Frame Relay - 简单介绍及基本配置

Frame Relay如今越来越不流行了，只是在过去的设计中被广泛应用。

所以工作上还是能常常见到的, 这篇博文从二层简单总结下FR的一些概念

在介绍Frame Relay之前，先了解下广播介质和非广播介质的差别：

Broadcast Media （广播介质）： Ethernet 、 Token-Ring、FDDI

特点：

• 源能够同一时候向全部连接（二层）目的地址通话
• 简化了三层到二层的解析

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

特点：

• 源不能同一时候和二层相连的全部目的地址通话
• 三层的广播通过二层反复的单播实现 （pseudo-broadcast）

========三层到二层的地址解析对照=======

广播介质的三层地址解析：远程二层MAC地址和远程三层地址绑定

- IPv4 Adress Resolution Protocol ( ARP)

• 广播请求 - 谁知道1.1.1.1的MAC地址
• 单播回复 - 我的MAC是abcd.abcd.abcd

广播介质的三层地址解析：远程或本地的二层MAC地址和远程三层地址绑定

• 本地FR DLCI和远程IPv4地址绑定
• 远端ISDN电话和原电IPX地址绑定

- 反向ARP用解析远程三层地址到本地二层地址

- 仅仅有直接相连的设备能够进行解析

NBMA port类型：

多点接口 - 可连接多个二层链路 & 须要三层到二层的解析 (FR & ATM main interface)

单点接口 - 连接一个二层链路 & 不须要三层到二层解析 （PPP link, FR p2p subinterface)

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

1. FR 简单介绍

应用了NBMA - 也就意味着潜在存在三层到二层解析问题

DLCI - Data link Connection Identifier (数字链路识别码）

• 二层寻址
• DLCI本地有效

LMI - Local Management Interface

• DTE or DCE 通讯
• 汇报虚拟链路的状态

2. FR LMI 配置

开启FR

 - encapsulation frame-relay

LMI的类型 ( 自己主动侦測）

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

- show frame-relay lmi

验证虚拟链接VC状态

- show frame-relay pvc

状态种类：

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

- inactive （中途出了问题）

- deleted （ 路由器试图用一些參数可是FR交换机不支持，配置错误等）

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

3. FR 地址解析

- 多点port须要三层到二层解析： 想要发包到IP a.b.c.d须要哪个DLCI ？

- 解析的种类

• 动态解析通过反向ARP请求 （inverse ARP）
• 静态解析通过命令frame-relay map

- 验证： sh frame-relay map

=======

FR inverse ARP

- 当配置了FR支持的协议，反向ARP自己主动开启

- 请求会在port配置的全部链路上发出

• 反向APR请求发送能够disable - no frame-relay inverse arp [protocol] [dlci]
• 回复不能被disable （这是个潜在的问题）

- 自己主动包含了对广播的支持

FR static Mapping

- 手动把本地的二层地址和远程的三层地址绑定

• frame-relay map [protocol] [address] dlci [broadcast]
• 和静态ARP的逻辑一样

- 静态配置会覆盖动态配置

- 想支持广播的话须要手动配置

========

路由器的Auto-install和Frame Relay可能引入问题：

 - 假设在NVRAM里没有配置文件，路由器会侦測port的封装模式和发送地址请求

LAN - DHCP

HDLC - SLARP

FR - BOOTP

- 当地址配置过后，路由器就会试图通过TFTP拿到配置

FR在这里出现的问题在第四步：

1. 侦測封装
2. 通过DLCI学习LMI
3. 给全部DLCIs发送BOOTP
4. 假设请求失败了，会自己主动配置0.0.0.0 mapping

• 这可能破坏之后的路由配置
• 解决方法就是save config and reload

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

最后要提的是点到点port

- 一个port对一条二层链路

- 不须要三层到二层的解析，仅仅须要配置链路号 - frame-relay interface-dlci [dlci]

- 更倾向于点到点配置（能够避免三层以上配置问题）

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

思科官方文档 - 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

验证

• 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

參考资料：

Comprehensive Guide to Configuring and Troubleshooting Frame Relay