Frame Relay
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By Cbrzana |
Frame Relay
Frame Relay Protocols
Frame Relay networks are Non-Broadcast Multiaccess Networks
Local Management Interface: protocol that defines keepalive messages and other messages.
Router's identify the Virtual Circuit by encapsulating that data in a Data-Link Connection Identifier (DLCI).
Frame Relay Terms
| Term | Description |
| Virtual Circuit (VC) | Logical path that connects routers |
| Permanent Virtual Circuit (PVC) | A predefined VC (equated to leased line) |
| Switched Virtual Circuit (SVC) | Dial connection in concept |
| Data Terminal Equipment (DTE) | Devices connected to Frame Relay Service |
| Data Communications Equipment (DCE) | Frame Relay switches, typically in service provider's network. |
| Access Link | Leased line between DTE and DCE |
| Access Rate (AR) | Speed at which access link is clocked. |
| Data-link Connection Identifier (DLCI) | Frame Relay address used in headers to identify the VC |
| Nonbroadcast Multiaccess (NBMA) | Broadcasts not supported, but more than 2 devices can be connected |
| Local Management Interface | Used between DCE and DTE, manages the connection (signaling messages, keepalive messages). |
Frame Relay Standards
| What specification defines | ITU Document | ANSI Document |
| Data-link specifications (LAPF header/trailer) | Q.922-A | T1.618 |
| PVC Management, LMI | Q.933-A | T1.617-D |
| SVC Signaling | Q.933 | T1.617 |
| Multiprotocol encapsulation | Q.933-E | T1.617-F |
LMI and Encapsulation Types
LMI is between Router (DTE) and Frame Relay Switch (DCE).
DLCI is between Router (DTE) and Router (DTE) encapsulation
LMI status inquiry messages perform two key functions:
- Perform keepalive function between the DTE and DCE ( implies link is down)
- Signal when PVC is active or inactive
There are three protocol types for LMI:
- Cisco (proprietary)
- ANSI (T1.617-D)
- ITU (Q.933-A)
DTE needs to know which LMI type to use (must be the same as the DCE type)
Autosense feature detects it automatically
To configure it manually, frame-relay lmi-type [type]
- Can use either cisco, ansi, or q933a as the type
Encapsulation defined by LAPF (Q.922-A)
| LAPF Header (DLCI, FECN, BECN, DE) | Information | LAPF Trailer (FCS) |
Doesn't provide protocol type field, can't support multiple protocols.
Two solutions developed to overcome lack of Protocol Type field.
1. Additional header created, 2-byte protocol type field
2. RFC 1490 (superseded by RFC 2427)
a. "Multiprotocol Interconnect over Frame Relay"
Protocol type field between LAPF header and L3 packet
| LAPF Header | Cisco or RFC 1490 | Packet | LAPF Trailer |
If Cisco used, type = cisco If other standard used, type = ietf
DLCI Addressing Details
DLCI's are locally significant, meaning they must have a unique value only on the local access link.
Analogy
There can only be one 150 ProProfs Avenue, Rochester, NY.
At the same time, every other city in the US can have a 150 ProProfs Avenue address.
Global Addressing makes the DLCI's appear as if they were unique LAN addresses, makes it easier to understand DLCI addressing.
- Switches change the DLCI before the receiver gets it
- Sender treats the DLCI as the destination address
- Receiver treats the DLCI as the source address
| Network Layer Concerns with Frame Relay * Choices for L3 Addresses on Frame Relay interfaces * Handling of broadcasts 3 Different Options for handling L3 packets: 1. One subnet for all DTEs * Usually used in a full mesh topology 2. One subnet per VC * Usually used in a partially meshed topology * Wastes some IP addresses (unless mask of 255.255.255.252 is used) 3. A hybrid of the 1'st 'two options * Used if you can create a sub-full mesh network between some router, but only a partial between others * Uses subinterfaces (logical interfaces on the same physical interface, with different IPs) Broadcast Handling: Cisco IOS sends copies of the broadcasts across each VC. To reduce lag in network, these are placed in different output queue than the one for user traffic, and you can limit the amount of bandwidth this consumes |
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| Frame Relay Service Interworking Most use Asynchronous Transfer Mode (ATM) within the core of the Frame Relay Network. * 53-byte cells * Better Quality of Service (QoS) * Service Interworking is the use of ATM between Frame Relay switches * FRF.5 is the specification that defines how to use ATM in Frame Relay * FRF.8 is the specification that defines how two routers communicate when one is using ATM, and the other is using Frame Relay |
Frame Relay Configuration
| Command | Description |
| cisco] | Frame relay encapsulation type |
| q933a | cisco} | LMI type configuration |
| bandwidth num | Configure bandwidth in kbps |
| cisco] | Statically defines mapping between L3 address and a DLCI |
| keepalive sec | Defines whether and how long LMI keepalives should be sent |
multipoint] | Creates a subinterface or references already existing one. |
| cisco] [voice-cir cir] [ppp virtual-template-name] | Links or correlates a DLCI to the subinterface. |
Show Commands
show interfaces
show frame-relay pvc [interface interface] [dlci]
show frame-relay lmi [type number]
| Fully-Meshed Network with Single Subnet interface serial0 encapsulation frame-relay ip address [address][mask] * Check that all serial interfaces are in the same subnet * LMI type automatically sensed * Encapsulation is cisco * PVC DLCIs are learned via LMIs * Inverse ARP enabled by default |
| More criteria given: * Router 1 requires IETF encapsulation (it is not a Cisco router) * Router 2's LMI type should be ANSI, no autosense used Router 1 interface serial0 encapsulation frame-relay frame-relay lmi-type ansi frame-relay interface-dlci 53 ietf ip address [address][subnet] Router 2 interface serial0 encapsulation frame-relay ietf ip address [address][subnet] |
| Frame Relay Address Mapping Creates a correlation between L3 address and corresponding L2 address * Statically configure the mapping * Dynamically learn mapping through Inverse ARP Inverse ARP: Announces L3 addresses as soon as the LMI signals the PVCs are up. Static Configuration no frame-relay inverse-arp frame-relay map ip [address][DLCI #] broadcast |
| Partially Meshed Network with 1 IP per VC Step 1: Configure Encapsulation interface serial0 encapsulation frame-relay Step 2: Configure sub-interfaces interface serial 0.1 point-to-point frame-relay interface-dlci [dlci #] Step 3: Configure Individual VCs interface serial0 encapsulation frame-relay interface serial 0.1 point-to-point ip address [address] [subnet] frame-relay interface-dlci [dlci #] Step 4: Verify Connectivity show frame-relay map show frame-relay pvc debug frame-relay lmi |
| Partially Meshed Network with some Fully Meshed Parts Step 1: Configure Encapsulation interface serial0 encapsulation frame-relay Step 2: Configure multipoint sub-interface(s) interface serial 0.1 multipoint ip address [address] [subnet mask] frame-relay interface-dlci [dlci #] first interface frame-relay interface-dlci [dlci #] second interface Step 3: Configure point-to-point interface(s) interface serial 0.2 point-to-point ip address [address] [subnet mask] frame-relay interface-dlci [dlci #] Step 4: Verify Connectivity show frame-relay map |
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