Point-to-Point Protocol PPP Advantages & Operations

Point-to-Point Protocol PPP Advantages & Operations

Synchronous Point-to-Point Protocol used to connect to non-Cisco devices, control link quality, provide authentication or group links for sharing. PPP uses HDLC to encapsulate datagrams. This article is on what is PPP and Point-to-Point Protocol Advantages & Operationsis. Remember that HDLC is the default serial encapsulation method when connecting two Cisco routers. With an added protocol type field, the Cisco HDLC version is exclusive. Therefore, Cisco HDLC can only work with other Cisco devices. However, when there is a need to connect to a non-Cisco router, PPP encapsulation should be used.

WHAT IS PPP?

PPP (Point-to-Point Protocol) encapsulation was carefully designed to maintain compatibility with the most used hardware that supports it.
PPP encapsulates data frames to transmit them through physical Layer 2 links. PPP establishes a direct connection via serial cables, telephone lines, trunk lines, cell phones, specialized radio links or fiber optic links.
The Point-to-Point Protocol contains three main components:

• HDLC style framework to transport multiprotocol packets through point-to- point links .
• Extensible Link Control Protocol (LCP) to establish, configure and test the data link connection.
• Family of network control protocols (NCP) to establish and configure different network layer protocols. PPP allows the simultaneous use of several network layer protocols.

Some of the most common NCPs are the Internet Protocol Control Protocol (IPv4), the IPv6 Control Protocol, the AppleTalk Control Protocol, Novell IPX, the Cisco Systems Control Protocol, the SNA Control Protocol and the compression control protocol

ADVANTAGES OF PPP

PPP originally emerged as an encapsulation protocol to transport IPv4 traffic through point-to-point links. PPP provides a standard method for transporting multiprotocol packets through point-to-point links.
The use of PPP has many advantages, including the fact that it is not exclusive. PPP includes many features that are not available in HDLC:

• The link quality management feature, which is shown in the illustration, controls the link quality. If too many errors are detected, PPP deactivates the link.
• PPP supports PAP and CHAP authentication. This feature is explained and practiced later in another section.

PPP LAYER ARCHITECTURE

A layered architecture is a model, a design, or a logical plane that helps in the communication of the interconnecting layers.
In the illustration, the PPP layered architecture is compared with the open system interconnection (OSI) model.
PPP and OSI share the same physical layer, but PPP distributes the functions of LCP and NCP differently.
On the physical layer, you can configure PPP in a range of interfaces, including the following:

• Asynchronous serial
• Synchronous serial
• HSSI
• ISDN

PPP operates through any DTE / DCE interface (RS-232-C, RS-422, RS-423 or V.35). The only absolute requirement imposed by PPP is a full-duplex circuit , whether dedicated or switched, that can operate in synchronous or asynchronous serial bit mode, transparent to PPP link layer frames.
PPP does not impose any restrictions with respect to the transmission rate in addition to those imposed by the specific DTE / DCE interface used.
Most of the work that PPP does is carried out by LCP and NCPs in the data link and network layers. LCP configures the PPP connection and its parameters, the NCPs handle the upper layer protocol configurations, and LCP terminates the PPP connection.

PPP: LINK CONTROL PROTOCOL (LCP)

LCP works within the data link layer and plays a role in the establishment, configuration and testing of the data link connection. In addition, it establishes the point-to-point link.
LCP also negotiates and configures the control options in the WAN data link, managed by the NCPs.
LCP provides automatic configuration of the interfaces at each end, including the following:

• Handling different limits on package size
• Detection of common configuration errors
• Link termination
• Determining when a link works correctly or when it fails

Once the link is established, PPP also uses LCP to automatically agree on encapsulation formats, such as authentication, compression and error detection .

PPP: NETWORK CONTROL PROTOCOL (NCP)

PPP allows several network layer protocols to work on the same communication link. For each network layer protocol that is used, PPP uses a separate NCP. For example, IPv4 uses the IP control protocol (IPCP) and IPv6 uses the IPv6 control protocol (IPv6CP).
NCP protocols include functional fields that contain standardized codes to indicate the network layer protocol that PPP encapsulates.

PPP FRAME STRUCTURE

PPP frames consist of six fields.

The following descriptions summarize the fields of the PPP frames, which are shown in the illustration:

• Flag or Indicator : a single byte that indicates the beginning and end of a frame. The Signaling field is formed by the binary sequence 01111110. In successive PPP frames only a single flag character is used.
• Address : a single byte containing the binary sequence 11111111, the standard broadcast address. PPP does not assign addresses to individual stations.
• Control : a single byte formed by the binary sequence 00000011, which requires the transmission of user data in a non-sequential frame.
• Protocol : two bytes that identify the protocol encapsulated in the frame information field. The 2-byte Protocol field identifies the PPP content protocol.
• Data : zero or more bytes containing the datagram for the protocol specified in the Protocol field. To find the end of the information field, the sequence of the completion indicator must be searched and 2 bytes left for the FCS field.
• Frame Check Sequence (FCS) : typically 16 bits (2 bytes). Through a prior agreement, with the acceptance of PPP implementations, a 32-bit FCS (4 bytes) can be used for greater error detection.

LCP protocols can negotiate modifications to the structure of the standard PPP frame. However, modified frames always differ from standard frames.

SESSIONS FOR PPP

There are three phases of establishing a PPP session, as shown in the illustration:

1. Phase 1 , link establishment and configuration negotiation : Before PPP exchanges any network layer datagram (such as IP) LCP must first open the connection and negotiate the configuration options. This phase is completed when the receiving router sends a configuration acknowledgment frame back to the router that initiates the connection.
2. Phase 2 , link quality determination (optional) : LCP tests the link to determine if its quality is sufficient to activate network layer protocols. LCP may delay the transmission of network layer protocol information until this phase is completed.
3. Phase 3 , negotiation of the network layer protocol configuration : once LCP has completed the link quality determination phase, the corresponding NCP protocol can configure the network layer protocols separately, activate and deactivate them at any time . If LCP closes the link, it informs the network layer protocols so that they can take appropriate measures.

The link remains configured for communications until explicit LCP or NCP frames close the link, or until an external event occurs, for example, an inactivity timer expires or an administrator intervenes.
LCP can terminate the link at any time. This is usually done when one of the routers requests termination, but it can happen due to a physical event, such as the loss of a carrier or the expiration of an inactive period timer.

Redundancy Protocols VRRP HSRP GLBP First Hop Redundancy Protocols

Redundancy Protocols VRRP HSRP First Hop Redundancy Protocols

Complete details on the purpose and operation of the Redundancy Protocols VRRP HSRP First Hop Redundancy Protocols are described. As well as the different varieties of first hop redundancy protocols. Here you will learn the different types of Redundancy Protocols and how to use Cisco IOS commands to configure HSRP and GLBP on routers with practical examples.

Default Gateway Limitations

Expansion tree protocols allow physical redundancy in a switched network. However, hosts in the access layer of a hierarchical network also benefit from alternative default gateways.
If a router or router interface (which functions as a default gateway) fails, hosts configured with that default gateway are isolated from external networks. A mechanism is needed to provide alternate default gateways in switched networks where two or more routers are connected to the same VLANs.
Note : For the purpose of analyzing the redundancy of the routers, there is no functional difference between a multilayer switch and a router in the distribution layer. In practice, it is common for a multilayer switch to function as a default gateway for each VLAN in a switched network.
In a switched network, each client receives only a default gateway. There is no way to configure a secondary gateway, even if there is a second route that transports packets outside the local segment.

In above figure, R1 is responsible for routing packets from PC1. If R1 is no longer available, routing protocols can converge dynamically. Now, R2 routes packets from external networks that would have gone through R1. However, internal network traffic associated with R1, including workstation, server and printer traffic that was configured with R1 as the default gateway, is still sent to R1 and discarded.
Typically, the terminals are configured with a single IP address for the default gateway. This address is not modified when the network topology changes. If that default gateway IP address cannot be reached, the local device cannot send packets outside the local network segment, which disconnects it completely from the rest of the network.

 Router redundancy

One way to avoid a single point of failure in the default gateway is to implement a virtual router . As shown in following figure, to implement this type of router redundancy, several routers are configured to work together and thus give the feeling that there is a single router to the hosts on the LAN. By sharing an IP address and a MAC address, two or more routers can function as a single virtual router.
The virtual router's IP address is configured as the default gateway for workstations of a specific IP segment. When frames are sent from host devices to the default gateway, hosts use ARP to resolve the MAC address associated with the IP address of the default gateway. The ARP resolution returns the MAC address of the virtual router.

The router currently active within the group of virtual routers can physically process the frames that are sent to the MAC address of the virtual router. The protocols are used to identify two or more routers as the devices responsible for processing frames that are sent to the MAC or IP address of a single virtual router. Host devices send traffic to the virtual router address. The physical router that forwards this traffic is transparent to host devices .
A redundancy protocol provides the mechanism to determine which router should fulfill the active function in traffic forwarding . In addition, it determines when a backup router must assume the forwarding function. The transition between forwarding routers is transparent to end devices.
The ability of a network to dynamically recover from the failure of a device that functions as a default gateway is known as " first hop redundancy ."

Steps To configure Router failover

When the active router fails, the redundancy protocol causes the backup router to assume the new active router role. These are the steps that are taken when the active router fails:

1. The backup router stops receiving greeting messages from the forwarding router.
2. The backup router assumes the role of the forwarding router.
3. Because the new forwarding router assumes both the IP address and the MAC address of the virtual router, the host devices do not perceive any interruption in the service.

Type of Redundancy Protocols

In the following list, the options available to defined protocols first - hop redundancy (FHRP - First Hop Redundancy Protocol), as shown in Figure.

Hot Standby Routing Protocol (HSRP)

It is an exclusive Cisco protocol designed to allow transparent failover of a first-hop IPv4 device.
Hot Standby Routing Protocol HSRP provides high network availability, as it provides first hop routing redundancy for IPv4 hosts on networks configured with a default gateway IPv4 address. HSRP is used in a group of routers to select an active device and a backup device.
In a group of device interfaces, the active device is one that is used to route packets, and the backup device is the one that takes control when the active device fails or when previously established conditions are met. The function of the HSRP backup router is to control the operational status of the Hot Standby Routing Protocol HSRP group and quickly assume responsibility for packet forwarding if the active router fails.

Hot Standby Routing Protocol HSRP for IPv6

Cisco exclusive FHRP that provides the same HSRP functionality but in an IPv6 environment. An IPv6 HSRP group has a virtual MAC address derived from the HSRP group number and a virtual link-local IPv6 address derived from the HSRP virtual MAC address.
When the HSRP group is active, periodic router (RA) ads are sent for the HSRP virtual link-local IPv6 address. When the group is no longer active, these RAs end after a last RA is sent.

Virtual Router Redundancy Protocol Version 2 (VRRPv2)

It is a non-exclusive protocol of choice that dynamically assigns the responsibility of one or more virtual routers to the VRRP routers on an IPv4 LAN. This allows several routers in a multiple access link to use the same virtual IPv4 address.

VRRP routers are configured to run the VRRP protocol in conjunction with one or more routers connected to a LAN. In a VRRP configuration, a router is chosen as the virtual master router, while the rest works as a backup in the event that the virtual master router fails.

VRRPv3

It provides the ability to support IPv4 and IPv6 addresses. VRRPv3 works in multi-vendor environments and is more scalable than VRRPv2.

Gateway Load Balancing Protocol (GLBP)

Cisco exclusive FHRP that protects data traffic against a router or circuit failure, such as HSRP and VRRP, while allowing load balancing (also called " load sharing ") between a group of redundant routers.

GLBP for IPv6

Cisco exclusive FHRP that provides the same GLBP functionality but in an IPv6 environment . GLBP for IPv6 provides automatic router backup for IPv6 hosts configured with a single default gateway on a LAN.

Several first hop routers on the LAN are combined to offer a single virtual first hop IPv6 router and, at the same time, share the IPv6 packet forwarding load.

ICMP router discovery protocol (IRDP)

It is specified in RFC 1256; It is an old FHRP solution. IRDP allows IPv4 hosts to locate routers that provide IPv4 connectivity to other (non-local) IP networks.

HSRP Verification

• An active HSRP router has the following characteristics:
• Respond to ARP requests from the default gateway with the MAC of the virtual router.
• It assumes active packet forwarding for the virtual router.
• Send greeting messages.
• Know the IP address of the virtual router.

A backup HSRP router has the following characteristics:

• Listen to periodic greeting messages.
• It assumes active packet forwarding if it does not perceive active router activity.
• HSRP Verification

Use the show standby command to verify the status of HSRP. In Image 5, the result shows that the router is in active state.

GLBP Verification

Although HSRP and VRRP provide recoverability to the gateway, for backup members of the redundancy group, upstream bandwidth is not used while the device is in backup mode.
Only the active router of the HSRP and VRRP groups sends traffic to the virtual MAC address. Resources that are not associated with the backup router are not fully utilized. It is possible to achieve load balancing with these protocols by creating several groups and assigning several default gateways, but this configuration generates an administrative burden.
GLBP is a Cisco proprietary solution that allows automatic selection and simultaneous use of several available gateways, in addition to automatic failover between those gateways.

In figure, several routers share the load of the frames that, from the client's perspective, are sent to a single default gateway address.
With GLBP, you can make maximum use of resources without the administrative burden of configuring multiple groups and managing various default gateway configurations. GLBP has the following characteristics:

• It allows full use of resources on all devices, without the administrative burden of creating several groups.
• It provides a single virtual IP address and several virtual MAC addresses.
• Route the traffic to the only gateway distributed through the routers.
• Allows you to reroute automatically in case of failure.
• Use the show glbp command to verify the status of GLBP. In Image 8, it is shown that the GLBP group 1 is in active state with the virtual IP address 192.168.2.100.