Friday, February 25, 2011


(Note that this is not an official Basic Draft article. It does not meet Basic Draft standards and will not be listed. If you've arrived through a search engine, please use the search function (top-left) to find a more suitable article).
Routing is the process a router goes through to forward a packet towards a destination network.

Switching is the process a switch goes through to switch a frame out of the correct interface (or interfaces in case of a broadcast).

A router determines the best path to take through the destination IP address (or any Layer 3 address) using the Routing Table.

An Ethernet switch determines the interface to forward frames out using the CAM Table.

For routers to make a proper decision, it must learn routes. These routes are stored in a routing table. Routes can be static (manually entered by an Administrator) or dynamic (learned through a Routing Protocol from other routers).

A Routed protocol is a protocol that is routed by a Routing protocol.

Here is a list of example Routed protocols:

A routed protocol basically worries about only the encapsulation of the packets and the fields within the encapsulation. It is concerned with storing information used by routers to make decisions. A routed protocol is routed through the Routing Table.

A Routing protocol is a protocol that routes the Routed protocols.

Here is a list of example Routing protocols:

A routing protocol supports a routed protocol. It is concerned with populating, updating and maintaining the Routing Table with information to correctly move the routed protocols around.

Routers can support multiple routing and routed protocols. In this way, they are capable of supporting many different types of networks. Each routed protocol, of course, has its own Routing Table relevant to it.

From the seminar, a Static route is a programmed route that the Administrator enters into the router. A Dynamic route is a route that a routing protocol adjusts automatically for topology or traffic changes.

A Static Route is a fixed path from source to destination. It is manually entered and so therefore must be manually updated.

Static route can be used to reveal information about hidden networks without entering it into the routing protocol.

Static route can also be used to specify stub networks - Networks that can only be accessed via one path (this is to avoid overhead related to dynamic routing).

Static routes can be entered from Global Configuration context using:

When specifying static routes, the administrative distance is an optional parameter to specify how reliable a route is. The lower the administrative distance, the more reliable a route is perceived to be. If there are multiple paths to the same network, then the one with the lower administrative distance is chosen. A static route can have administrative distance of 0 or 1 depending on whether an outgoing interface or a next-hop address is specified, respectively.

As an administrative distance is an 8 bit value, it can range from 0 to 255.

Here's a list of the common Administrative Distances:
Connected - 0
Static Outgoing Interface - 0
Static Next-Hop - 1
EIGRP - 90
IGRP - 100
OSPF - 110
RIP - 120
Unknown - 255

Using this Administrative Distance concept, we can create backup routes using static routes by specifying a static route with a lower administrative distance for an existing route.

For example, if you have an existing RIP route of 120 AD, then we can create a static route of 121 AD. The static route will be taken if the RIP one fails.

A default route is a special route that is taken when the routing table does not contain an entry that matches the destination address requested. To declare a default route, simply use for both the NETWORK_ADDRESS and SUBNET_MASK in the "ip route" command.

This is typically used by routers to forward Internet-bound traffic to the ISP Provider Edge routers.

Dynamic Routing Protocols are used to automatically create, update and maintain tables when there are network:

Dynamic Routing finds all possible routes, and puts the Best Route into the routing table. Routes are removed when they are no longer valid.

A network is said to have converged when all routers operate with the same knowledge, and routing updates have stabilized. Fast convergence is desirable.

If a Dynamic Routing protocol finds two equal paths, both paths would enter the routing table and they would perform load balancing over them. The Dynamic Routing protocol defines the set of rules routers follow when exchanging information.

The two goals of Dynamic Routing protocols are:
-Maintenance of a routing table
-Timely distribution of knowledge (routing updates)

There are three classes of Distance Vector routing protocols learned in the seminar.

RIP and IGRP are Distance Vector protocols. DV protocols determine the distance and direction to the networks. DV protocols typically send broadcast periodic updates (30 seconds RIP, 90 seconds IGRP) in the form of the entire routing table to neighbors (RIP version 2 uses multicasts instead; not tested). It accumulates distances found in the updates (e.g. If it's 5 hops away from the neighbor, it's 6 hops away from itself). It does not know the exact topology, and it only knows what its neighbors know. Typically DV are slow to converge. The updates are the only keepalives.

OSPF and IS-IS are Link State protocols. LS protocols learn the whole network topology, and find their way around by computing the cost to get to each network using the SPF algorithm. The lowest cost routes are used. LS uses triggered updates and send only the required updates in the form of LSAs (containing many LSPs, one for each route). Every time an update occurs, every router in the domain recomputes the SPF tree with itself as the root. It also uses Hello packets as a keepalive.

Link State requires more processor and memory, but is more bandwidth efficient in the end (the seminar says that Link State is bandwidth inefficient?).

Metric is what a router uses to determine what path is better. In case of the above examples, Hop Count and Cost are the metrics. Other metrics include Bandwidth, Delay, Load, Reliability and Ticks.

Advanced DV (referred to as Hybrid in the seminar) is a class of Routing Protocols that use DV metrics, but exhibit LS behaviors such as the maintenance of a Topology Database, the use of multicast Triggered Updates and Fast Convergence. As Advanced DV doesn't need to compute an SPF tree, but instead (in the case of EIGRP) uses more efficient algorithms such as DUAL, it is more processor efficient. It also does not need as much memory as the LS protocols. As with LS, other than the initial flooding, it is very bandwidth efficient. It uses the Hello as the keepalive mechanism.

Some characteristics of RIPv1 are: DV, Uses Hop Count, Broadcasts Updates every 30 seconds, Route Timeout 180 seconds, Hop Count cannot be more than 15, Classful.

Some characteristics of IGRP are: DV, Proprietary, handles Complex Topologies well, uses Bandwidth and Delay (can be configured to include Load and Reliability), Broadcasts Updates every 90 seconds, Classful.

Some characteristics of OSPF are: LS. (That's all?)

Some characteristics of EIGRP are: ADV (or "Hybrid"), Proprietary, keeps Successors and Feasible Successors for backup routes.

An AS is a group of networks under a common administration, and sharing a common routing strategy. IGRP, EIGRP and BGP requires AS numbers, and they must be the same for neighbors (applies to IGRP and EIGRP only) to exchange information.

IGPs are protocols used within an organization. Examples are RIP, OSPF, IGRP, EIGRP, IS-IS. The AS numbers for IGRP and EIGRP can be arbitrary as long as they are the same for devices meant to exchange information.

EGPs are protocols used to connect networks of different organizations together. Examples are EGP (a routing protocol succeeded by BGP, not referring to the class) and BGP. AS numbers for each organization in this case must be unique and assigned by an Internet authority.

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