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5Then prefer the route with the shortest AS Path.

6If this is equal, prefer the route with origin set to originated (via BGP); IGP is preferred to EGP and then incomplete.

7If the origin codes are the same, prefer the route with the lowest MED.

8If the MED is the same, prefer EBGP over IBGP.

9Then prefer the path that is the closest.

10Finally, if all else is equal, prefer the path with the lowest BGP router ID.

Configuring BGP

To start the BGP process on a Cisco router requires the following command:

router bgp autonomous-system-number

To define networks to be advertised, apply the following command:

network network-number mask network-mask

You must be aware that the network command is not used the same way you apply networks in OSPF or EIGRP. With BGP, the network command advertises networks that are originated from the router and should be advertised via BGP. For more Cisco IOS examples of BGP, please visit Chapter 9, “CCIE Security Self-Study Lab.”

To identify peer routers, apply the following command:

neighbor {ip-address | peer-group name} remote-as autonomous-system-number

NOTE Route redistribution allows routing information discovered through one routing protocol to be distributed in the update messages of another routing protocol. Whenever redistribution is configured on Cisco routers, the routing metric must also be converted. For example, with redistribution from a RIP domain into OSPF, the RIP network inserted into OSPF requires an OSPF cost metric.

ISDN

Integrated Services Digital Network (ISDN) is a digital service that enables network users to send and receive data, voice, and video transmissions over a network. ISDN offers a variety of link speeds, ranging from 64 kbps to 2.048 Mbps. Many smalland medium-sized companies find that ISDN is a viable network solution.

80 Chapter 2: General Networking Topics

Basic Rate and Primary Rate Interfaces

ISDN can be supplied by a carrier in two main forms: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). An ISDN BRI consists of two 64-kbps services (B channels) and one 16-kbps signaling channel (D channel). An ISDN PRI consists of 23 B or 30 B channels, depending on the country. In North America and Japan, a PRI service consists of 23 B channels. In Europe and Australia, a PRI service consists of 30 B channels. A signaling channel (or D channel) is used in a PRI service and is a dedicated 64-kbps channel. The B channel sends data and the D channel primarily controls signaling.

NOTE The effective throughput of a PRI service with 23 channels is 1.472 Mbps (23 × 64 kbps). With 30 B channels, the effective throughput is 1.920 Mbps (30 × 64 kbps). The International Telecommunications Union (ITU) defines the standards for ISDN. The specified standard is ITU-T Q.921.

ISDN Framing and Frame Format

The ISDN physical layer provides the ability to send outbound traffic and receive inbound traffic by transmitting binary bits over the physical media. The ISDN data link layer provides signaling, which ensures that data is sent and received correctly. The signaling protocol used in ISDN is called the Link Access Procedure on the D channel (LAPD).

ISDN Layer 2 Protocols

ISDN can use a number of Layer 2 encapsulation types. Point-to-Point Protocol (PPP) and high-level data link control (HDLC) are the only methods tested in the qualification exam.

NOTE X.25 is not tested in the CCIE Security written exam.

HDLC

High-level data link control is a WAN protocol encapsulation method that allows point-to-point connections between two remote sites. Typically, HDLC is used in a leased-line setup. HDLC is a connectionless protocol that relies on upper layers to recover any frames that have encountered errors across a WAN link. HDLC is the default encapsulation on Cisco serial interfaces.

Cisco routers use HDLC encapsulation, which is proprietary. Cisco added an address field in the HDLC frame, which is not present in the HDLC standard. This field is used by Cisco devices to indicate the type of payload (protocol). Cisco routers use the address field in an

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HDLC frame to indicate a payload type, but other routers or manufacturers that implement the HDLC standard do not use the address field. HDLC cannot be used to connect a Cisco router with another vendor.

Figure 2-18 displays the HDLC frame format, which shares a common format with the PPP frame format discussed in the next section.

Figure 2-18 HDLC Frame Format

SAPI C/R EA TEI EA

SAPI - Service Access Point Identifier

C/R - Command/Response

EA - Extended Address

TEI - Terminal Endpoint Identifier (All 1s indicate a broadcast.)

Point-to-Point Protocol (PPP)

PPP was designed to transport user information between two WAN devices (also referred to as point-to-point links). PPP was designed as an improvement over Serial Line Internet Protocol (SLIP). When PPP encapsulation is configured on a Cisco WAN interface, the network administrator can carry protocols such as IP and IPX, as well as many others. Cisco routers support PPP over asynchronous lines, High-Speed Serial Interfaces (HSSIs), ISDN lines, and synchronous serial ports. PPP has the added function of allowing authentication to take place before any end user data is sent across the link.

The following three phases occur in any PPP session:

•Link establishment—Link Control Protocol (LCP) packets are sent to configure and test the link.

•Authentication (optional)—After the link is established, authentication can ensure that link security is maintained.

•Network layers—In this phase, Network Control Protocol (NCP) packets determine which protocols are used across the PPP link. An interesting aspect of PPP is that each protocol (IP, IPX, and so on) supported in this phase is documented in a separate RFC that discusses how it operates over PPP.