Archive for June, 2012

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CCDA – Designing for CISCO Internetwork Solutions (DESGN) ….

June 15, 2012

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Dynamic Routing
Dynamic routing allows the network to adjust to changes in the topology automatically, without administrator involvement. A static route cannot dynamically respond to changes in the network.
If a link fails, the static route is no longer valid if it is configured to use that failed link, so a new static route must be configured. If a new router or new link is added, that information must also be configured on every router in the network. In a very large or unstable network, these changes can lead to considerable work for network  administrators. It can also take a long time for every router in the network to receive the correct information. In situations such as these, it might be better to have the routers receive information about networks and links from each other using a dynamic routing protocol. Dynamic routing protocols must do the following:
■ Find sources from which routing information can be received (usually neighboring routers)
■ Select the best paths toward all reachable destinations, based on received information
■ Maintain this routing information
■ Have a means of verifying routing information (periodic updates or refreshes)
When using a dynamic routing protocol, the administrator configures the routing protocol on each router. The routers then exchange information about the reachable networks and the state of each network. Routers exchange information only with other routers running the same routing protocol. When the network topology changes, the new information is dynamically propagated throughout the network, and each router updates its routing table to reflect the changes. network. Routers exchange information only with other routers running the same routing protocol.
When the network topology changes, the new information is dynamically propagated throughout the network, and each router updates its routing table to reflect the changes.

Interior Versus Exterior Routing Protocols
An autonomous system (AS), also known as a domain, is a collection of routers that are under a common administration, such as a company’s internal network or an Internet service provider’s (ISP’s) network.

Different types of protocols are required for the following reasons:
■ Inter-AS connections require more options for manual selection of routing characteristics.
EGPs should be able to implement various policies.
■ The speed of convergence (distribution of routing information) and finding the best path to the destination are crucial for intra-AS routing protocols.
Therefore, EGP routing protocol metrics (used to measure paths to a destination) include more parameters to allow the administrator to influence the selection of certain routing paths. EGPs are slower to converge and more complex to configure. IGPs use less-complicated metrics to ease configuration and speed up the decisions about best routing paths for faster convergence. The “Routing Protocol Metrics” section later in this chapter defines and explains routing protocol metrics.

IGP and EGP Example
Figure 7-2 shows three interconnected autonomous systems (domains). Each AS uses an IGP for intra-AS (intra-domain) routing.

KEY
POINT

Because the Internet is based on the AS concept, two types of routing protocols are
required:
■ Interior gateway protocols (IGP) are intra-AS (inside an AS) routing protocols.
Examples of IGPs include Routing Information Protocol (RIP) version 1 (RIPv1),
RIP version 2 (RIPv2), Open Shortest Path First (OSPF), Integrated Intermediate
System-to-Intermediate System (IS-IS), and Enhanced Interior Gateway Routing
Protocol (EIGRP).
■ Exterior gateway protocols (EGP) are inter-AS (between autonomous systems)
routing protocols. Border Gateway Protocol (BGP) is the only widely used EGP
protocol on the Internet. BGP version 4 (BGP-4) is considered the acceptable version
of BGP on the Internet. It is discussed in the “Border Gateway Protocol” section.

IGP and EGP Example
Figure 7-2 shows three interconnected autonomous systems (domains). Each AS uses an IGP for intra-AS (intra-domain) routing.

KEY
POINT

Because the Internet is based on the AS concept, two types of routing protocols are
required:
■ Interior gateway protocols (IGP) are intra-AS (inside an AS) routing protocols.
Examples of IGPs include Routing Information Protocol (RIP) version 1 (RIPv1),
RIP version 2 (RIPv2), Open Shortest Path First (OSPF), Integrated Intermediate
System-to-Intermediate System (IS-IS), and Enhanced Interior Gateway Routing
Protocol (EIGRP).
■ Exterior gateway protocols (EGP) are inter-AS (between autonomous systems)
routing protocols. Border Gateway Protocol (BGP) is the only widely used EGP
protocol on the Internet. BGP version 4 (BGP-4) is considered the acceptable version
of BGP on the Internet. It is discussed in the “Border Gateway Protocol” section.
Routing Protocol Features 433
Figure 7-2 Interior Protocols Are Used Inside and Exterior Protocols Are Used Between Autonomous Systems
The autonomous systems require some form of interdomain routing to communicate with each other. Static routes are used in simple cases; typically, an EGP is used.
BGP-4 is the dominant EGP currently in use; BGP-4 and its extensions are the only acceptable version of BGP available for use on the public Internet.
Multihoming is when an AS has more than one connection to the Internet (for redundancy or to increase performance). BGP is particularly useful when an AS multihomes to the Internet via multiple ISPs, as illustrated in Figure 7-2. To comply with the contractual requirements from specific ISPs, an administrator uses BGP to apply specific policies—for example, to define traffic exit points, return traffic paths, and levels of quality of service (QoS).