Part 4: Frame Relay Cloud

Berhubung gue lagi males bikin tulisan dalam bahasa inggris, kali ini gue buat dalam bahasa indonesia aja lah, dengan gaya bahasa yang tidak terlalu formal tentunya agar mudah "dicerna" dan tidak kaku. To the point aja, setelah baca tutorial, ebook dan googling sana-sini, gue mau bikin sedikit ringkasan mengenai cloud Frame Relay. Pada umumnya, ketika customer menghubungkan router-router yang ada di kantor cabang dan pusat melalui cloud Frame Relay milik ISP. Mereka tidak tahu apa yang ada di dalam cloud tersebut, cloud Frame Relay akan nampak transparan dari sisi mereka.  Kira-kira seperti inilah topologi Frame Relay yang nampak dari sisi customer.

Topologi Logic Frame Relay Cloud

Setelah melihat topologi logic di atas, mari kita lihat topologi fisik-nya seperti gambar di bawah ini:

Topologi Fisik Frame Relay Cloud

Dari topologi fisik di atas, cloud Frame Relay terdiri atas empat buah switch , yaitu FR1, FR2, FR3 dan FR4. Tidak ada serial line antara FR2 dan FR4, Jadi kita akan gunakan tunneling untuk melewatkan frame Frame Relay dari FR2 ke FR4. Berikut konfigurasi masing-masing switch Frame Relay di atas:

FR1:

 !  
 hostname FR1  
 !  
 frame-relay switching  
 !  
 interface Serial1/0  
  description to R1  
  no ip address  
  encapsulation frame-relay  
  serial restart-delay 0  
  frame-relay intf-type dce  
  frame-relay route 102 interface Serial1/1 112  
  frame-relay route 103 interface Serial1/1 113  
 !  
 interface Serial1/1  
  description to FR2  
  no ip address  
  encapsulation frame-relay  
  serial restart-delay 0  
  frame-relay intf-type nni  
  frame-relay route 112 interface Serial1/0 102  
  frame-relay route 113 interface Serial1/0 103  
 !  

Part 3: Frame Relay Switching

A Router, by default is DTE devices, it can be configured as a Frame Relay Switch by changing the interface to a DCE. Frame Relay Switch will forward Frame Relay frames based on upon their DLCI numbers. Let's see the Logical Frame Relay network below.

Logical Frame Relay Network

Frame Relay Cloud can be one or a group of Frame Relay switch or a router configured as Frame Relay switch. On the network topology below, we'll see the router functioned as Frame Relay switch.

Part 2: Frame Relay Point-to-Point and Multipoint

Now, i'll make some notes about the issues in Frame Relay. When we connect multiple sites through a single router interface, we'll face the routing problem caused by split horizon. Split horizon is a mechanism in routing protocol that designed to eliminate routing loop by blocking routing updates to be sent out of the router interface. See the picture below:

Split Horizon in Frame Relay

R1 received routing updates through serial S0 from R4, but it won't be sent out to R2 or R3 because of split horizon. And how to resolved this problem? Use subinterface. There are two type of subinterfaces support by Cisco:

• Point-to-point, used when we have a separate subnet for each VC. Only one DLCI can be configured per point-to-point subinterfaced. Here's the example of the point-to-point configuration:

     interface Serial1/0

      encapsulation frame-relay

      serial restart-delay 0

    !

    interface Serial1/0.102 point-to-point

      ip address 10.1.1.1 255.255.255.252

      frame-relay interface-dlci 102

    !

• Multipoint, by default, on cisco router, the physical interfaces are the multipoint interfaces. When we created a multipoint subinterfaces under the physical interfaces, we need to specifically assign DLCI to multipoint subinterfaces. Here's the example of the point-to-point configuration:

     interface Serial1/0

      encapsulation frame-relay

      serial restart-delay 0

    !

    interface Serial1/0.103 multipoint

     ip address 10.1.2.1 255.255.255.248

     frame-relay interface-dlci 103

     frame-relay interface-dlci 104

    !