any IEEE standards on hdlc, ppp, or frame relay. Bunch of RFC's though.

    HDLC is a standard of ITU-T, now ISO seems like. Unfortunately ISO's publication is not free online...

    Actually my interest with HDLC is how ARP or similar protocols are implemented on router's HDLC interface. Googled without luck....
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    HDLC is a standard of ITU-T, now ISO seems like. Unfortunately ISO's publication is not free online...

    Actually my interest with HDLC is how ARP or similar protocols are implemented on router's HDLC interface. Googled without luck....

    Well that's easy - they aren't. HDLC and PPP are traditionally deployed as point to point. With P2P links, there's no need for layer 2 to layer 3 resolution. There are two points connected, so if traffic is going out one end, the only possible place it has to go is to the other point (HDLC does have capability to be deployed as point to multipoint, so I imagine there must be some form of layer 2 to 3 resolution and mapping possible, but I have no idea what the mechanism is, nor have I ever encountered a P2MP HDLC setup in real life or in any lab scenario, so I'm pretty comfortable ignoring it).

    In the case of Frame Relay, the layer 2 to layer 3 mappings are handled via the DLCI's.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    And after further review, while it appears that the HDLC standard does support point to multipoint, Cisco's implementation of HDLC is proprietary, and looks like it will only function as a point to point protocol.
  • SharkDiverSharkDiver Posts: 844Member
    I work for an ISP, and we used to install alot of frame relay, but that has now been overtaken by MPLS.
    To program the customer edge router, you just use an HDLC point-to-point connection to the provider edge router.
  • drkatdrkat Posts: 703Banned
    There are multiple configurations that'll work I guess.

    You can have HDLC between a cisco edge and cisco cpe

    your choice of p2p is PPP, HDLC or frame-relay encapsulation. There is an inherent difference between frame-relay at the physical layer vs frame-relay at layer 2.

    with a T1 encapsulated as frame you can carry internet and mpls over a single link with the use of dlci's - this makes it very affordable for a customer to have both mpls and internet. Where has the other 2 protocols only allow for a specific type of traffic unless there is a special config in the MPLS cloud which says "hey point 0.0.0.0 here." I believe global crossing (level 3) has a "Secure Internet" product that essentially has an MPLS cloud and a default route statement out one of their cores to the internet.

    Frame at the physical layer where we have multiple subscribers over a single vc is going by the way-side.

    I'm in the ISP business and most of the providers are using these setups now - for scalability and cost
    Married to the game but she broke her vows. That's why my bars are full of broken bottles And my night stands are full of open bibles
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