Frame Relay is more cost effective then dedicated lines. It also has very little overhead, it provides no error correction and leaves that to the endpoints.
Ethernet L2 -vs- Frame-Relay L2
http://ccieblog.co.uk/ospf/ospf-non-broadcast-nbma-network
Ethernet:
Ethernet have Source and Destination for L2 addresses
When PC (10.0.0.2) need to send packet to R1 (10.0.0.1), PC needs to know L2 address of R1.
For that it sends L2 broadcast to find out L2 address of R1.
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| Click to Maximize |
Frame-Relay DOSNOT have Source and Destination for L2 addresses.
It is using DLCI (Identifier of ONE circuit). So Broadcast and Multicast could not be specified.
We can see our DLCI mappings are already discovered by the routers using Inverse-ARP with the command “Show frame-relay map“
As you can see in the graphic below, there is alot of similarity between an Ethernet ARP request and a Frame-Relay Inverse ARP.
Frame Relay Recap
from http://blogat.centilin.com/tag/non-broadcast-multi-access/
There is a lot of topics and confusion over Frame Relay, and I’d like to take some time to quickly make up a summary.
Frame Relay
Frame Relay is a Non-broadcast Multi-access (NBMA) network, meaning that multiple routers can connect to a central switch, creating a network. But, these nodes can’t really send multicast and broadcast to each other, the technology imitate the behavior of multicast and broadcast by making a copy of the packet and sending them to each router in unicast fashion.
Inverse ARP
Layer 2 addressing in Frame Relay uses data link connection identifier (DLCI) that’s locally significant to each router and its Frame Relay Switch only. Mapping between DLCI and IP address is done using a special type of ARP called inverse ARP. Alternatively, operators can manually specify static mapping (that overrides dynamic inverse ARP entries).
By default, all inverse ARP entries are “broadcast” enabled, whereas static mapping must explicitly use “broadcast” keyword to achieve imitation of multicast and broadcast behavior. Inverse ARP is enabled by default when you enable Frame Relay.
Note: when a router enables inverse ARP on its Frame Relay interface, it’s enabling the process of requesting inverse ARP information from other routers. The process of replying to inverse ARP information is always on; replying is not turned off when inverse ARP is turned off.
LMI
LMI is the status reporting protocol that exchanges information between the router (DTE) and Frame Relay Switch (DCE). There are 3 versions, or implementations of Frame Relay LMI: Cisco, ANSI, and Q933a. Nothing too much to worry about, since most or all Cisco devices can automatically detect the LMI protocol to use.
Its main purpose is to report status of VCs, which can be:
Active: DLCI is configured on local router, and DLCI is available at Frame Relay switch
Inactive: DLCI is not configured on local router, but DLCI is available at Frame Relay switch
Deleted: DLCI is configured on local router, but DLCI is not available at Frame Relay switch
Remember that LMI is only a mean to communicate VC status information between Frame Relay switch and the router; it reports local status rather than end-to-end status. For end-to-end status, you need to configure a map class.
Type of interfaces
When talking about Frame Relay, the most confusing topic may be the behavior of interfaces. Generally speaking, there are 2 types of interfaces: multipoint (multipoint sub-interfaces and main interface) and point-to-point (point-to-point sub-interfaces).
| Property | Multipoint | Point-to-point |
| Layer 2 Circuit | Multiple | One |
| Circuit Definition | With inverse ARP or ‘(config-if)#frame-relay interface-dlci DLCI’ |
with ‘(config-if)#frame-relay interface-dlci DLCI’ command |
| Resolution | Necessary | Not necessary |
Frame Relay Terms and Concepts
ANSI - American National Standards Institute
ITU - International Telecommunication Union
Virtual circuit (VC) - A logical concept that represents the path between 2 DTEs. VCs are particularly useful when comparing Frame Relay to leased physical circuits.
Permanent virtual circuit (PVC) - A predefined VC. A PVC can be equated to a leased line in concept.
Switched virtual circuit (SVC) - A VC that is set up dynamically when needed. An SVC can be equated to a dial connection in concept.
Data terminal equipment (DTE) - DTEs are connected to a Frame Relay service (client)
Data communications equipment (DCE) - Frame Relay switches (in the service provider's network)
Access link - The leased line between the DTE and DCE.
Access rate (AR) - The speed at which the access link is clocked. This choice affects the connection's price.
Data-link connection identifier (DLCI) - A Frame Relay address used in Frame Relay headers to identify the VC.
Nonbroadcast multiaccess (NBMA) - A network in which broadcasts are not supported, but more than two devices can be connected.
Local Management Interface (LMI) - The protocol used between a DCE and DTE to manage the connection. Signaling messages for SVCs, PVC status messages, and keepalives are all LMI messages.
Frame Relay Configuration Commands
[] - optional
{} - mandatory
Global configuration mode#
interface serial number.sub [point-to-point | multipoint]
Interface configuration mode#
encapsulation frame-relay [ietf | cisco]
frame-relay lmi-type {ansi | q933a | cisco}
bandwidth number - used by some routing protocols to influence the metric
frame-relay map {protocol protocol-address dlci} payload-compression frf9 stac caim [element-number] [broadcast] [ietf | cisco]
keepalive sec
frame-relay interface-dlci dlci [ietf | cisco] [voice-cir cir] [ppp virtual-template-name]
Privilege EXEC mode#
show interfaces [type number]
show frame-relay pvc [interface interface][dlci] - management information (counters, FECN, BECN)
show frame-relay lmi [type number]
show frame-relay map - lists mapping information
Debug (privilege EXEC CLI mode)
debug frame-relay lmi
debug frame-relay events
Frame Relay Technology
The connection between two DTEs on a frame relay network is called a Virtual Circuit (VC).
There are two types of VCs:
- SVCs (dynamically created)- PVCs (pre-configured by carrier).
VCs are identified by DLCIs (is ID from DTE to DCE switch).
A data link connection identifier (DLCI) (datalink layer address) is a channel number which is attached to frame relay data frames to tell the network how to route the data. This 10-bit field defines the destination address of a packet. The address is local on a link-by-link basis.
DLCIs are assigned by the Frame Relay service provider.
DLCIs have local significance and are not unique to the WAN.
DLCI is a 10 bit number. For each VC a DLCI is locally assigned. DLCIs are usually assigned as 16-1007, 0-15 and 1008-1023 are assigned special purposes.
Frame Relay encapsulates the Layer 3 protocol with a header and trailer. The header contains the DLCI and a checksum, plus a flag field of 0x7E is added to identify the start of frame.
Network Topologies
• Star topology
• Mesh topology (theoretical VC limit of 1000)
• Partial mesh topology
Before a Cisco router is able to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination.
The Inverse Address Resolution Protocol, also called Inverse ARP, obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks.
On Cisco routers, Inverse ARP is enabled by default for all protocols enabled on the physical interface.
To map between a next hop protocol address and DLCI destination address, use this command:
frame-relay map protocol protocol-address dlci [broadcast] [ietf] [cisco]
ex: frame relay map ip 10.1.1.2 102 broadcast cisco
Local Management Interface (LMI) is a protocol that provided additional capabilities for complex inter-networking environments (provide communication and synchronization between Frame Relay DTE and DCE devices).
The DTE and DCE are concerned that they use the same LMI. A DCE does not care about which encapsulation you choose but both DTE endpoints do. Different LMI types do not work with each other. Cisco routers support 3 LMI types:
• Cisco Original LMI extension
• ANSI
• q933a (ITU)
show frame-relay lmiRecent versions (11.2+) of the Cisco IOS by default use LMI auto sense, where the DTE detects which protocol the frame relay switch is using. You can also use frame-relay lmi-type [cisco | ansi | q933a] to manually set the LMI type. When manually setting the LMI type you must also set a keepalive value so there is not a large mismatch between the two devices. By default the keepalive value is set as 10.
Frame Relay Configuration
config tFrame-Relay Cisco IOS default settings
int s0/0
ip address 10.1.1.1 255.255.255.0
encapsulation frame-relay (cisco by default or ietf)
bandwidth 64000 (notes: set in kbps and is used by routing protocols)
frame-relay lmi-type cisco (optional because cisco routers auto sense)
- LMI type is auto sensed (could be cisco - default, ansi, q933),
- The default encapsulation is Cisco instead of IETF (IETF when in frame relay netw are non-Cisco routers),
- PVC DLCIs are learned via LMI status messages,
- Inverse ARP is enabled (by default) and is triggered when status message declaring the VCs are up is received.
Frame Relay, ATM, and X.25 are non-broadcast multi-access networks (NBMA). They will not allow multicast or broadcast messages over their links. When using RIP, EIGRP, or OSPF you will need to further configure these protocols to work with Frame Relay.
Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface.
You can use static mappings to work this situation. Return to the serial 0/0 interface and add the following configurations.
frame-relay map ip (destination int) (dlci) broadcastExample: frame-relay map ip 10.1.1.2 102 broadcast
Frame Relay Sub-interfaces
Point-to-point - A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router.
Multipoint - A single multipoint subinterface establishes multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All the participating interfaces are in the same subnet.
If you are using a multipoint subinterface, you will need to configure frame-relay maps and you cannot rely on inverse-arp.
If you are using a point-to-point subinterface, you will need to assign a DLCI to the subinterface.
This is only for point-to-point subinterfaces; this is not needed on the main interface or on multipoint subinterfaces. To assign a DLCI to a point-to-point subinterface, enter the following command under the subinterface:
frame-relay interface-dlci dlciUsing sub-interfaces to configure Frame Relay.
int s0/0.[dlci] [multipoint | point-to-point]Example:
int s 0/0/0Paying for Frame Relay based on:
no ip add
encap fr
no sh
int s0/0/0.102 point-to point
ip add 10.1.1.1 255.255.255.252
band 64
fr interface-dlci 102
int s0/0/0.103 point-to point
ip add 10.1.1.5 255.255.255.252
band 64
fr interface-dlci 103
• Access rate or port speed – 56kbps, T1,…
• Committed Information Rate (CIR) - Service provider guarantees customer can send data at the CIR.
Frame Relay shares media with all DTEs connected to the switch. If the bandwidth is not being used by everyone you have the ability to use more bandwidth than your access rate, which is called a burst.
• PVC: This cost component is based on the PVCs. Once a PVC is established, the additional cost to increase CIR is typically small and can be done in small (4 kb/s) increments.
Bursting - A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge.
Committed Burst Information Rate (CBIR) - is a negotiated rate above the CIR which the customer can use to transmit for short burst. The duration of a burst transmission should be short, less than three or four seconds.
Excess Burst (BE) size - the bandwidth available above the CBIR up to the access rate of the link.
Troubleshooting Commands
show interfacesInverse ARP
show frame-relay lmi
show frame-relay pvc
show frame-relay map
debug frame-relay lmi
Frame Relay uses Inverse ARP to map DCLIs to the IP addresses of remote locations.
Dynamic address mapping relies on Inverse ARP to resolve a next hop network protocol address to a local DLCI value. The Frame Relay router sends out Inverse ARP requests on its PVC to discover the protocol address of the remote device connected to the Frame Relay network.
Inverse ARP - you know the DLCI of the remote router but you don’t know its IP address.
Inverse ARP sends a request to obtain that IP address and map it to the Layer 2 frame-relay DLCI.
However, it is always recommended to disable & not use inverse ARP
because your frame end points could get different IP’s if the network were to be ‘bounced’.
Commands:
• frame-relay inverse-arp
• clear frame-relay-inarp
Inverse ARP States:
• Active: DTE to DTE connectivity
• Inactive: Successful connection to switch but not to remote DTE
• Deleted: DTE configured for DLCI switch does not recognize
Troubleshoot WAN
Congestion troubleshoointg - FECN/BECN concept
LAYER 1
# show controllers serial 0
MK5 unit 0, NIM slot 1, NIM type code 7, NIM version 1
idb = 0x6150, driver structure at 0x34A878, regaddr = 0x8100300
IB at 0x6045500: mode=0x0108, local_addr=0, remote_addr=0
N1=1524, N2=1, scaler=100, T1=1000, T3=2000, TP=1
buffer size 1524
DTE V.35 serial cable attached
RX ring with 32 entries at 0x45560 : RLEN=5, Rxhead 0
00 pak=0x6044D78 ds=0x6044ED4 status=80 max_size=1524 pak_size=0
# show controllers s1/0
M8T-X.21: show controller:
PAS unit 0, subunit 0, f/w version 3-101, rev ID 0x2800001, version 1
idb = 0x66BC53CC, ds = 0x66BC6514, ssb=0x66BC68D8
Clock mux=0x0, ucmd_ctrl=0x1C, port_status=0x7B
Serial config=0x8, line config=0x302
maxdgram=1608, bufpool=32Kb, 64 particles
DCD=up DSR=up DTR=up RTS=up CTS=up
line state: up
cable type : V.11 (X.21) DCE cable, received clockrate 2015232
base0 registers=0x3C800000, base1 registers=0x3C802000
mxt_ds=0x671DFD04, rx ring entries=37, tx ring entries=64
LAYER 2
# show interface serial 0PPP
Serial 0 is up, line protocol is up
Hardware is MCI Serial
Internet address is 192.168.10.203, subnet mask is 255.255.255.0
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input 0:00:07, output 0:00:00, output hang never
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Five minute input rate 0 bits/sec, 0 packets/sec
Five minute output rate 0 bits/sec, 0 packets/sec
16263 packets input, 1347238 bytes, 0 no buffer
Received 13983 broadcasts, 0 runts, 0 giants
2 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 2 abort
1 carrier transitions
# show interface serial 1/0
Serial1/0 is up, line protocol is down
Hardware is M8T-X.21
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY IETF, crc 16, loopback not set
Keepalive set (10 sec)
Restart-Delay is 0 secs
LMI enq sent 33, LMI stat recvd 0, LMI upd recvd 0, DTE LMI down
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
FR SVC disabled, LAPF state down
..
# sh inte ser 1/0 | in line|ANSI|FRAME
Serial1/0 is up, line protocol is up
Encapsulation FRAME-RELAY, crc 16, loopback not set
LMI DLCI 0 LMI type is ANSI Annex D frame relay DTE segmentation inactive
# sh frame-relay lmi
LMI Statistics for interface Serial1/0 (Frame Relay DTE) LMI TYPE = ANSI
# debug ppp authentication
# debug ppp negotiation
*Mar 1 00:40:03.835: %LINK-3-UPDOWN: Interface Serial0/0, changed state to up
*Mar 1 00:40:03.839: Se0/0 PPP: Using default call direction
*Mar 1 00:40:03.839: Se0/0 PPP: Treating connection as a dedicated line
*Mar 1 00:40:03.839: Se0/0 PPP: Session handle[F300006A] Session id[119]
*Mar 1 00:40:03.839: Se0/0 PPP: Phase is ESTABLISHING, Active Open
*Mar 1 00:40:03.839: Se0/0 PPP: Authorization required
*Mar 1 00:40:03.839: Se0/0 LCP: O CONFREQ [Closed] id 189 len 15
*Mar 1 00:40:03.839: Se0/0 LCP: AuthProto CHAP (0x0305C22305)
*Mar 1 00:40:03.839: Se0/0 LCP: MagicNumber 0x0030B4D6 (0x05060030B4D6)
*Mar 1 00:40:03.851: Se0/0 LCP: I CONFREQ [REQsent] id 67 len 10
*Mar 1 00:40:03.851: Se0/0 LCP: MagicNumber 0x0030B1AA (0x05060030B1AA)
*Mar 1 00:40:03.851: Se0/0 LCP: O CONFACK [REQsent] id 67 len 10
*Mar 1 00:40:03.855: Se0/0 LCP: MagicNumber 0x0030B1AA (0x05060030B1AA)
*Mar 1 00:40:03.855: Se0/0 LCP: I CONFACK [ACKsent] id 189 len 15
*Mar 1 00:40:03.855: Se0/0 LCP: AuthProto CHAP (0x0305C22305)
*Mar 1 00:40:03.855: Se0/0 LCP: MagicNumber 0x0030B4D6 (0x05060030B4D6)
*Mar 1 00:40:03.855: Se0/0 LCP: State is Open
*Mar 1 00:40:03.855: Se0/0 PPP: Phase is AUTHENTICATING, by this end
*Mar 1 00:40:03.855: Se0/0 CHAP: O CHALLENGE id 20 len 23 from "R1"
*Mar 1 00:40:03.871: Se0/0 CHAP: I RESPONSE id 20 len 23 from "R2"
*Mar 1 00:40:03.871: Se0/0 PPP: Phase is FORWARDING, Attempting Forward
*Mar 1 00:40:03.875: Se0/0 PPP: Phase is AUTHENTICATING, Unauthenticated User
*Mar 1 00:40:03.875: Se0/0 PPP: Sent CHAP LOGIN Request
*Mar 1 00:40:03.879: Se0/0 PPP: Received LOGIN Response PASS
*Mar 1 00:40:03.879: Se0/0 PPP: Phase is FORWARDING, Attempting Forward
*Mar 1 00:40:03.879: Se0/0 PPP: Phase is AUTHENTICATING, Authenticated User
*Mar 1 00:40:03.879: Se0/0 PPP: Sent LCP AUTHOR Request
*Mar 1 00:40:03.879: Se0/0 PPP: Sent IPCP AUTHOR Request
*Mar 1 00:40:03.879: Se0/0 LCP: Received AAA AUTHOR Response PASS
*Mar 1 00:40:03.879: Se0/0 IPCP: Received AAA AUTHOR Response PASS
*Mar 1 00:40:03.879: Se0/0 CHAP: O SUCCESS id 20 len 4
*Mar 1 00:40:03.879: Se0/0 PPP: Phase is UP
*Mar 1 00:40:03.879: Se0/0 IPCP: O CONFREQ [Closed] id 1 len 10
*Mar 1 00:40:03.883: Se0/0 IPCP: Address 150.1.12.1 (0x030696010C01)
*Mar 1 00:40:03.883: Se0/0 PPP: Sent CDPCP AUTHOR Request
*Mar 1 00:40:03.883: Se0/0 PPP: Process pending ncp packets
*Mar 1 00:40:03.883: Se0/0 CDPCP: Received AAA AUTHOR Response PASS
*Mar 1 00:40:03.883: Se0/0 CDPCP: O CONFREQ [Closed] id 1 len 4
*Mar 1 00:40:03.891: Se0/0 do IPCP: I CONFREQ [REQsent] id 1 len 10
*Mar 1 00:40:03.891: Se0/0 IPCP: Address 150.1.12.2 (0x030696010C02)
*Mar 1 00:40:03.891: Se0/0 AAA/AUTHOR/IPCP: Start. Her address 150.1.12.2, we want 0.0.0.0
*Mar 1 00:40:03.895: Se0/0 PPP: Sent IPCP AUTHOR Request
*Mar 1 00:40:03.895: Se0/0 CDPCP: I CONFREQ [REQsent] id 1 len 4
*Mar 1 00:40:03.895: Se0/0 CDPCP: O CONFACK [REQsent] id 1 len 4
*Mar 1 00:40:03.895: Se0/0 AAA/AUTHOR/IPCP: Reject 150.1.12.2, using 0.0.0.0
*Mar 1 00:40:03.895: Se0/0 AAA/AUTHOR/IPCP: Done. Her address 150.1.12.2, we want 0.0.0.0
*Mar 1 00:40:03.895: Se0/0 IPCP: O CONFACK [REQsent] id 1 len 10
*Mar 1 00:40:03.895: Se0/0 IPCP: Address 150.1.12.2 (0x030696010C02)
*Mar 1 00:40:03.899: Se0/0 IPCP: I CONFACK [ACKsent] id 1 len 10
*Mar 1 00:40:03.903: Se0/0 IPCP: Address 150.1.12.1 (0x030696010C01)
*Mar 1 00:40:03.903: Se0/0 IPCP: State is Open
*Mar 1 00:40:03.903: Se0/0 CDPCP: I CONFACK [ACKsent] id 1 len 4
*Mar 1 00:40:03.903: Se0/0 CDPCP: State is Open
*Mar 1 00:40:03.907: Se0/0 IPCP: Install route to 150.1.12.2
*Mar 1 00:40:04.879: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up
Frame-Relay loopback test, CSU/DSU and Modem
“Serial0/0 is down, line protocol is down”
1. Set encapsulation hdlc and keepalive 10;
2. Place CSU/DSU or modem in local loop mode.
- If line protocol comes up, the problem is beyond the local CSU/DSU,
- If it does not, then problem is in CSU/DSU, modem, cable connection or router. In most cases, problem is in CSU/DSU or modem;
3. Ping your own IP address from CSU/DSU or modem looped.
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