GNS3 - VitualBox Part 7: OSPF over Frame Relay Switches

An explanation of mapped and Inverse ARP Frame Relay configurations and how to implement OSPF over them.

Introduction

Frame Relay is a Non Broadcast Multiple Access (NBMA) technology.  It operates at Layer 2 -- Datalink -- of the OSI Networking Model and relies upon connected nodes (in this case Cisco routers) to implement Layer 3 and above (in this case, TCP/IP).  NBMA technologies do not support broadcast and multicast traffic.  OSPF uses multicast packets for LSAs.  Additional configuration is required to implement OSPF on Frame Relay switched networks.

Frame Relay Basics

In Ethernet networks, this is implemented with Media Access Control (MAC) addresses -- 12-hexadecimal digit unique identifiers typically "burned in" to the hardware by the manufacturer.  When two nodes on an Ethernet network, they translate Layer 3 (e.g. IP) addresses into Layer 2 (MAC) addresses using Address Resolution protocol.  The Ethernet devices (e.g. hubs and switches) and supporting hardware (e.g. cabling, wireless) determine the path and access at Layer 2.  This sequence operates dynamically because the end nodes and networks support Address Resolution Protocol (ARP), which maps Layer 3 IP addresses to Layer 2 MAC addresses.

Frame Relay does not operate at Layer 2 the same way Ethernet does.  Frame Relay uses Data Link Connection Identifiers (DLCIs) assigned to physical interfaces rather than MAC addresses.  The switched Frame Relay network manages connections between DLCIs with mappings -- associating one DLCI with a corresponding one.  DLCIs are typically assigned by the Internet Service Provider (ISP).  A pathway from one DLCI to a corresponding DLCI is a Virtual Circuit (VC).  For this example, a unique and unchanging DLCI and data path is assigned for each VC, creating a Permanent Virtual Circuit (PVC).  Although not demonstrated in this article, DLCIs and paths may be assigned dynamically to endpoints (analogous to dial-up networking) and the resulting VCs are Switched Virtual Circuits (SVCs).

Each PVC requires a unique DLCI-DLCI mapping on the Frame Relay switched network.  This implies other differences between Frame Relay and Ethernet Layer 2 technologies:  Frame Relay must be manually configured with one or more unique Layer 2 addresses while Ethernet is configured with (typically) permanent Layer 2 addresses and the switches establish endpoint connections automatically.  Each Frame Relay endpoint must have a unique DLCI assigned for each connection to another endpoint, so a single endpoint will have multiple DLCIs if it connect to more than one other endpoint.

Example Frame Relay Layer 2 Topologies

The illustration at the top of the page depicts the Layer 1 Physical topology -- each router has a single serial T-1 connection to a Frame Relay switch port.  There are many Layer 2 -- DLCI-DLCI PVC mappings -- that may be implemented.

Hub and Spoke Topology

A hub and spoke topology applicable to a Data Center to five Branch Offices is to create a PVC connection from each Branch Office to the Data Center.  This is implemented by assigning five DLCIs to the Data Center endpoint (port 1) mapped to a each of a single DLCI assigned to each Branch Office (ports 2 through 6).  There are a total of five PVCs.  The illustration above depicts the logical Layer 2 Frame Relay Hub and Spoke DLCI-DLCI PVC mappings.

The format of the DLCIs used in the GNS3 Frame Relay switch example is "<source port> 0 <destination port>.  DLCI 102 is assigned to the PVC from interface 1 to interface 2 and is mapped to the corresponding DLCI 201  assigned to the PVC from interface 2 to interface 1.

Mesh Topology

A mesh topology applicable to a Data Center and five Branch Offices is to create a PVC connection between every endpoint.  Each endpoint has five unique DLCIs and there are a total of 15 PVCs.  The illustration above depicts the logical Layer 2 Frame Relay Mesh DLCI-DLCI PVC mappings.

Establishing Router-To-Router Connections Over Frame Relay

Ethernet uses ARP to map IP to MAC addresses.  When an IP node establishes a connection to another IP node, it first consults its local ARP cache (list of IP address to MAC adress mappings).  If there is no corresponding IP address to MAC address mapping, it sends out an ARP broadcast packet to determine the MAC address to use.  Frame Relay, as an NBMA technology, does not support broadcast packets.

Lacking broadcast support, establishing router-to-router IP connections over Frame Relay connections is a different process.

Manual IP Address-to-DCLI Mappings

Cisco IOS provides commands to manually map IP addresses to DLCIs.  Two important ones are frame-relay map and frame-relay interface-dlci.

Addresses Assigned to the Physical Interface

When an IP address is assigned to the physical serial interface (e.g. s0/0, s0/1, s1/0, etc.), Cisco IOS recognizes the assigned DLCIs in much the same way operating systems recognize Ethernet MAC addresses.  That is, the Cisco IOS on the router already has a mapping for its local DLCI assigned by the ISP to its physical interface.  However, it lacks a DLCI to remote IP address mapping (analogous to Ethernet's ARP cache).  This mapping may be established by manually specifying the remote IP address associated with the local DLCI using the frame-relay map command, whose syntax is:

frame-relay map <protocol> <destination IP address> <local DLCI> <option>

The goal is to support OSPF over the link and NBMA networks do not support broadcasts.  The operating system must specify that broadcast packets are forwarded to this DLCI, at which point the Frame Relay switched network delivers it to the corresponding DLCI and receiving device accepts the packet.  To do so, add the "broadcast" option to each configuration.  For example:

On Router 1:

frame-relay map ip 192.168.0.2 102 broadcast

On Router 2:

frame-relay map ip 192.168.0.1 201 broadcast

A complete example, mapping Router 1 with IP address 192.168.0.1/30 and DLCI 102 to Router 2 with IP address 192.168.0.2/30 and DLCI 201, is:

On Router 1:

config t
interface s0/0
no shutdown
ip address 192.168.0.1 255.255.255.252
encapsulation frame-relay
frame-relay map ip 192.168.0.2 102 broadcast

On Router 2:

config t
interface s0/0
no shutdown
ip address 192.168.0.2 255.255.255.252
encapsulation frame-relay
frame-relay map ip 192.168.0.1 201 broadcast

Addresses Assigned to Logical Subinterfaces

Logical subinterfaces (e.g. s0/0.1, s0/0.2, etc.) operate differently than physical interfaces.  They are logical devices created by the Cisco IOS, not physical ones recognized by the Cisco IOS.  One implication of this is that subinterfaces do not recognize any of the DLCIs assigned to their parent physical interfaces.  Before mapping remote IP addresses to local DLCIs, the subinterface must first have one or more DLCIs from the parent physical interface assigned to the logical subinterface.  Using the example above:

On Router 1:

config t
interface s0/0
no shutdown
encapsulation frame-relay
interface s0/0.1multipoint
ip address 192.168.0.1 255.255.255.252
frame-relay interface-dlci 102
frame-relay map ip 192.168.0.2 102 broadcast

On Router 2:

config t
interface s0/0
no shutdown
encapsulation frame-relay
interface s0/0.1 multipoint
ip address 192.168.0.2 255.255.255.252
frame-relay interface-dlci 201
frame-relay map ip 192.168.0.1 201 broadcast

Inverse ARP IP Address-To-DLCI Mappings

Frame Relay Inverse ARP is another way to map addresses.  ARP translates Layer 3 IP addresses to Layer 2 MAC addresses; Inverse ARP (as the name implies) translates Layer 2 DLCI addresses to Layer 3 IP addresses.  In operation, when a router requests a remote IP address, it sends Inverse ARP packets by matching the associated network and using any local IP addresses to decide where to send it.  It then sends the Inverse ARP packets to every DLCI mapped to candidate IP addresses.  On the receiving devices, the receiving DLCI that corresponds to the requested remote IP address responds, creating the PVC and DLCI-IP address mappings.

Consider the section above explaining how physical and logical interfaces interact with Cisco IOS.  When an IP address is assigned to a physical interface, the corresponding DLCIs are available to IOS.  However, when an IP address is assigned to a logical subinterface, there is no DLCI mapping until one is specified.  So, Inverse ARP works by default on physical interfaces as soon as an IP address is assigned.  Inverse ARP will not work on a logical subinterface until both an IP Address and DLCI assigned to the parent physical interface are assigned.  In fact, the frame-relay map command is disallowed on subinterfaces because if it is point-to-point because there is only one possible destination DLCI; multipoint subinterfaces still require mappings.

Once properly configured for Inverse ARP, the IP Address - DLCI mappings will be configured upon the router requesting the remote IP address.  Pinging the remote address will trigger the Inverse ARP request and dynamic DLCI mappings created.

Configuring OSPF over Frame Relay

The OSPF network type, by default, for frame relay is non-broadcast.  To manually set the OSPF network type to broadcast:

config t
interface s0/0
ip ospf network broadcast

At that point, the two routers will form an adjacency.

Optionally, you may manually specify the IP Address of neighboring routers using the neighbor command. 

The video below illustrates how to configure the above OSPF over Frame Relay options.