Routing: Split Horizon

Split horizon

Split horizon is a technique used by routing protocols to help prevent routing loops. The split-horizon rule states that an interface will not send routing information out an interface from which the routing information was originally received. Split horizon can cause problems in some topologies, such as hub-and-spoke Frame Relay configurations.

Routing: Route Summarization

Route - Summarization

Summarization is the process in which the administrator collapses many routes with a long mask to form another route with a shorter mask. Route summarization reduces the size of routing tables and makes routing function more efficiently. Route summarization also helps make networks more stable by reducing the number of updates that are sent when subnets change state. Route summarization makes classless inter-domain routing (CIDR) possible. Variable-length subnet masking (VLSM) promotes the use of route summarization. Some dynamic routing protocols engage in route summarization automatically for changes in a major classful network, whereas others do not.

MEMORY ELEMENTS OF ROUTER

RAM(random access memory) Used to hold packet buffers, ARP cache, routing 

tables, and also the software and data structures that allow the router to

function.Running--config is stored in RAM, and most routers expand the IOS

from flash into RAM upon boot.

Flash Memory Stores the IOS by default. Flash memory is not erased when the

router is reloaded. It is EEPROM(electronically erasable programmable read-only

memory) made by intel.

NVRAM(non-volatile RAM) Used to hold the router and switch configuration.

NVRAM is not erased when the router or switch is reloaded. Does not store an

IOS.

ROM(read-only memory) Used to start maintain the router. Holds the POST and mini IOS.

Routing Protocols: Classification of Routing Protocols

Classification of Routing Protocols

Distance vectorExamples: Routing Information Protocol Version 1 (RIPv1), RIPv2, Interior Gateway Routing Protocol (IGRP) Features periodic transmission of entire routing tables to directly connected neighbors Mathematically compares routes using some measurement of distance Features hop-count limitation



Link StateExamples: Open Shortest Path First (OSPF), Intermediate System- to- Intermediate System (IS-IS). Sends local connection information to all nodes in the internetwork. Forms adjacencies with neighboring routers that speak the same protocol; sends local link information to these devices. Note that although this is flooding of information to all nodes, the router is sending only the portion of information that deals with

the state of its own links. Each router constructs its own complete “picture” or “map” of the network from all of the information received.



Hybrid
Example: Enhanced Interior Gateway Routing Protocol (EIGRP) Features properties of both distance vector and link-state routing protocols



Path vector protocolExample: Border Gateway Protocol (BGP). Path vector protocols are a subset of distance vector protocols; BGP uses “path vectors” or a list of all the autonomous systems a prefix has crossed to make metric decisions and to ensure a loop free environment. In addition to the autonomous system path list, an administrator can use many other factors to affect the forwarding or receipt of traffic using BGP.

Routing Protocols: EIGRP Concepts

EIGRP Concepts

Successor

A successor for a particular destination is a next hop router that satisfies these two conditions:
it provides the least distance to that destination
it is guaranteed not to be a part of some routing loop

The first condition can be satisfied by comparing metrics from all neighboring routers that advertise that particular destination, increasing the metrics by the cost of the link to that respective neighbor, and selecting the neighbor that yields the least total distance. The second condition can be satisfied by testing a so-called Feasibility Condition for every neighbor advertising that destination. There can be multiple successors for a destination, depending on the actual topology.

The successors for a destination are recorded in the  topology table and afterwards they are used to populate the routing table as next-hops for that destination.


Feasible Successor

A feasible successor for a particular destination is a next hop router that satisfies this condition:
it is guaranteed not to be a part of some routing loop

Thus, every successor is also a feasible successor. However, in most references about EIGRP the term "feasible successor" is used to denote only those routers which provide a loop-free path but which are not successors (i.e. they do not provide the least distance). From this point of view, for a reachable destination there is always at least one successor, however, there might not be any feasible successors.
Active and Passive State

A destination in the topology table can be marked either as Passive or Active. A Passive state is a state when the router has identified the successor(s) for the destination. The destination changes to Active state when current successor no longer satisfies the Feasibility Condition and there are no feasible successors identified for that destination (i.e. no backup routes are available).


Reported Distance and Feasible Distance

Reported Distance (RD) is the total metric along a path to a destination network as advertised by an upstream neighbor

A Feasible Distance (FD) is the lowest known distance from a router to a particular destination.

This diagram shows the example of Feasible and Reported distance of EIGRP network topolgy description as above

*Feasibility Condition

If, for a destination, a neighbor router tells us that it is closer to the destination than we have ever been, then this neighbor lies on a loop-free route to this destination.
In exact terms, every neighbor that satisfies the relation RD < FD for a particular destination is on a loop-free route to that destination.

Routing Protocols: EIGRP Metric

EIGRP Metric

Bandwidth

Minimum Bandwidth (in kilobits per second) along the path from router to destination network

Load

Load (number in range 1 to 255; 255 being saturated)

Delay

Total Delay (in 10s of microseconds) along the path from router to destination network

Reliability

Reliability (number in range 1 to 255; 255 being the most reliable)

MTU

Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)

Formula

Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)])

The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.

Routing Protocol: EIGRP TABLES

EIGRP Tables

1-Neighbor Table: 

Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.

Topology Table: 

Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tables gathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.

Routing table: 

Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionally feasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.

Routing Protocol: Enhanced Interior Gateway Routing Protocol

EIGRP

EIGRP is an enhanced version of IGRP. The same distance vector technology found in IGRP is also used in EIGRP, and the underlying distance information remains unchanged. The convergence properties and the operating efficiency of this protocol have improved significantly. This allows for an improved architecture while retaining existing investment in IGRP.

The Diffusing Update Algorithm (DUAL) is the algorithm used to obtain loop-freedom at every instant throughout a route computation. This allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in the recomputation. The convergence time with DUAL rivals that of any other existing routing protocol.



EIGRP has been extended to be network-layer-protocol independent, thereby allowing DUAL to support other protocol suites.

EIGRP Characteristics

• Fast convergence.

• Support for VLSM.

• Partial updates conserve network bandwidth.

• Support for IP, AppleTalk, and IPX.

• Runs directly over IP, using protocol number 88.

• Support for all Layer 2 (data link layer) protocols and topologies.

• Sophisticated metric that supports load-balancing across unequal-cost paths .

• Use of multicast (and unicast where appropriate) instead of broadcasts.

• Support for authentication.

• Manual summarization at any interface.

• Uses multicast 224.0.0.10.