PPP (4 main Components)

Hi All,

Reading up on PPP and the Sybex book states -
PPP contain 4 main components

EIA/TIA-232-C, V.4, V.35 and ISDN
HDLC
LCP
NCP

I thought HDLC was a total different form of encapsulation? I really don't think I'm grasping the WAN side of things at all.
Are PPP, HDLC and Frame Relay independent of each other?

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Comments

boredgamelad

From what I understand, the encapsulation that PPP uses for frames was modeled/designed after HDLC and is called HDLC-like framing. This is sometimes referred to, in the context of discussing PPP requirements, as simply HDLC (confusing if you ask me). Someone can correct me if I'm wrong or if it's more complicated than that but that's what I've come to understand after skimming RFC 1662 and Wikipedia after seeing a question about this on the Boson ICND1 practice exam.

Futura

Totally agrree that this is confusing, I was also going to post a similar question. I guess that the HDLC used in PPP is non proprietry, unlike the standalone HDLC protocol.

ChickenNuggetz

control wrote: »

Hi All,

Reading up on PPP and the Sybex book states -
PPP contain 4 main components

EIA/TIA-232-C, V.4, V.35 and ISDN
HDLC
LCP
NCP

I thought HDLC was a total different form of encapsulation? I really don't think I'm grasping the WAN side of things at all.
Are PPP, HDLC and Frame Relay independent of each other?

PPP and HDLC are point-to-point protocols and Frame Relay is a packet-switching service. PPP and HDLC are very similar but have some key differences. From what I remember PPP is used more heavily as it is not proprietary like HDLC. Also, PPP uses authentication whereas HDLC does not.

Forsaken_GA

control wrote: »

I thought HDLC was a total different form of encapsulation? I really don't think I'm grasping the WAN side of things at all.
Are PPP, HDLC and Frame Relay independent of each other?

Yes, they are all separate layer 2 encapsulations supported by a serial interface. Once upon a time, the layer 2 encapsulation was not married to the choice of layer 1 media.

control

So are PPP and HDLC just different methods of encapsulating frames across a P2P link?

Layer 2 frames are encapsulated via either method before being put across the actual P2P link?

Roguetadhg

control wrote: »

So are PPP and HDLC just different methods of encapsulating frames across a P2P link?

Layer 2 frames are encapsulated via either method before being put across the actual P2P link?

Yes, PPP and HDLC - are different encapsulating standards.

In terms of the semi-larger picture:

Routing protocols ride over Encapsulation protocols.

You can have RIP (1&2), OSPF,... over PPP or HDLC.

HDLC will be enabled by default for cisco routers. Which is something to keep inmind when mixing non-cisco routers, like Juniper.

Encapsulation types:

R2(config)#[B]int s0/0[/B]
R2(config-if)#[COLOR=#0000ff][B]encapsulation ?[/B][/COLOR]
  atm-dxi         ATM-DXI encapsulation
  bstun           Block Serial tunneling (BSTUN)
  frame-relay     Frame Relay networks
[B][COLOR=#ff0000]  hdlc            Serial HDLC synchronous[/COLOR][/B]
  lapb            LAPB (X.25 Level 2)
[COLOR=#ff0000][B]  ppp             Point-to-Point protocol[/B][/COLOR]
  sdlc            SDLC
  sdlc-primary    SDLC (primary)
  sdlc-secondary  SDLC (secondary)
  smds            Switched Megabit Data Service (SMDS)
  stun            Serial tunneling (STUN)
  x25             X.25

Verifying Encapsulation for interfaces:

R2#[COLOR=#0000ff][B]show int s0/0[/B][/COLOR]
Serial0/0 is up, line protocol is up
  Hardware is M4T
  Internet address is 10.1.23.2/30
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
[COLOR=#ff0000][B]Encapsulation HDLC,[/B][/COLOR][COLOR=#B22222] crc 16, loopback not set[/COLOR]

Problems you may have with mixed encapsulations are flapping interfaces. Interfaces that bounce from Up/up to Up/down over time. Or they may just be up/down.

R2(config)#[COLOR=#0000ff][B]do[/B][/COLOR] [B][COLOR=#0000ff]show ip int brief[/COLOR][/B]
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  10.1.23.2       YES manual up                    up
[COLOR=#b22222] Serial0/1                  10.1.12.2       YES manual up  [/COLOR][B][COLOR=#FF0000]                  up[/COLOR][/B]
Serial0/2                  unassigned      YES unset  administratively down down
Serial0/3                  unassigned      YES unset  administratively down down
Loopback0                  10.1.2.2        YES manual up                    up
Loopback1                  10.1.222.2      YES manual up                    up
R2(config)#[B][COLOR=#0000ff]int s0/1[/COLOR][/B]
R2(config-if)#[B][COLOR=#0000ff]encapsulation ppp[/COLOR][/B]
R2(config-if)#
*Mar  1 00:33:05.943: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.12.1 (Serial0/1) is down: interface down
R2(config-if)#
[COLOR=#b22222]*Mar  1 00:33:08.875: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down[/COLOR]
R2(config-if)#[B][COLOR=#0000ff]do show ip int brief[/COLOR][/B]
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  10.1.23.2       YES manual up                    up
[COLOR=#b22222] Serial0/1                  10.1.12.2       YES manual up [/COLOR][COLOR=#FF0000][B]                   down[/B][/COLOR]
Serial0/2                  unassigned      YES unset  administratively down down
Serial0/3                  unassigned      YES unset  administratively down down
Loopback0                  10.1.2.2        YES manual up                    up
Loopback1                  10.1.222.2      YES manual up                    up
R2(config-if)#

control

Roguetadhg wrote: »

R2(config)#[B]int s0/0[/B]
R2(config-if)#[COLOR=#0000ff][B]encapsulation ?[/B][/COLOR]
  atm-dxi         ATM-DXI encapsulation
  bstun           Block Serial tunneling (BSTUN)
  frame-relay     Frame Relay networks
[B][COLOR=#ff0000] hdlc            Serial HDLC synchronous[/COLOR][/B]
  lapb            LAPB (X.25 Level 2)
[COLOR=#ff0000][B] ppp             Point-to-Point protocol[/B][/COLOR]
  sdlc            SDLC
  sdlc-primary    SDLC (primary)
  sdlc-secondary  SDLC (secondary)
  smds            Switched Megabit Data Service (SMDS)
  stun            Serial tunneling (STUN)
  x25             X.25

Verifying Encapsulation for interfaces:

R2#[COLOR=#0000ff][B]show int s0/0[/B][/COLOR]
Serial0/0 is up, line protocol is up
  Hardware is M4T
  Internet address is 10.1.23.2/30
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
[COLOR=#ff0000][B]Encapsulation HDLC,[/B][/COLOR][COLOR=#b22222] crc 16, loopback not set[/COLOR]

Problems you may have with mixed encapsulations are flapping interfaces. Interfaces that bounce from Up/up to Up/down over time. Or they may just be up/down.

R2(config)#[COLOR=#0000ff][B]do[/B][/COLOR] [B][COLOR=#0000ff]show ip int brief[/COLOR][/B]
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  10.1.23.2       YES manual up                    up
[COLOR=#b22222]Serial0/1                  10.1.12.2       YES manual up  [/COLOR][B][COLOR=#ff0000]                  up[/COLOR][/B]
Serial0/2                  unassigned      YES unset  administratively down down
Serial0/3                  unassigned      YES unset  administratively down down
Loopback0                  10.1.2.2        YES manual up                    up
Loopback1                  10.1.222.2      YES manual up                    up
R2(config)#[B][COLOR=#0000ff]int s0/1[/COLOR][/B]
R2(config-if)#[B][COLOR=#0000ff]encapsulation ppp[/COLOR][/B]
R2(config-if)#
*Mar  1 00:33:05.943: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.12.1 (Serial0/1) is down: interface down
R2(config-if)#
[COLOR=#b22222]*Mar  1 00:33:08.875: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down[/COLOR]
R2(config-if)#[B][COLOR=#0000ff]do show ip int brief[/COLOR][/B]
Interface                  IP-Address      OK? Method Status                Protocol
Serial0/0                  10.1.23.2       YES manual up                    up
[COLOR=#b22222]Serial0/1                  10.1.12.2       YES manual up [/COLOR][COLOR=#ff0000][B]                  down[/B][/COLOR]
Serial0/2                  unassigned      YES unset  administratively down down
Serial0/3                  unassigned      YES unset  administratively down down
Loopback0                  10.1.2.2        YES manual up                    up
Loopback1                  10.1.222.2      YES manual up                    up
R2(config-if)#

Thanks Rouge, Big help!
Cheers

fsanyee

HDLC is a layer 2 protocol, created by ISO. Cisco made its own version, which is proprietary and default encapsulation on Cisco routers serial interface.
PPP use some original "HDLC functions".

Forsaken_GA

control wrote: »

So are PPP and HDLC just different methods of encapsulating frames across a P2P link?

Layer 2 frames are encapsulated via either method before being put across the actual P2P link?

HDLC actually isn't a strict point to point protocol. The ISO standard for it has provisions for it to run as point to multipoint. Cisco's implementation, however, does not.

But as I said - PPP, HDLC, and Frame Relay are all layer 2 encapsulations. You can use any of the three with a serial link. Obviously it has to be compatible with what's on the other side of the WAN.

The book is, strictly speaking, incorrect when it says HDLC is part of PPP. RFC 1662 covers PPP being encapsulated in HDLC like frames.

HDLC has been borrowed and extended and modified in a number of protocols, but HDLC as a layer 2 encapsulation is distinct from PPP.

control

Forsaken_GA wrote: »

HDLC actually isn't a strict point to point protocol. The ISO standard for it has provisions for it to run as point to multipoint. Cisco's implementation, however, does not.

But as I said - PPP, HDLC, and Frame Relay are all layer 2 encapsulations. You can use any of the three with a serial link. Obviously it has to be compatible with what's on the other side of the WAN.

The book is, strictly speaking, incorrect when it says HDLC is part of PPP. RFC 1662 covers PPP being encapsulated in HDLC like frames.

HDLC has been borrowed and extended and modified in a number of protocols, but HDLC as a layer 2 encapsulation is distinct from PPP.

Appreciate all the info on this. Thanks

Roguetadhg

Forsaken_GA wrote: »

HDLC actually isn't a strict point to point protocol. The ISO standard for it has provisions for it to run as point to multipoint. Cisco's implementation, however, does not.

But as I said - PPP, HDLC, and Frame Relay are all layer 2 encapsulations. You can use any of the three with a serial link. Obviously it has to be compatible with what's on the other side of the WAN.

The book is, strictly speaking, incorrect when it says HDLC is part of PPP. RFC 1662 covers PPP being encapsulated in HDLC like frames.

HDLC has been borrowed and extended and modified in a number of protocols, but HDLC as a layer 2 encapsulation is distinct from PPP.

I want to be you when I grow up, Forsaken.

ChickenNuggetz

Roguetadhg wrote: »

I want to be you when I grow up, Forsaken.

He's my hero too