Cisco ICND1 Flashcard: Router RIP and IGRP

ICND1 Topic: RIP
ICND2 Topics: RIPv2 VLSM, single area OSPF, EIGRP


Routing is taking a packet from one device and sending it over the network to another device on a different network. Routing requires routers.  Routers learn about remote networks from neighbor routers (dynamic) or programmed static routes by the administrator.  All Cisco routers are classful routers, that means expecting a default subnet mask on each interface on the router, otherwise the packet will be dropped. For default routing, the command "ip classless" must be used.

Two types of interior routing protocols
Static, Default and Dynamic Routes

IP Routing requirements

• destination address and a source address
• routing table for destination
• neighbor

See other article about Static Route vs. Dynamic Routes

On directly connected networks, hosts and routers find the destination MAC address for the destination IP addresses using ARP.

Strategies to prevent routing loops:

Split Horizon Rule: the router will not advertise a route back through the same interface from which the route was learned

Route Poisoning: to poison a route, set the metric to infinite; for RIP that number would be 16 for the maximum number of hops. When used with Split Horizon, this strategy is called Split-Horizon with Poison Reverse.

Holddown: holddown timer is used in route convergence, by preventing  route from reinstating the route until all routers have enough time to "age out" the route from the tables. Prevents routes from advertising bad or expired routing information.

Routing Protocols QUIZ

Most common metrics used by routing protocols to determine a network path
hop count
bandwidth - data capacity. Say 10 Mbps is better than 64 kbps leased line
delay
load
reliability - bit error rate of each network link
cost - configuraable, by default based on bandwidth
not packet length, distance.

1. What are three types of routing protocols?
a) hop count based
b) link state
c) cost based
d) distance-vector
e) advanced distance-vector (hybrid)
f) bndwidth based
answer 1. b,d,e


Name two IGP (interior gateway protocol) Distance Vector protocols: RIP, IGRP (Cisco proprietary)

• uses hop count as a metric
• sends entire routing table to neighbor, called routing by rumour and periodic updates
• lower CPU overhead than link-state protocols
• uses split horizon, route poisoning, and holddown to protect from routing loops

RIP - Routing Information Protocol

Configure RIP to run on the router:
conf t usage:
router rip
network 192.5.5.0
network 10.0.0.0 0.255.255.255

• RIP v1 and RIP v2
• RIP Classful
• RIP v2 supports VLSM (variable length subnet mask), route authentication
• distance vector protocol
• can load balance up to 6 equal cost paths
• uses hop count as the metric. The maximum hop count is 15, therefore 16 is infinite and unreachable
• sends periodic update every 30 seconds
• RIP timers include: update, invalid, holddown, flush (and garbage collection).

A handy command to create an interface to learn routes but not advertise
In router config mode usage: passive-interface interface#

IGRP - Cisco Proprietary
Interior Gateway Routing Protocol.  All routers within the same autonomous system should use the same AS number, and all routers should be Cisco if IGRP is used. IGRP was created to overcome the problems with RIP such as 15 hop count limit.  The maximum hop count for IGRP is 255 though 100 is the default. The IGRP metric (value to calculate best route) based on bandwidth and delay of the line called a composite metric; others factors optional but not required are reliability, load, maximum transmission unit (MTU). Sends a periodic update every 90s

usage: conf t
router igrp 10
network 172.16.0.0

IGRP can load balance up to six unequal links to a remote network (RIP networks must have the same hop count to load balance).


Summary 
How does RIP work in the internetwork?
RIP uses hop counts to determine the best route to a network, with an upper hop count limit of 15

Know the RIP commands
router rip
show ip route
show ip route rip

Administrative distance
Rates the trustworthiness of routing information received on a router from a neighbor; an integer between 0 to 255, where 0 is the most trusted and 255 means no traffic will pass through this route.
Default Administrative Distances
Route Source, Default Distance
Connected interface = 0
Static route = 1
EIGRP = 90
IGRP = 100
OSPF = 110
RIP = 120
External EIGRP = 170
Unknown = 255 not routable
      
What is AS, autonomous system
An AS is a group of routers that share the same routing information.

Know the difference between RIP and IGRP
Describe the metrics used. RIP only uses hop count.
Others are hop count, bandwidth, delay.

Three truths, from a quiz:
Routers evaluate the available paths to a destination
The routing process uses metrics and administrative distances when evaluating network paths
Dynamic routing occurs when information is learned using routing information that is obtained from routing protocols.

Note: the routing table is only displaying the information to the user
Routing table provides an ordered list of known network addresses.
Routing tables contain metrics that are used to determine the desirability of the route
Routing table associations tell a router that a particular destination is either directly connected to the router (C) or that it can reached via another router (the next-hop router or exit interface) on the way to the final destination

Link State Routing Protocols
Link state routing protocol respond quickly to network changes
Link state routing protocols send peridoic updates (link state refreshes) at long time intervals, about once every 30 minutes!!
In link state routing protocols, every router tries to build its own internap map of the network topology