In preparation of our CCNA exam, we want to make sure we cover the various concepts that we could see on our Cisco CCNA exam. So to assist you, below we will discuss Cisco CCNA Routing Cliff Notes.

Routing

Below we are going to review at a high level various routing protocols that will run on your Cisco routers. Routing is at the core of what you are learning for your Cisco CCNA exam and if you don’t understand how a Cisco router routes, well you have no chance at passing the Cisco CCNA exam. As I am sure you are aware, Cisco is the global leader in networking because of their Cisco router and Cisco switches.

The first thing you must be aware of is the difference between a routing protocol and a routable protocol. An example of a routing protocol is RIP or OSPF. They exchange information among Cisco routers to generate routing tables. An example of a routable protocol is TCP/IP or IPX/SPX. Netbeui is an example of a non-routable protocol. Which in essence means it does not understand different subnets. It only thinks the network is one big subnet that can’t be divided into multiple or smaller subnets. This is why in such a network you have lots of collisions as you have no way to segment the network to break up the network traffic into difference broadcast domains.

Routing Methods
Static Routing: Directs traffic to a specified network or group of networks. The syntax for this command is:

Router (config)# ip route destination_netwak mask (next_hop_ip I out going interface)

[administrative_distance] [permanent]

The default “administrative_dislance” is 0 when using next_hop__ip, or 1 when using outgoing interface.
The optional “permanent” is used to keep this route in the routing table if the interface shuts down else the route wilt be no longer available.

Default Routing: Directs traffic when destination network is unknown. This is similar to your default gateway in your computer configuration. The sytax for this command is:

Router(config)# ip classless Enable the router to toward traffic to subnets or supernets.
Router(config)# ip route 0.0.0.0 0.0.0.0 next-hop. If destination unknown, send frames to next-hop IP.

Dynamic Routing:(RIP, OSPF, EIGRP & IGRP)
There are three classes of Dynamic Routing Protocols with the following characteristics:
1. Distance-Vector Routing Protocols:

  • Passes complete routing table to neighboring routers (second hand routing information.)
  • Uses hop count for metric, like RIP.
  • Slow convergence in Distance-Vector causes inconsistent routing tables among routers causing a problem called Routing Loops (also known as Counting to Infinity), which causes traffic to get stuck in a loop.

    Routing Loops Solutions:
    Maximum Hop Count:     Drops traffic if destination network's hop count exceeds the maximum hop count allowed (RIP has max hop count of 15.)
    Split Horizon: Route information learned from one interface cannot be sent back out of the same int.
    Route Poisoning: A router will poison a lost route in the routing table by setting cost to infinity meaning the network is no longer reachable.
    Holddown: A router will not reinstate a route that has gone down for a specific time to allow network conversion to complete.

    2. Link-State Routing Protocols:
    Typically called “shortest path first”. Open Shortest Path First (OSPf) is IP's Link-State Protocol. Each router creates three separate tables:
    A- Entire topology table.
    B- Routing table.
    C- Directly attached neighbors' table.

    3. Hybrid Routing Protocol: An advance Distance Vector with Link State features, like EIGRP.

    Routing Protocols
    Routing Information Protocol(RIP)
    RIP uses the Bellman-Ford algorithm which simply counts the number of hops, or routers to a destination network path. The then chooses the path with the least number of hops as the preferred path. If any destination is greater than 15 hops, it is considered unreachable and the packets are dropped. Below are the characteristics of RIP:

  • Uses hop count as metric. The max hop count allowed in RIP is 15.)
  • RIP v1 does not support Variable Length Subnet Mask (VLSM). RIP v2 supports VLSM.
  • When hop count is equal, RIP can load balance using round-robin with up to six equal-cost links, load balance on different bandwidth links will fail.

    Default RIP Timer:
    Route update timer: Complete copy of the routing table sent to neighbors every 30 sec.
    Route invalid timer: a route is considered invalid if an update is not received for 90 sec.
    Route flush timer: when a route becomes invalid, it will remain in the routing table for 240 sec before it is removed (flushed).

    Configuring RIP Routing:
    Router (config)# router rip Configure router to receive and send RIP routing update.
    Router (ccnfig-router)# network network-address Specify the networks to be advertised in a classful format.

    Optional RIP Commands:
    Router(config-router)#passive-interface Serial 0
    Sets Serial 0 to accept RIP updates but it won't send any route updates out that interface.

    Interior Gateway Routway Protocol(IGRP)
    IGRP is a proprietary classful distance vector routing protocol that was supposed to defeat the limitations of RIP and IGRP was created by Cisco.

  • Uses Reliability, Load and Maximum Transmission Unit (MTU.)
  • Maximum Hop Count allowed is up to 255 (but 100 is the default setup for max hop count.)
  • Loads balance up to six unequal links, uses bandwidth to determine how to load-balance.

    Default IGRP Timers:
    Route update timer: The router sends complete copy of the routing table to its neighbors every 90 sec.
    Route invalid timer: A route is considered invalid if 3 route update intervals passed; 90 * 3 = 270 sec.
    Holddown timer: Specifies the holddown period, three times the update intervals plus 10 sec, (90 *3) +10 = 280 sec.
    Route flush timer: When a route becomes invalid, it will remain in the routing table for 7 update intervals (630 sec), before it is removed.

    Configuring IGRP Routing:
    Router(config}# router igrp process, .id
    Configure router to receive and send IGRP routing updates.

    Router(config-router)#network network-address
    Specifies the networks to be advertised

    Optional IGRP Commands:
    Router(config-router)#passive-interface Serial 0
    Set Serial 0 to accept IGRP updates, but it won't send them out that interface.

    Administrative Distances
    It is the default cost of a route valued from 0 (most trusted) to 255 (not used.)
    Connected Interface 0
    Static Route 1
    EIGRP Route 90
    IGRP Route 100
    OSPF Route 110
    RIP Route 120
    External EIGRP 170

    I hope you found this article to be of use and it helps you prepare for your Cisco CCNA certification. Achieving your CCNA certification is much more than just memorizing Cisco exam material. It is having the real world knowledge to configure your Cisco equipment and be able to methodically troubleshoot Cisco issues. So I encourage you to continue in your studies for your CCNA exam certification.