Frame relay

Some facts:

• It’s set of standarts.
• Based on X.25 technology.
• It is is a packet-switched technology.
• Frame Relay networks are called nonbroadcast multiaccess (NBMA) networks. More than 2 devices can be attached to the network, but you can't send broadcast. Also, because Frame Relay is multiaccess, it requires the use of an address that identifies to which remote router each frame is addressed.
• DTE sends keepalive messages using Local Management Interface (LMI) protocol.
• The routers are considered DTE, and the Frame Relay switches are data communications equipment (DCE).
• Predefined VCs are called permanent virtual circuits (PVC).
• Routers use the data-link connection identifier (DLCI) as the Frame Relay address
• 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 from a telecommunications company. They typically reside at sites used by the company buying the Frame Relay service.
• Data communication equipment (DCE) - Frame Relay switches are DCE devices. DCEs are also known as data circuit-terminating equipment. DCEs are typically 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.
• Committed Information Rate (CIR) - The speed at which bits can be sent over a VC, according to the business contract between the customer and provider.
• Data-link connection identifier (DLCI) - A Frame Relay address used in Frame Relay headers to identify the VC. In DLCI the header has a single DLCI field, not both Source and Destination DLCI fields.
• 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.
• Each VC has a CIR, which is a guarantee by the provider that a particular VC gets at least that much bandwidth.
• When there are PVCs between each pair of sites; this is called a full-mesh Frame Relay network.
• When not all pairs have a direct PVC, it is called a partial-mesh network.

There are 3 LMI standards:

• Cisco
• ITU
• ANSI

Frame-relay lmi-type interface subcommand is used to configure LMI type. A Frame Relay-connected router encapsulates each Layer 3 packet inside a Frame Relay header and trailer before it is sent out an access link. The header and trailer are defined by the Link Access Procedure Frame Bearer Services (LAPF) specification, ITU Q.922-A.

LAPF Header Information LAPF trailer
LAPF header consist of:

• DLCI – 10 bits


• Forward Explicit Congestion Notification (FECN) – 1 bit. This bit is set by router or DCE. It means, that this frame itself has experienced congection.


• Backward Explicit Congestion Notification (BECN) – 1bit - is sent to router, when this router makes congestion.


• Discard Eligibility (DE) – 1 bit - is set to frames with unimportant traffic, the provider can discard this packets, if there is congestion.

BUT there is no protocol type field, there are 2 solutions about this:

• LAPF header, Cisco, Packet,LAPF trailer


• LAPF header, RFC 1490, Packet, LAPF trailer

Two types of encapsulation:

• encapsulation frame-relay ietf


• encapsulation frame-relay cisco

DTEs use and react to the fields specified by these two types of encapsulation, but Frame Relay switches ignore these fields. Because the frames flow from DTE to DTE, both DTEs should agree on the encapsulation used. The switches don’t care. However, each VC can use a different encapsulation. In the configuration, the encapsulation created by Cisco is called cisco, and the other one is called ietf.

Addressing

Local addressing is significant only on local access link on one PVC. Global addressing is simply a way of choosing DLCI numbers when planning a Frame Relay network so that working with DLCIs is much easier. Because local addressing is a fact, global addressing does not change these rules. Source DTE puts DLCI of destination DTE to the header, and the last switch, that is directly connected to the destination DTE, changes DLCI to the source DTE.

The sender treats the DLCI field as a destination address, using the destination’s global DLCI in the header.
The receiver thinks of the DLCI field as the source address, because it contains the global DLCI of the frame’s sender.
It is possible address to physical interface, multipoint or point-to-point subinterfaces.

Network Layer Concerns

3 ways of assigning ip addresses:

• One subnet containing all Frame Relay DTEs


• One subnet per VC - hub and spoke technologie - it avoids split horizon issues.


• A hybrid of the first two options

There is no capability exists for a Frame Relay DTE to send a single frame into the Frame Relay network and have that frame replicated and delivered across multiple VCs to multiple destinations. However, routers need to send broadcasts for several features to work. In particular, routing protocol updates are either broadcasts or multicasts. To map DLCI to ip Inverse ARP is used. Inverse ARP dynamically creates a mapping between the Layer 3 address (for example, the IP address) and the Layer 2 address (the DLCI). After the VC is up, each router announces its network layer address by sending an Inverse ARP message over that VC. To map staticly, you need to enter interface sub command, for instant:

frame-relay map ip 199.1.1.2 52 broadcast

The broadcast keyword is required when the router needs to send broadcasts or multicasts to the neighboring router — for example, to support routing protocol messages such as Hellos.

Configuring Frame Relay

Full meshed with 1 i p network:

interface serial0/0/0

encapsulation frame-relay

ip address 199.1.1.2 255.255.255.0

The LMI type is automatically sensed.

The (default) encapsulation is Cisco instead of IETF.

PVC DLCIs are learned via LMI status messages.

Inverse ARP is enabled (by default) and is triggered when the status message declaring that the VCs are up is received.

Change lmi:

frame-relay lmi-type ansi

Change encapsulation on all VCs on interface:

interface serial0/0/0

encapsulation frame-relay ietf

Change encapsulation for one VC:

frame-relay interface-dlci 53 ietf

Static mapping:

no frame-relay inverse-arp

frame-relay map ip 199.1.1.2 52 broadcast

The broadcast keyword is required when the router needs to send broadcasts or multicasts to the neighboring router—for example, to support routing protocol messages such as Hellos

A Partially Meshed Network with One IP Subnet Per VC

interface serial 0/0/0.1 point-to-point

ip address 140.1.1.2 255.255.255.0

frame-relay interface-dlci 51

Another way to map dlci to subinterface:

frame-relay map ip 140.1.1.2 52 broadcast

First physical interface subcommand encapsulation frame-relay. Then configure ip address(ip address interface subcommand). Define lmi type with frame relay lmi-type command. Manually map ip to DLCI frame-relay map ip.

The LMI setting is a per-physical-interface setting, even if subinterfaces are used, so the frame-relay lmi-type command is always a subcommand under the physical interface.

Show commands:

Show interfaces – shows all info about interface

Show frame-relay pvc - shows information about all PVCs.

Show frame-relay map - it shows IP-to-DLCI mappings.

Show frame-relay lmi

Interface serial 0/1/0.0 point-to point Frame-relay interface-dlci 53 - This command is typically used for subinterfaces DLCI is configured for every sub interface. Cisco IOS software needs to associate the correct PVC with the correct subinterface. This is accomplished with the frame-relay interface-dlci command. In case of sub interfaces DLCI is destination DLCI. This is Global sheme of addressing. If there is only one DLCI on main site router, than this is Global Addressing.

WAN

WAN technologies

• Frame Relay – old technology, it is replaced by MPLS nowadays
• ISDN
• LAPB
• LAPD
• HDLC
• PPP
• PPPoE
• Cable
• DSL
• MPLS
• ATM

Some facts:

• T1 - 1,544 Mbit/s


• T3 - 44.736 Mbit/s


• Local loop - telco cable between your home and telco central office


• Analog modems and DSL both support symmeric and asymmenric transfers


• Default encapsulation is HDLC


• All HDLC versions are proprietary


• PPP is industry standard. It can be used to create point-to-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol and allows authentication and multi-link connections to be run over asynchronous and synchronous links


• DCE – modems


• DTE – computers, laptops, routers


• DCE side determined by cable


• Add clocking to DCE side only


• A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit, for example a T1 or T3 line


• A CSU/DSU operates at the physical layer (layer 1) of the OSI model. CSU/DSUs are also made as separate physical products: CSUs and DSUs


• RS-232 end of the cable connets to CSU/DSU, on the router side cable is router specific


• Digital lines require both a channel service unit (CSU) and a data service unit (DSU). The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the data encoded in the digital circuit into synchronous serial data for connection to a DTE device.

user#config t

user(config)#interface serial 0

user(config-if)#clock rate 64000

• By default, Cisco routers are all data terminal equipment (DTE) devices, which means that you must configure an interface to provide clocking if you need it to act like a DCE device. Again, you would not provide clocking on a production T1 connection, for example, because you would have a CSU/DSU connected to your serial interface.

Todd(config-if)#clock rate ?

Speed (bits per second)

• To determine the type of the device: DTE or DCE, show controllers command is used:

user#sh controllers s0/2/0

Interface Serial0/2/0

Hardware is GT96K

DCE V.35, clock rate 1000000

• New ISR routers automatically detect DCE connections and set the clock rate to 2000000.


• Setting bandwith:

user#config t

user(config)#int s0/0/0

user(config-if)#bandwidth ?

1-10000000 Bandwidth in kilobits

inherit Specify that bandwidth is inherited

receive Specify receive-side bandwidth

user(config-if)#bandwidth 1000

• Bandwidth command is configured in kilobits.

FastEthernet0/0 is up, line protocol is up

• The first parameter refers to the Physical layer, and it’s up when it receives carrier detect.


• The second parameter refers to the Data Link layer, and it looks for keepalives from the con- necting end. (Keepalives are used between devices to make sure connectivity has not dropped.)


• Default band-width (BW) on all Cisco serial links: 1.544Kbps

VLAN

Some facts:

• Breaks up broadcast domains in a layer 2 switch internetwork


• Inter-Switch Link (ISL) - it’s used for Fast Ethernet and Gigabit Ethernet links only


• Minimum speed for trunk – 100 Mbps


• ISL fully encapsulates the original Ethernet frame in a trunking header


• Configuring vlans on switch is possible in transparent and server mode


• Access link is only part of one VLAN and is referred to as the “native VLAN” of the port


• All switches must use the same native VLAN. Native VLAN is connected with a port, not with a switch


• “router on a stick.” – when router routes all vlans


• VLANs can greatly simplify adding, moving, or changing hosts on the network

VLAN Trunking Protocol (VTP) Layer 2

• Default VTP mode is server


• The link between the switches must be operating as a VLAN trunk (ISL or 802.1Q).


• The two switches’ case-sensitive VTP domain name must match.


• If configured on at least one of the switches, the two switches’ case-sensitive VTP password must match.


• VTP only learns about normal-range VLANs, with VLAN IDs 1 to 1005; VLANs with IDs greater than 1005 are called extended-range VLANs and they’re not stored in the VLAN database.


• VLAN IDs 1 and 1002 to 1005 are automatically created on all switches and can’t be removed.


• VTP gives you a way to preserve bandwidth by configuring it to reduce the amount of broadcasts, multicasts, and unicast packets. This is called pruning.


• Extended-range VLANs (VLAN IDs1006 to 4094) can’t be pruned


• VTP servers and clients also send periodic VTP messages every 5 minutes, in case any newly added switches need to know the VLAN configuration.


• The default VTP configuration on Cisco switches is VTP server mode with a null domain name.


• After configuring a domain name, that switch immediately starts sending VTP updates over all its trunks.


• If a switch that still has a (default) null domain name receives a VTP update—which by definition lists a domain name—and no password was used by the sending switch, the receiving switch starts using that VTP domain name.

Dynamic Tranking protocol (DTP)

• Dynamic Trunking protocol, port may be in auto, desirable or on state.

(interface) switchport mode dynamic [auto | desirable]

• on - ntunking is on and sending DTP packets.


• desireable - These links would like to become trunk links and will send DTP signals that attempt to initiate a trunk. They will only become trunk links if the other side responds to the DTP signal.


• auto – doesn’t send DTP packets, becomes trunk, if receives DTP packet.

Troubleshooting

R1#show vlan
R1#show vlan brief
R1#show interfaces switchport
R1#show interfaces trunk

Ethernet & Cisco

Most popular Ethernet standards:

• IEEE 802.3u – Fast Ethernet
• IEEE 802.3z – Gigabit Ethernet over fiber optic
• IEEE 802.3ab – Gigabit Ethernet over twisted pair
• IEEE 802.3ae – 10 Gigabit Ethernet over fiber
• IEEE 802.3an – 10 Gigabit Ethernet over UTP

Some facts:

• There are no collisions in full-duplex mode.
• A dedicated switch port is required for each full-duplex node.
• The host network card and the switch port must be capable of operating in full-duplex mode.
• Runts are packets that are discarded because they are smaller than the medium's minimum packet size. Any Ethernet packet that is less than 64 bytes is considered a runt. In half-duplex environments, it is possible for both the switch and the connected device to sense the wire and transmit at exactly the same time and result in a collision. Collisions can cause runts, FCS, and alignment errors, caused when the frame is not completely copied to the wire, which results in fragmented frames. Runts are the result of collisions, faulty NIC's, duplex mismatch, IEEE 802.1Q (dot1q), or an Inter-Switch Link Protocol (ISL) configuration issue.
• Microsegmentation - segmentation of a collision domain into as many segments as there are circuits, minus one (#segments = #circuits - 1). This microsegmentation performed by the switch cuts the collision domain down so that only two nodes coexist within each collision domain. This way, collisions are decreased and only the two NICs which are directly connected via a point-to-point link are contending for the medium.
• If you want to implement a network medium that is not susceptible to elec-tromagnetic interference (EMI), fiber-optic cable provides a more secure, long-distance cable that is not susceptible to EMI at high speeds.
• Hub and switch can enlarge the area covered by a single LAN segment.
• If the duplex settings do not match on the ends of an Ethernet segment, the switch interface will still be in a connect (up/up) state. In this case, the interface works, but it may work poorly, with poor performance, and with symptoms of intermittent problems.
• Only show interfaces status shows whether or not auto negotiation is on.
• Autonegotiation is disabled if speed and duplex is configured.
• If the speed is not known, use 10 Mbps, half duplex.
• If the speed is somehow known to be 10 or 100 Mbps, default to use half duplex.
• If the speed is somehow known to be 1000 Mbps, default to use full duplex.
• Cisco switches can determine speed in a couple of ways even when IEEE standard autonegotiation fails. First, the switch knows the speed if the speed interface subcommand was manually configured. Additionally, even when IEEE autonegotiation fails, Cisco switches can automatically sense the speed used by the device on the other end of the cable, and can use that speed based on the electrical signals on the cable.

Lan problems

• They usually can be found using counters of  show interfaces
• Excessive interference on the cable - can cause the various input error counters to keep growing larger, especially the CRC counter. In particular, if the CRC errors grow, but the collisions counters do not, the problem may simply be interference on the cable.
• Duplex mismatch - collisions and late collision counters could keep growing
• Jabber - collisions and late collision counters could keep growing
• In particular, a significant problem exists if the collision counters show that more than 1% of all the output frames have collided.

MAC

MAC address is 48 bit or 6 bytes:

• Individual Group Bit
• Global/local bit or universal/local bit: 0 administered by IEEE, 1 by local
• 22 bits left of Organizational Unique Identifier (OUI)
• 24 bits vendor assigned

The most popular type of frame is Ethernet II:

• Dest addr 6 bytes
• Source addr 6 bytes
• Type 2 bytes - type of protocol
• Data
• FCS – frame check sequence - crc

Three types of twisted pair:

• Straight-through cable (PC, router, access point to switch, hub)
• Crossover cable (PC to PC, PC to router, switch to switch, hub to hub, switch to hub, router to roter)
• Rolled cable (PC to cisco console)

Network topologies

There are three basic categories of network topologies:

• physical topologies
• signal topologies
• logical topologies

There are six basic types of topology in networks:

• Bus topology
• Star topology
• Ring topology: double ring - max redundancy
• Mesh topology - max redundancy
• Tree topology
• Hybrid topology

OSI, DoD & TCP/IP

Some notes on this topic, made during CCNA preparation
OSI model
There are 7 layers:

Application layer
Application layer is acting as an interface between the actual application programs. It chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery. The Application layer is responsible for finding the network resources broadcast from a server and adding flow control and error control

VoIP needs better jitter, loss and delay, then for a example HTTP. VoIP typically requires less bandwidth than data applications.

Presentation layer
Presents data, for example encryption.

Session layer
Keeps different applications data separate.

Transport layer
Provides reliable or unreliable delivery, performs error correction before retransmit.

The Transport layer segments and reassembles data into a data stream.
Flow Control - The purpose of flow control is to provide a means for the receiver to govern the amount of data sent by the sender. Types of flow control:

• Buffering
• Windowing
• Congestion avoidance
• Connection-Oriented Communication
• Multiplexing/Demultiplexing
• Windowing - Windows are used to control the amount of outstanding, unacknowledged data segments.

Acknowledgments
TCP uses a concept called forward acknowledgment, in which the acknowledgment field in the header lists the next-expected byte, not the last-received byte.

P.S UDP provides only multiplexing/demultiplexing and it is very useful, when no error recovery is needed(VoIP). UDP also require less network and CPU resources. UDP does not provide ordered data transfer.

Network layer
Provides logical addressing

Datalink layer

• Combines packets into bytes and bytes into frames
• Provides access to media using MAC(Media Access Control) address
• Performs error detection, BUT not correction
• This layer handles error notification, network topology, and flow control(optional)
• Media Access Control (MAC) 802.3 defines how packets are placed on the media
• Logical Link Control (LLC) 802.2 is responsible for identifying Network layer protocols and then encapsulating them.
• An LLC header tells the Data Link layer what to do with a packet once a frame is received.
• Data Link (LLC sublayer) uses service access points.

Physical layer
Moves bits between devices, specifies voltage, wire speed and pin-out of cables.

DoD model

There are 3 layers:

• Application
• Host to host
• Internet

TCP/IP model

There are 4 layers:

• Application
• Transport
• Internet
• Network access