CCNA – Routing concepts

Routing concepts

Routers can be configured to route traffic based on static routes that have to be manually entered by an administrator or by dynamic routes that are created dynamically by a routing protocol.

  • Static routing is a good choice for networks that: never change, are small in size or have only one router, or have only one way out of the network.
  • Dynamic routing is a good choice if a network has multiple routers, is part of a larger network, or if the network changes frequently.

Dynamic routing protocols

The dynamic routing protocols are useful only when your topology is redundant, and you need automatic failover. For instance, in a situation where the network changes/ network goes down

  • With a dynamic routing protocol, the routers will inform each other automatically through the routing protocol, and the route will be removed from the routing table.
  • With static routing, the administrator will have to go in and remove the static route manually.

Distance Vector Link State Path-Vector Routing
Routers communicate with neighbor routers adverting networks as measures of distance and vector Routers communicate with all other routers exchanging link state information to build a topology or the entire network BGP is the routing protocol of the Internet, used to route traffic across the Internet.

BGP is relevant to network administrators of large organizations which connect to two or more ISPs, as well as to Internet Service Providers (ISPs) who connect to other network providers.

If you are the administrator of a small corporate network, or an end user, then you probably don’t need to know about BGP.

Distance = metric

Vector = Direction (interface)

Link state = interface connections or links to other routers and networks
Best for simple, flat design, non-hierarchical networks

Minimum administrator knowledge

Convergence time is not an issue

Best for large, hierarchical networks

Advanced administrator knowledge

Convergence time is crucial

Knowledge of network from directly connected neighbors Routers have a complete view of network/ the entire topology
Send periodic updates of entire routing table Send triggered partial updates

Routed Protocols and Routing Protocols

Routed Protocols

Routing Protocols

IPX/SPX (Novell – no longer in use)
Apple Talk (Apple – no longer in use)
RIP v1, v2: IETF – RFC1058, open standard
EIGRP: Cisco proprietary
OSPF : IETF, open standard
ISIS, BGP : covered in CCNP

Administrative distance

An administrative distance is an integer from 0 to 255. A routing protocol with a lower administrative distance is more trustworthy than one with a higher administrative distance

Route Source

Default Distance

Connected 0
Static 1
eBGP 20
EIGRP (internal) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIGRP (external) 170
iBGP 200
EIGRP summary route 5
Unknown or unbelievable 255 (will not be added to the routing table to pass traffic)


This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.

VLSM – Variable Length Subnet Masks

VLSM is subnetting. VLSM means that you can divide a classful network like /24 into networks of different sizes and subnet masks

CIDR – Classless Interdomain Routing

CIDR is supernetting. CIDR basically means that when routing you are not limited to networks based on subnet masks. The main purpose of CIDR is aggregate route it is also known as supernetting or summarization. For example you have four C class subnets:


We can add 4 routes on the router or we summarize all of them and add a CIDR route which is:

  • /22 and covers all the above 4 routes.

CIDR addresses reduce the size of routing tables.

  • CIDR= make one of 6 Network = route summarization
    VLSM= Make 6 of 1 Network


Classful is based on the default Class A, B or C networks.

  • Class A: 0 – 127 with a mask of (/8)
  • Class B: 128 – 191 with a mask of (/16)
  • Class C: 192 – 223 with a mask of (/24)

Protocol choice

The following table is a summary of the essential characteristics of the major routing protocols and can be used as part of your evaluation criteria. It has the most common protocols and a number of points of evaluation.











Convergence Time










Bandwidth Consumption





Resource Consumption





Multi-path Support





Scales Well










  • Convergence Time: Convergence time is the point at which all routers on your network know about all current routes for the network. When a router is added or removed from a network, a certain amount of time — convergence time — must pass before this change is propagated to all routers on the network.
  • Variable length subnet masks (VLSM): This term refers to whether all routers on the network are required to use the same subnet mask. This requirement reduces your flexibility in assigning IP address network IDs to the network segments on your network.
  • Bandwidth Consumption: This term refers to the amount of necessary network bandwidth to maintain and distribute routing table information on the network. To share and distribute routing table information, all routing protocols need to send an amount of data over the network, and some send more than others.
  • Resource Consumption: In calculating and maintain routing table information on a router, a certain amount of processing power and memory is used.
  • Multi-path Support: When routes are discovered on the network that have loops in their paths, some segments have two possible routes, which represent multiple paths. Some routing protocols have support for multiple paths, by storing alternative paths in their routing information.
  • Scales Well: Some routing protocols operate well on small networks, but as the number of routers increases on the network, the routing protocol does not function as well. Routing protocols that can be used on small to very large networks scale well in size.
  • Proprietary: The routing protocol based on open standards or a proprietary protocol owned by one company can affect the level of support and the speed of changes.

In terms of which protocol type you should use, it depends on your requirements. In general, unless you are being forced to do so by a vendor, RIP should not be used. If you are running an all Cisco network, EIGRP can be brought up with very little manual configuration. If interoperability between vendors is a requirement, OSPF may be a better choice. As mentioned in another answer, if you are going to be exchanging routes with a 3rd party, BGP is the protocol of choice.

Personally I would choose your routing protocol not the way it works. Now days the right answer is practically always OSPF if it’s an internal network. If it’s an external network then the answer is probably

RIP can still be used on tiny networks or to redistribute routing from simple devices to more complex devices (or to inject default routes)


About Terri

System Administrator @Netpower Datacenter

Posted on 24.09.2014, in Basic & Networking, Technical Articles and tagged , . Bookmark the permalink. Leave a comment.

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