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L2 Protocols

  • For P2P links, most popular are HDLC and PPP
  • ISO standard for HDLC has no type field
  • 2-byte type added in IOS for multiple protocols
  • HDLC default on serial

HDLC

  • Encap not shown in config
  • With back to back serial, router connected to DCE end of cable
  • Provides clcok signal for serial
  • clockrate
  • show controllers - Verify DCE or DTE

PPP

  • RFC 1661
Feature HDLC PPP
Error Detection yes yes
Error Recovery no Yes (IOS defaults to not use reliable PPP feature
Standard Protocol Type Field No Yes
Default on IOS Serial links Yes no
Supports sync and async links No yes
  • RFC 1662
  • Defines PPP Framing using HDLC header and trailer for most parts
  • Adds protocol field and optional padding field
  • Padding field ensures even number of bytes

PPP Link Control Protocol

  • Controls features independent of L3 protocol

  • Each L3 protocol has an NCP (network control protocol)

  • IPCP defines dynamic IP assignment etc

  • When PPP comes up (i.e. router sends a Clear TO Send, Data Send Read and Data Carrier Detect to bring up physical)

  • LCP parameter negotiation

  • Auth methods controlled by LCP, in what order

  • After LCP negotiation done, considered up

  • L3 CP then begins

LCP Features

  • Link Quality Monitoring (LQM) - Exchange statistics about percentage errors, link dropped if below a config'd threshold
  • Looped link detection - Random magic number picked, if sees own, link looped, might take down
  • Layer 2 load balancing - MLP frags each frame into one per link
  • Auth - CHAP and PAP

Basic LCP and PPP config

username R4 password 0
rom 838

int Se0/1/0
 ip address 10.1.34.3 255.255.255.0
 encapsulation ppp
 ppp quality 80
 ppp authenticaiton chap

Multilink PPP

  • Original for multiple ISDN B-channels
  • Can balance across any p2p serial link
  • Frag each data link frame
  • Based on link number or config'd frag delay
  • Frags sent across differet links
  • Header added including seq number, and flags with beginning and endging fragment
  • Can use multilink ints or VTs
int Multilink 1
 ip address 10.1.34.3 255.255.255.0
 encapsulation ppp
 ppp multilink
 ppp multilink group 1

int Se0/1/0
 no ip address
 encapsulation ppp
 ppp multilink group 1

int Se0/1/1
 no ip address
 encapsulation ppp
 ppp multilink group 1
  • show int multilink1

MLP LFI

  • Cisco QoS tool
  • Small delay-sensitive packets interleaved into big packets
  • Big packets fragged, small packets after portion of longer
  • Key elements fragmentation, interleave possibility, and queueing scheduler

MLP supports LFI: -

  • ppp multilink interleave - Allows interleaving, per int
  • ppp multilink fragment-delay x - Frag size indirectly (delay in ms, size = x * bandwidth)
  • Can be used on one link or multiple
  • Queue scheduler determines next packet, often uses LLQ
int multilink 1
 bandwidth 256
 ip address 10.1.34.3 255.255.255.0
 encapsulation ppp
 ppp multilink group 1
 ppp multilink fragment-delay 10
 ppp multilink interleave
 service-policy out queue-on-dscp

PPP Compression

  • Can neg to use L2 payload compression, or TCP/RTP header compression
  • Payload better on larger packets
  • Header better n smaller, header compression achieves big ratios (10:1 to 20:1 compression)

PPP L2 Payload Compression

  • Three options, LZS (Lempel-Ziv stacker), Microsoft P2P Compression (MPPC) and Predictor

  • First two use same LZ algorithm

  • Predictors algorithm is predictor

  • LZ more cpu use, better ratio

  • When ATM-FR interworking used, MLPS must be used, so all payload compression types supported by PPP supported for interworking

Feature Stack MPPC Predictor
Uses LZ Yes Yes no
Predictor No no Yes
HDLC Yes no no
PPP yes yes yes
FR Yes no no
ATM and ATM-to-FR interworking Yes yES Yes
  • Use compression command on each interface
  • After config'd, PPP starts Compression CP, negs and manages compression

Header Compression

  • Two styles, TCP and RTP

  • 40 bytes of G.279 packets usually headers (packet 60 bytes)

  • Above reduces packet by more than 50%

  • For TCP, 40 bytes to 3 or 5, helps on smaller payloads

  • Legacy commands

  • ip tcp header-compression [passive]

  • ip rtp header-compression [passive]

  • Passive awaits negotiation, otherwise tries to enable it

  • All flows compressed when added

  • MQC

policy-map cb-compression
 class voice
  bandwidth 82
  compress header ip rtp
 class critical
  bandwidth 110
  compression header ip tcp

int Multilink 1
 bandwidth 256
 service-policy out cb-compression

PPPoE

  • connects hosts to remote aggregator
  • Allows Network Service provider to own customer
  • Emulates PPP link across shared medium

Server config

  • Broadband Aggregate (bba) group createdm handles incoming PPPoE config
bba-group pppoe BBA-GROUP
 virtual-template 1
 session per-mac limit 2
  • Above has few macs per session (allows a drop and reconnect with a different mac, and thats it)
int virtual-template 1
 ip address 10.0.0.1 255.255.255.0
 peer default ip address pool PPPOE_POOL
  • When client initiates session, router dynamically creates virtual int
ip local pool PPPOE_POOL 10.0.0.2 10.0.0.254
  • Enable group on int to client
int Fa0/0
 no ip address
 pppoe enable group MyGroup
 no shutdown

Client Config

int dialer 1
 dialer pool 1
 encapsulation ppp
 ip address negotiated
  • 8 byte header overhead, so drop MTU of dialer to 1492
int Dialer 1
 mtu 1492

int Fa0/0
 no ip address
 pppoe-client dial-pool-number 1
 no shutdown
  • show pppoe session

Auth

  • PAP or CHAP, latter preferred
username PPP password PPPpassword

int virtual-template 1
 ppp authentication chap callin

int dialer 1
 ppp chap password MyPassword

Ethernet WAN

  • Many solutions (VPLS, MPLS, AToM, QinQ, Metro-E)

VPLS

  • Brings various WAN connections as logical ethernet network

  • Multipoing ethernet LAN services, referred to as Transparent LAN service (TLS)

  • CE communicates directly with all other CE nodes

  • Usually in ATM< CE node designated to be hub for all spokes

  • IETF VPLS links ethernet bridges with Pseudo-Wires

  • Operation same as IEEE 802.1 Bridges

  • Self learns source MAP, frames forwarded on dest

  • Unknown macs flooded on all ports

Other functions: -

  • Autodiscovery of PE associated with particular VPLS instance
  • Signalling of PWs to interconenction VPLS VSIs
  • Loop avoidance
  • MAC address withdrawal

Metro-Ethernet

  • Ethernet on MAN can be used as Ethernet, EoMPLS, or Eo Dark Fibre

  • MPLS-based Metro_E use MPLS in SPs network

  • Subscriber gets ethernet interface

  • Packets transported over MPLS network

  • Ethernet underlying technology transporting MPLS

  • LDP signals site-to-site inner label

  • RSVP-TE or LDP for outer label

  • Metro-E has star or mesh topology