Frame Relay - 簡單介紹及基本配置

Frame Relay現在愈來愈不流行了，只是在過去的設計中被普遍應用。

因此工做上仍是能經常見到的, 這篇博文從二層簡單總結下FR的一些概念

在介紹Frame Relay以前，先了解下廣播介質和非廣播介質的差異：

Broadcast Media （廣播介質）： Ethernet 、 Token-Ring、FDDI

特色：

• 源可以同一時候向所有鏈接（二層）目的地址通話
• 簡化了三層到二層的解析

Non-Broadcast Multi Access （NBMA）： Frame Relay、ISDN、ATM etc。

特色：

• 源不能同一時候和二層相連的所有目的地址通話
• 三層的廣播經過二層反覆的單播實現 （pseudo-broadcast）

========三層到二層的地址解析對照=======

廣播介質的三層地址解析：遠程二層MAC地址和遠程三層地址綁定

 - IPv4 Adress Resolution Protocol ( ARP)

• 廣播請求 - 誰知道1.1.1.1的MAC地址
• 單播回覆 - 個人MAC是abcd.abcd.abcd

廣播介質的三層地址解析：遠程或本地的二層MAC地址和遠程三層地址綁定

• 本地FR DLCI和遠程IPv4地址綁定
• 遠端ISDN電話和原電IPX地址綁定

- 反向ARP用解析遠程三層地址到本地二層地址

- 僅僅有直接相連的設備可以進行解析

NBMA port類型：

多點接口 - 可鏈接多個二層鏈路 & 需要三層到二層的解析 (FR & ATM main interface)

單點接口 - 鏈接一個二層鏈路 & 不需要三層到二層解析 （PPP link, FR p2p subinterface) 

======================================

1. FR 簡單介紹

應用了NBMA - 也就意味着潛在存在三層到二層解析問題

DLCI - Data link Connection Identifier (數字鏈路識別碼）

• 二層尋址
• DLCI本地有效

LMI - Local Management Interface

• DTE or DCE 通信
• 彙報虛擬鏈路的狀態

2. FR LMI 配置

開啓FR

 - encapsulation frame-relay

LMI的類型 ( 本身主動偵測）

 - frame-relay lmi-type 【cisco | ansi | q933a】

- show frame-relay lmi

驗證虛擬連接VC狀態

- show frame-relay pvc

狀態種類：

- active （E2E是好的，至少router 到 switch是ok的）

- inactive （中途出了問題）

- deleted （ 路由器試圖用一些參數但是FR交換機不支持，配置錯誤等）

- static （LMI is disabled，用於back to back FR)

3. FR 地址解析

- 多點port需要三層到二層解析： 想要發包到IP a.b.c.d需要哪一個DLCI ？

- 解析的種類

• 動態解析經過反向ARP請求 （inverse ARP）
• 靜態解析經過命令frame-relay map

- 驗證： sh frame-relay map

=======

FR inverse ARP

- 當配置了FR支持的協議，反向ARP本身主動開啓

- 請求會在port配置的所有鏈路上發出

• 反向APR請求發送可以disable - no frame-relay inverse arp [protocol] [dlci]
• 回覆不能被disable （這是個潛在的問題）

- 本身主動包含了對廣播的支持

FR static Mapping

- 手動把本地的二層地址和遠程的三層地址綁定

• frame-relay map [protocol] [address] dlci [broadcast]
• 和靜態ARP的邏輯同樣

- 靜態配置會覆蓋動態配置

- 想支持廣播的話需要手動配置

========

路由器的Auto-install和Frame Relay可能引入問題：

 - 假設在NVRAM裏沒有配置文件，路由器會偵測port的封裝模式和發送地址請求

LAN - DHCP

HDLC - SLARP

FR - BOOTP

- 當地址配置事後，路由器就會試圖經過TFTP拿到配置

FR在這裏出現的問題在第四步：

1. 偵測封裝
2. 經過DLCI學習LMI
3. 給所有DLCIs發送BOOTP
4. 假設請求失敗了，會本身主動配置0.0.0.0 mapping

• 這可能破壞以後的路由配置
• 解決方法就是save config and reload

=================

最後要提的是點到點port

- 一個port對一條二層鏈路

- 不需要三層到二層的解析，僅僅需要配置鏈路號 - frame-relay interface-dlci [dlci]

- 更傾向於點到點配置（可以避免三層以上配置問題）

================

思科官方文檔 - 基本配置

An interface is by default a "multipoint" interface so, frame-relay inverse-arp is on (for point-to-point, there is no Inverse ARP). IP split horizon checking is disabled by default for Frame Relay encapsulation, so routing updates come in and out the same interface. The routers learn the data-link connection identifiers (DLCIs) they need to use from the Frame Relay switch via LMI updates. The routers then Inverse ARP for the remote IP address and create a mapping of local DLCIs and their associated remote IP addresses.

Spicey

Spicey#show running-config Building configuration... Current configuration : 1705 bytes ! version 12.1 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname Spicey ! ! ! interface Ethernet0 ip address 124.124.124.1 255.255.255.0 ! interface Serial0 ip address 3.1.3.1 255.255.255.0 encapsulation frame-relay frame-relay interface-dlci 140 ! ! router rip network 3.0.0.0 network 124.0.0.0 ! line con 0 exec-timeout 0 0 transport input none line aux 0 line vty 0 4 login ! end

Prasit

Prasit#show running-config Building configuration... Current configuration : 1499 bytes ! version 12.1 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname Prasit ! ! ! interface Ethernet0 ip address 123.123.123.1 255.255.255.0 ! ! interface Serial1 ip address 3.1.3.2 255.255.255.0 encapsulation frame-relay frame-relay interface-dlci 150 ! ! router rip network 3.0.0.0 network 123.0.0.0 ! ! ! line con 0 exec-timeout 0 0 transport input none line aux 0 line vty 0 4 login ! end

驗證

• show frame-relay map

• show frame-relay pvc

• show frame-relay lmi

• ping <device name>

• show ip route

Spicey#show frame-relay map
Serial0 (up): ip 3.1.3.2 dlci 140(0x8C,0x20C0), dynamic,
              broadcast,, status defined, active

Spicey#show frame-relay pvc
PVC Statistics for interface Serial0 (Frame Relay DTE)
              Active     Inactive      Deleted       Static
  Local          1            0            0            0
  Switched       0            0            0            0
  Unused         0            0            0            0

DLCI = 140, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0
  input pkts 83            output pkts 87           in bytes 8144      
  out bytes 8408           dropped pkts 0           in FECN pkts0         
  in BECN pkts 0           out FECN pkts 0          out BECN pkts0         
  in DE pkts 0             out DE pkts 0         
  out bcast pkts 41         out bcast bytes 3652      
  pvc create time 01:31:50, last time pvc status changed 01:28:28
Spicey#show frame-relay lmi
LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = CISCO
  Invalid Unnumbered info 0             Invalid Prot Disc 0
  Invalid dummy Call Ref 0              Invalid Msg Type 0
  Invalid Status Message 0              Invalid Lock Shift 0
  Invalid Information ID 0              Invalid Report IE Len 0
  Invalid Report Request 0              Invalid Keep IE Len 0
  Num Status Enq. Sent 550              Num Status msgs Rcvd 552
  Num Update Status Rcvd 0              Num Status Timeouts 0
Spicey#ping 123.123.123.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 123.123.123.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/36/40 ms
Spicey#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS
inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
 
Gateway of last resort is not set
     3.0.0.0/24 is subnetted, 1 subnets
C       3.1.3.0 is directly connected, Serial0
     124.0.0.0/24 is subnetted, 1 subnets
C       124.124.124.0 is directly connected, Ethernet0
R    123.0.0.0/8 [120/1] via 3.1.3.2, 00:00:08, Serial0

Prasit#show frame-relay map
Serial1 (up): ip 3.1.3.1 dlci 150(0x96,0x2460), dynamic,
              broadcast,, status defined, active

Prasit#show frame-relay pvc
PVC Statistics for interface Serial1 (Frame Relay DTE)
              Active     Inactive      Deleted       Static
  Local          1            0            0            0
  Switched       0            0            0            0
  Unused         0            0            0            0
DLCI = 150, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1
  input pkts 87            output pkts 83           in bytes 8408      
  out bytes 8144           dropped pkts 0           in FECN pkts 0         
  in BECN pkts 0           out FECN pkts 0          out BECN pkts 0         
  in DE pkts 0             out DE pkts 0         
  out bcast pkts 38         out bcast bytes 3464      
  pvc create time 01:34:29, last time pvc status changed 01:28:05

Prasit#show frame-relay lmi
LMI Statistics for interface Serial1 (Frame Relay DTE) LMI TYPE = CISCO
  Invalid Unnumbered info 0             Invalid Prot Disc 0
  Invalid dummy Call Ref 0              Invalid Msg Type 0
  Invalid Status Message 0              Invalid Lock Shift 0
  Invalid Information ID 0              Invalid Report IE Len 0
  Invalid Report Request 0              Invalid Keep IE Len 0
  Num Status Enq. Sent 569              Num Status msgs Rcvd 570
  Num Update Status Rcvd 0              Num Status Timeouts 0

Prasit#ping 124.124.124.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 124.124.124.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/36/36 ms
Prasit#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS
inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
Gateway of last resort is not set
     3.0.0.0/24 is subnetted, 1 subnets
C       3.1.3.0 is directly connected, Serial1
R    124.0.0.0/8 [120/1] via 3.1.3.1, 00:00:19, Serial1
     123.0.0.0/24 is subnetted, 1 subnets
C       123.123.123.0 is directly connected, Ethernet0

參考資料：

Comprehensive Guide to Configuring and Troubleshooting Frame Relay