ccna study guide by sybex phần 8 docx

Frame Relay 491 7. The CO receives the frame and sends it through the Frame Relay “cloud” to its destination. This cloud can be dozens of switching offices—or more! It looks for the destination IP address and DLCI number. It typically can find the DLCI number of the remote device or router by looking up an IP-to-DLCI mapping. Frame Relay mappings are usually created statically by the service provider, but they can be created dynamically using the Inverse ARP (IARP) protocol. Remem- ber that before data is sent through the cloud, the virtual circuit is cre- ated from end to end. 8. Once the frame reaches the switching office closest to the destination office, it is sent through the local loop. The frame is received at the demarc and then is sent to the CSU/DSU. Finally, the router extracts the packet, or datagram, from the frame and puts the packet in a new LAN frame to be delivered to the destination host. The frame on the LAN will have the final destination hardware address in the header. This was found in the router’s ARP cache, or an ARP broad- cast was performed. Whew! The user and server do not need to know, nor should they know, every- thing that happens as the frame makes its way across the Frame Relay net- work. The remote server should be as easy to use as a locally connected resource. Frame Relay Encapsulation When configuring Frame Relay on Cisco routers, you need to specify it as an encapsulation on serial interfaces. There are only two encapsulation types: Cisco and IETF (Internet Engineering Task Force). The following router output shows the two different encapsulation methods when choosing Frame Relay on your Cisco router: RouterA(config)#int s0 RouterA(config-if)#encapsulation frame-relay ? ietf Use RFC1490 encapsulation <cr> The default encapsulation is Cisco unless you manually type in IETF, and Cisco is the type used when connecting two Cisco devices. You’d opt for the IETF-type encapsulation if you needed to connect a Cisco device to a Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com 492 Chapter 10  Wide Area Networking Protocols non-Cisco device with Frame Relay. So before choosing an encapsulation type, check with your ISP and find out which one they use. (If they don’t know, hook up with a different ISP!) Data Link Connection Identifiers (DLCIs) Frame Relay virtual circuits (PVCs) are identified by DLCIs. A Frame Relay service provider, such as the telephone company, typically assigns DLCI val- ues, which are used by Frame Relay to distinguish between different virtual cir- cuits on the network. Because many virtual circuits can be terminated on one multipoint Frame Relay interface, many DLCIs are often affiliated with it. For the IP devices at each end of a virtual circuit to communicate, their IP addresses need to be mapped to DLCIs. This mapping can function as a mul- tipoint device—one that can identify to the Frame Relay network the appro- priate destination virtual circuit for each packet that is sent over the single physical interface. The mappings can be done dynamically through IARP or manually through the Frame Relay map command. Frame Relay uses DLCIs the same way that X.25 uses X.121 addresses, and every DLCI number can be given either global or local meaning every- where within the Frame Relay network. Sometimes a provider can give a site a DLCI that is advertised to all remote sites as the same PVC. This PVC is said to have a global significance. For example, a corporate office might have a DLCI of 20. All remote sites would know that the corporate office is DLCI 20 and use this PVC to com- municate to the corporate office. However, the customary implementation is to give each DLCI local meaning. What does this mean? It means that DLCI numbers do not necessarily need to be unique. Two DLCI numbers can be the same on different sides of a link because Frame Relay maps a local DLCI number to a virtual circuit on each interface of the switch. Each remote office can have its own DLCI number and communicate with the corporate office using unique DLCI numbers. DLCI numbers, used to identify a PVC, are typically assigned by the pro- vider and start at 16. Configuring a DLCI number to be applied to an inter- face is shown below: RouterA(config-if)#frame-relay interface-dlci ? <16-1007> Define a DLCI as part of the current subinterface RouterA(config-if)#frame-relay interface-dlci 16 Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com Frame Relay 493 Local Management Interface (LMI) The Local Management Interface (LMI) was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corpora- tion and became known as the Gang-of-Four LMI or Cisco LMI. This gang took the basic Frame Relay protocol from the CCIT and added extensions onto the protocol features that allow internetworking devices to communi- cate easily with a Frame Relay network. The LMI is a signaling standard between a CPE device (router) and a frame switch. The LMI is responsible for managing and maintaining status between these devices. LMI messages provide information about the following: Keepalives Verify data is flowing Multicasting Provides a local DLCI PVC Multicast addressing Provides global significance Status of virtual circuits Provides DLCI status Beginning with IOS version 11.2, the LMI type is auto-sensed. This enables the interface to determine the LMI type supported by the switch. If you’re not going to use the auto-sense feature, you’ll need to check with your Frame Relay provider to find out which type to use instead. The default type is Cisco, but you may need to change to ANSI or Q.933A. The three dif- ferent LMI types are depicted in the router output below. RouterA(config-if)#frame-relay lmi-type ? cisco ansi q933a As seen in the output, all three standard LMI signaling formats are supported: Cisco LMI defined by the Gang of Four (default) ANSI Annex D defined by ANSI standard T1.617 ITU-T (q933a) Annex A defined by Q.933 Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com 494 Chapter 10  Wide Area Networking Protocols Routers receive LMI information on a frame-encapsulated interface and update the virtual circuit status to one of three different states: Active state Everything is up and routers can exchange information. Inactive state The router’s interface is up and working with a connec- tion to the switching office, but the remote router is not working. Deleted state This means that no LMI information is being received on the interface from the switch. It could be a mapping problem or a line failure. Subinterfaces You can have multiple virtual circuits on a single serial interface and yet treat each as a separate interface. These are known as subinterfaces. Think of a subinterface as a hardware interface defined by the IOS software. An advan- tage gained through using subinterfaces is the ability to assign different Net- work layer characteristics to each subinterface and virtual circuit, such as IP routing on one virtual circuit and IPX on another. Partial Meshed Networks You can use subinterfaces to mitigate partial meshed Frame Relay networks and split horizon protocols. For example, say you were running the IP pro- tocol on a LAN network. If, on the same physical network, Router A can talk to Router B, and Router B to Router C, you can usually assume that Router A can talk to Router C. Though this is true with a LAN, it’s not true with a Frame Relay network, unless Router A has a PVC to Router C. In Figure 10.5, Network 1 is configured with five locations. To be able to make this network function, you would have to create a meshed network as shown in Network 2. However, even though Network 2’s example works, it’s an expensive solution—configuring subinterfaces as shown in the Net- work 3 solution is much more cost-effective. In Network 3, configuring subinterfaces actually works to subdivide the Frame Relay network into smaller subnetworks—each with its own network number. So locations A, B, and C connect to a fully meshed network, while locations C and D, and D and E, are connected via point-to-point connec- tions. Locations C and D connect to two subinterfaces and forward packets. Subinterfaces also solve the problem with routing protocols that use split horizon. As you may recall, split horizon protocols do not advertise routes Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com Frame Relay 495 out the same interface they received the route update on. This can cause a problem on a meshed Frame Relay network. However, by using subinter- faces, routing protocols that receive route updates on one subinterface can send out the same route update on another subinterface. FIGURE 10.5 Partial meshed network examples Creating Subinterfaces You define subinterfaces with the int s0.subinterface number com- mand as shown below. You first set the encapsulation on the serial interface, then you can define the subinterfaces. RouterA(config)#int s0 RouterA(config)#encapsulation frame-relay Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com 496 Chapter 10  Wide Area Networking Protocols RouterA(config)#int s0.? <0-4294967295> Serial interface number RouterA(config)#int s0.16 ? multipoint Treat as a multipoint link point-to-point Treat as a point-to-point link You can define an almost limitless number of subinterfaces on a given physical interface (keeping router memory in mind). In the above example, we chose to use subinterface 16 because that represents the DLCI number assigned to that interface. However, you can choose any number between 0 and 4,292,967,295. There are two types of subinterfaces: Point-to-point Used when a single virtual circuit connects one router to another. Each point-to-point subinterface requires its own subnet. Multipoint Used when the router is the center of a star of virtual cir- cuits. Uses a single subnet for all routers’ serial interfaces connected to the frame switch. An example of a production router running multiple subinterfaces is shown below. Notice that the subinterface number matches the DLCI num- ber. This is not a requirement but helps in the administration of the inter- faces. Also notice that there is no LMI type defined, which means they are running either the default of Cisco or using autodetect if running Cisco IOS version 11.2 or newer. This configuration was taken from one of my cus- tomers’ production routers (used by permission). Notice that each interface is defined as a separate subnet, separate IPX network, and separate Apple- Talk cable range (AppleTalk is beyond the scope of this course): interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay ! interface Serial0.102 point-to-point ip address 10.1.12.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 12-12 12.65 appletalk zone wan2 Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com Frame Relay 497 appletalk protocol eigrp no appletalk protocol rtmp ipx network 12 frame-relay interface-dlci 102 ! interface Serial0.103 point-to-point ip address 10.1.13.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 13-13 13.174 appletalk zone wan3 appletalk protocol eigrp no appletalk protocol rtmp ipx network 13 frame-relay interface-dlci 103 ! interface Serial0.104 point-to-point ip address 10.1.14.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 14-14 14.131 appletalk zone wan4 appletalk protocol eigrp no appletalk protocol rtmp ipx network 14 frame-relay interface-dlci 104 ! interface Serial0.105 point-to-point ip address 10.1.15.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 15-15 15.184 appletalk zone wan5 appletalk protocol eigrp no appletalk protocol rtmp ipx network 15 frame-relay interface-dlci 105 ! interface Serial0.106 point-to-point Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com 498 Chapter 10  Wide Area Networking Protocols ip address 10.1.16.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 16-16 16.28 appletalk zone wan6 appletalk protocol eigrp no appletalk protocol rtmp ipx network 16 frame-relay interface-dlci 106 ! interface Serial0.107 point-to-point ip address 10.1.17.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 17-17 17.223 appletalk zone wan7 appletalk protocol eigrp no appletalk protocol rtmp ipx network 17 frame-relay interface-dlci 107 ! interface Serial0.108 point-to-point ip address 10.1.18.1 255.255.255.0 no ip directed-broadcast appletalk cable-range 18-18 18.43 appletalk zone wan8 appletalk protocol eigrp no appletalk protocol rtmp ipx network 18 frame-relay interface-dlci 108 Mapping Frame Relay As we explained earlier, in order for IP devices at the ends of virtual circuits to communicate, their addresses must be mapped to the DLCIs. There are two ways to make this mapping happen:  Use the Frame Relay map command.  Use the inverse-arp function. Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com Frame Relay 499 Here’s an example using the Frame Relay map command: RouterA(config)#int s0 RouterA(config-if)#encap frame RouterA(config-if)#int s0.16 point-to-point RouterA(config-if)#no inverse-arp RouterA(config-if)#ip address 172.16.30.1 255.255.255.0 RouterA(config-if)#frame-relay map ip 172.16.30.17 16 ietf broadcast RouterA(config-if)#frame-relay map ip 172.16.30.18 17 broadcast RouterA(config-if)#frame-relay map ip 172.16.30.19 18 Here’s what we did: First, we chose configured interface serial 0 to use the encapsulation type of Cisco (default), then we created our subinterface. We then turned off inverse arp and mapped three virtual circuits and their cor- responding DLCI numbers. Notice that we changed the encapsulation type for the first mapping. The frame map command is the only way to configure multiple frame encapsu- lation types on an interface. The broadcast keyword at the end of the map command tells the router to forward broadcasts for this interface to this specific virtual circuit. Remember that Frame Relay is a nonbroadcast multiaccess (NBMA) encap- sulation method, which will not broadcast routing protocols. You can either use the map command with the broadcast keyword or the neighbor com- mand within the routing process. Instead of putting in map commands for each virtual circuit, you can use the inverse-arp function to perform dynamic mapping of the IP address to the DLCI number. This makes our configuration look like this: RouterA(config)#int s0.16 point-to-point RouterA(config-if)#encap frame-relay ietf RouterA(config-if)#ip address 172.16.30.1 255.255.255.0 Yes, this configuration is a whole lot easier to do, but it’s not as stable as using the map command. Why? Sometimes, when using the inverse-arp function, configuration errors occur because virtual circuits can be insidi- ously and dynamically mapped to unknown devices. Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com 500 Chapter 10  Wide Area Networking Protocols Frame Relay Congestion Control In this section we will define how the Frame Relay switch handles congestion problems. DE (Discard Eligibility) When a Frame Relay router detects congestion on the Frame Relay network, it will turn the DE bit on in a Frame Relay packet header. If the switch is congested, the Frame Relay switch will dis- card the packets with the DE bit set first. If your bandwidth is configured with a CIR of zero, the DE will always be on. FECN (Forward-Explicit Congestion Notification) When the Frame Relay network recognizes congestion in the cloud, the switch will set the FECN bit to 1 in a Frame Relay packet header. This will indicate to the destination DCE that the path just traversed is congested. BECN (Backward-Explicit Congestion Notification) When the switch detects congestion in the Frame Relay network, it will set the BECN bit in a Frame Relay packet and send it to the source router, telling it to slow down the rate at which it is transmitting packets. Committed Information Rate (CIR) Frame Relay provides a packet-switched network to many different custom- ers at the same time. This is a great idea because it spreads the cost of the switches among many customers. However, Frame Relay is based on the assumption that not all customers need to transmit constant data all at the same time. Frame Relay works best with bursty traffic. Think of Frame Relay as a party line. Remember party lines? That is when many people on your block had to share the same phone number. Okay, I am showing my age here, but understand that party lines were created on the assumption that few people needed to use the phone each day. If you needed to talk excessively, you had to pay for the more expensive dedicated circuit. Frame Relay works somewhat on the same principle, except many devices can transmit at the same time. However, if you need a constant data-stream connection, then Frame Relay is not for you. Buy a dedicated, point-to-point T-1 instead. Frame Relay works by providing a dedicated bandwidth to each user, who is committed to that bandwidth at any given time. Frame Relay provid- ers allow customers to buy a lower amount of bandwidth than what they Copyright ©2000 SYBEX , Inc., Alameda, CA www.sybex.com [...]... 255.255.255.0 no ip directed-broadcast encapsulation ppp dialer idle-timeout 300 dialer string 83 586 61 dialer load-threshold 2 either dialer-group 1 isdn switch-type basic-ni isdn spid1 083 586 6201 83 586 62 isdn spid2 083 586 6401 83 586 64 hold-queue 75 in ! ip classless Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com Dial-on-Demand Routing (DDR) 515 ip route 172.16.30.0 255.255.255.0 172.16.60.1 ip... configure the five steps: 80 4A#config t 80 4A(config)#int bri0 80 4A(config-if)#ip address 172.16.60.1 255.255.255.0 80 4A(config-if)#no shut 80 4A(config-if)#encapsulation ppp 80 4A(config-if)#dialer-group 1 80 4A(config-if)#dialer-string 83 50661 Instead of the dialer-string command, you can use a dialer map, which provides more security 80 4A(config-if)#dialer map ip 172.16.60.2 name 80 4B 83 50661 The dialer map... input pkts 5097 787 6 output pkts 4 182 289 2 in bytes 3137403144 out bytes 34 080 47602 dropped pkts 5 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 9393 out DE pkts 0 pvc create time 7w3d, last time pvc status changed 7w3d DLCI = 18, DLCI USAGE =LOCAL,PVC STATUS =ACTIVE,INTERFACE = Serial0.3 input pkts 30572401 output pkts 3113 983 7 in bytes 1797291100 out bytes 3227 181 474 dropped... dial to get there Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com 514 Chapter 10 Wide Area Networking Protocols Take a look at the following configuration of an 80 4 router: 80 4B#sh run Building configuration Current configuration: ! version 12.0 no service pad service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname 80 4B ! ip subnet-zero ! isdn switch-type... administrator using the dialer-list global configuration command defines interesting packets Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com Dial-on-Demand Routing (DDR) 513 The command to turn on all IP traffic is shown as follows: 80 4A(config)#dialer-list 1 protocol ip permit 80 4A(config)#int bri0 80 4A(config-if)#dialer-group 1 The dialer-group command sets the access list on the BRI interface Extended... load-threshold 125 either RouterA(config-if)#dialer idle-timeout 180 The dialer load-threshold 125 tells the BRI interface to bring up the second B channel if either the inbound or outbound traffic load is 50 percent The dialer idle-timeout 180 changes the default disconnect time from 120 to 180 seconds Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com 516 Chapter 10 Wide Area Networking Protocols DDR... for example, to only e-mail or Telnet Here is an example of how you define the dialer list to use an access list: 80 4A(config)#dialer-list 1 list 110 80 4A(config)#access-list 110 permit tcp any any eq smtp 80 4A(config)#access-list 110 permit tcp any any eq telnet 80 4A(config)#int bri0 80 4A(config-if)#dialer-group 1 In the preceding example, you configure the dialer-list command to look at an access... mappings RouterB#show frame map Serial0 (up): ipx 20.0007. 784 2.3575 dlci 16(0x10,0x400), dynamic, broadcast,, status defined, active Serial0 (up): ip 172.16.20.1 dlci 16(0x10,0x400), dynamic, broadcast,, status defined, active Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com 504 Chapter 10 Wide Area Networking Protocols Serial1 (up): ipx 40.0007. 784 2.153a dlci 17(0x11,0x410), dynamic, broadcast,, status... commands, one per line End with CNTL/Z RouterA(config)#isdn switch-type basic-ne1 RouterA(config)#int bri0 RouterA(config-if)#encap ppp (optional) RouterA(config-if)#isdn spid1 086 506610100 86 50661 RouterA(config-if)#isdn spid2 086 506620100 86 50662 The isdn switch-type command can be configured in either global configuration or interface configuration mode Configuring the switch type global will set the switch... RouterB(config)#username RouterA password todd 8 Enable CHAP or PAP authentication on each interface RouterA(config)#int s0 RouterA(config-if)#ppp authentication chap RouterB(config)#int s0 RouterB(config-if)#ppp authentication chap 9 Verify the PPP configuration on each router by using these two commands: sh int s0 debug PPP authentication Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com 526 Chapter 10 Wide . interface-dlci 107 ! interface Serial0.1 08 point-to-point ip address 10.1. 18. 1 255.255.255.0 no ip directed-broadcast appletalk cable-range 18- 18 18. 43 appletalk zone wan8 appletalk protocol eigrp . LOCAL,PVC STATUS =ACTIVE,INTERFACE = Serial0.1 input pkts 5097 787 6 output pkts 4 182 289 2 in bytes 3137403144 out bytes 34 080 47602 dropped pkts 5 in FECN pkts 0 in BECN pkts 0 out FECN pkts. supported: Cisco LMI defined by the Gang of Four (default) ANSI Annex D defined by ANSI standard T1.617 ITU-T (q933a) Annex A defined by Q.933 Copyright ©2000 SYBEX , Inc., Alameda, CA www .sybex. com 494 Chapter