Chapter 6

Enhanced Interior Gateway Routing Protocol (EIGRP)

A Cisco proprietary protocol. So, although EIGRP is popular in Cisco-only networks, it is less popular in mixed-vendor networks. Like OSPF, EIGRP, is an IGP with very fast convergence and high scalability. EIGRP is considered to be an advanced distance vector or a hybrid routing protocol.

Link State

A category of routing protocol that maintains a topology of a network and uses an algorithm to determine the shortest path to a destination network.

Distance Vector

A category of routing protocol that sends a full copy of its routing table to its directly attached neighbors.

Default Static Route

A default static route is an administratively configured entry in a router's routing table that specifies where traffic for all unknown networks should be sent.

Routing Information Protocol (RIP)

A distance-vector routing protocol that uses a metric of hop count. The maximum number of hops between wo routers in an RIP-based network is 15. Therefore, a hop count of 16 is considered to be infinite. RIP is considered to be an IGP.

Intermediate System-to-Intermediate System (IS-IS)

A link-state routing protocol similar in its operation to OSPF. IS-IS uses a configurable, yet dimensionless, metric associated with an interface and runs Dijkstra's shortest path first algorithm. Although using IS-IS as an IGP offers the scalability, fast convergence, and vendor interoperability benefits of OSPF, it has not been deployed as widely as OSPF.

Open Shortest Path First (OSPF)

A link-state routing protocol that uses a metric of cost, which is based on the link speed etween two routers. OSPF is a popular IGP became of its scalability, fast convergence, and vendor interoperability.

Internet Group Management Protocol (IGMP)

A multicast protocol used between clients and routers to let routers know which of their interfaces has a multicast receiver attached.

Protocol Independent Multicast (PIM)

A multicast protocol used between multicast-enabled routers to construct a multicast distribution tree.

Routing Protocol

A routing protocol (for example, RIP, OSPF, or EIGRP) that advertises route information between routers, which desribes how to reach specified destination networks.

Exterior Gateway Protocol (EGP)

A routing protocol that operates between autonomous systems, which are networks under different administrative control. Border Gateway Protocol (BGP) is the only EGP in widespread use today.

Interior Gateway Protocol (IGP)

A routing protocol that operates within an autonomous system, which is a network under a single administrative control. OSPF and EIGRP are popular examples of IGPs.

Administrative Distance (AD)

A routing protocol's index of believability. Routing protocols with a smaller AD are considered more believable than routing protocols with a higher AD.

Metric

A value assigned to a route. Lower metrics are preferred over higher metrics.

Static NAT (SNAT)

A variant of NAT in which an inside local IP address is statically mapped to an inside global IP address. SNAT is useful for servers inside a network that need to be accessible from an outside network.

Dynamic NAT (DNAT)

A variant of NAT in which inside local addresses are automatically assigned an inside global address from a pool of available addresses.

Port Address Translation (PAT)

A variant of NAT in which multiple inside local IP addresses share a single inside global IP address. PAT can distinguish between different flows based on port numbers.

Network Address Translation (NAT)

Allows private IP addresses (as defined in RFC 1918) to be translated into Internet-routable IP addresses (public IP addresses).

Route Redistribution

Allows routes learned by one routing protocol to be injected into the routing process of another routing protocol.

Address Resolution Protocol (ARP)

An ARP request is a broadcast asking for the MAC address corresponding to a known IP address. An ARP reply contains the requested MAC address.

Next-Hop Routed Protocol

An IP address on the next router to which traffic should be forwarded.

Hold-Down Timer

Can speed the convergence process of a routing protocol. After a router makes a change to a route entry, the hold-down timer prevents subsequent updates for a specified period of time. This approach can help stop flapping routes (which are routes that oscillate between being available and unavailable) from preventing convergence.

Link-State Advertisement (LSA)

Sent by routers i a network to advertise the networks the routers know how to reach. Routers use those LSAs to construct a topological map of a network. The algorithm run against this topological map is Dijkstra's shortest path first algorithm.

Time to Live (TTL)

The TTL field in an IP header is decremented once for each router hop. Therefore, if the value in a TTL field is reduced to 0, a router discards the frame and sends a time exceeded ICMP message back to the source.

Border Gateway Protocol (BGP)

The only EGP in widespread use today. In fact, BGP is considered to be the routing protocol that runs the Internet, which is an interconnection of multiple autonomous systems. BGP is a path-vector routing protocol, meaning that it can use as its metric the number of autonomous system hops that must be transited to reach a destination network, as opposed to the number of required router hops.

Poison Reverse

This feature of a distance-vector routing protocol causes a route received on one interface to be advertised back out of that same interface with a metric considered to be infinite.

Split Horizon

This feature of a distance-vector routing protocol prevents a route learned on one interface from being advertised back out of that same interface.