INE R&S ATC023 - Frame Relay

Broadcast Overview

• Broadcast Medias
– Ethernet
– Token-Ring
– FDDI

• Native “broadcast” support
– Source can address all connected destinations simultaneously
– Simplifies layer 3 to layer 2 resolution

NBMA Overview

• Non-Broadcast Multi Access
– Frame Relay, ISDN, ATM, etc.

• No native “broadcast” support
– Source cannot address all connected destinations simultaneously
  • E.g. you can’t call every possible phone number in the ISDN cloud
– Layer 3 broadcast sent as layer 2 replicated unicast
  • AKA “pseudo-broadcast”
– Implies Layer 3 to Layer 2 resolution issues


Layer 3 Resolution on Broadcast

• Layer 3 resolution needed to bind remote layer 2 address to remote layer 3 address
– Example: MAC Address to IPv4 address

• IPv4 Address Resolution Protocol (ARP)
– Broadcast Request
  • What’s the MAC address of IP 1.2.3.4?
– Unicast Reply
  • My MAC is 1234.5678.9012

Layer 3 Resolution on NBMA

• Layer 3 resolution needed to bind remote orlocallayer 2 address to remote layer 3 address
– Examples:
  • Local Frame Relay DLCI to remote IPv4 address
  • Remote ISDN phone number to remote IPX address
• Inverse ARP used to resolve remote layer 3 address to locallayer 2 address

• Only directly connected devices can be resolved
– Implies additional issues with partial mesh NBMA networks

NBMA Interface Types

• Multipoint interfaces
– Can terminate multiple layer 2 circuits
– Requires layer 3 to layer 2 resolution
– Examples:
  • Frame Relay & ATM main interface
  • Frame Relay & ATM multipoint subinterface
  • ISDN Legacy DDR

NBMA Interface Types

• Point-to-Point interfaces
– Can terminate only one layer 2 circuit
– Does not require layer 3 to layer 2 resolution
  • Only one circuit, only one possible layer 2 address
– Example:
  • Frame Relay point-to-point subinterface
  • ATM point-to-point subinterface
  • ISDN Dialer Profile

Frame Relay Overview

• Non-Broadcast Multi Access (NBMA)
– Address resolution issues implied

• Data Link Connection Identifier (DLCI)
– Layer 2 addressing
– DLCI number only locally significant

• Local Management Interface (LMI)
– DTE / DCE (router / switch) communication
– Reports virtual circuit (VC) status

Frame Relay LMI

• Enabled automatically with Frame Relay
– encapsulation frame-relay

• LMI Types
– Automatically detected
– frame-relay lmi-type [cisco | ansi| q933a]
– show frame-relay lmi

Frame Relay LMI

• LMI Advertises VC Status
– show frame-relay pvc

• Status can be…
– Active
– Inactive
– Deleted
– Static

FrameRelay Address Resolution

• Multipoint interfaces require layer 3 to layer 2 resolution
– What DLCI do I use to reach IP address A.B.C.D?

• Resolution occurs…
– Dynamically via Inverse-ARP
– Statically via frame-relay map
• Verify resolution
– show frame-relay map
– Same logic as show arp

Frame Relay Inverse-ARP

• Enabled automatically when a supported protocol is configured
– E.g. ipaddress 1.2.3.4 255.255.255.0

• Requests are sent out all circuits assigned to the interface for all supported protocols
– Request can be disabled
  • no frame-relay inverse-arp[protocol] [dlci]
– Reply cannot be disabled
• Automatically includes broadcast support

Frame Relay Static Mappings

• Manual binding of local layer 2 address to remote layer 3 address
– frame-relay map [protocol] [address] dlci[broadcast]
– Same logic as a static ARP entry
• Static mappings override dynamic mappings
– InARPdisabled when static mapping configured
• Broadcast support must be manually configured

Frame Relay and Auto-Install

• Auto-Install is used to automate loading of configfor a new router

• If no configin NVRAM, detect interface encapsulation and send address request
– LAN via DHCP
– HDLC via SLARP
– Frame Relay via BOOTP
• Once address assigned, attempt to get configvia TFTP

Frame Relay and Auto-Install

• For Frame Relay…
– Detect encapsulation
– Learn DLCIs via LMI
– Send BOOTP requests out all DLCIs

• If request fails, results in 0.0.0.0 mappings
– Can break routing topology later
– Save configand reload

Point-to-Point Interfaces

• Only one layer 2 circuit on the interface

• Does not require layer 3 to layer 2 resolution
• Simply assign the circuit number
– frame-relay interface-dlci[dlci]
• Preferred design to avoid Layer 3+ issues
– More on this later

Interface Types and Layer 3 Resolution

• Interface type and protocol resolution is only locally significant

• Any combinationis supported
– Main Interface –Main Interface
– Main Interface –Multipoint Subinterface
– Main Interface –Point-to-Point Subinterface
– Multipoint Subinterface –Multipoint Subinterface
– Multipoint Subinterface –Point-to-Point Subinterface
– Point-to-Point Subinterface –Point-to-Point Subinterface

Partial Mesh NBMA

• Topology where not all devices have layer 2 circuits to all other devices
– I.E. not fully meshed circuits
– Hub-and-Spoke is a type of Partial Mesh

• Design problems occur when layer 3 network does not map exactly to layer 2 network
– Devices without direct layer 2 circuits cannot resolve each other via Inverse-ARP
– Some higher layer protocols (OSPF, PIM, etc.) do not understand this disconnect
• Ideally layer 3 is 1:1 with layer 2 network
– Separate IPv4/IPv6 subnet and point-to-point subinterface for each DLCI