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RIP, IGRP, and Static Route Concepts and Configuration

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RIP, IGRP, and Static Route Concepts and Configuration

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Cisco CCNA ICND Study Guide

By Cbrzana

Table Of Contents


RIP, IGRP, and Static Route Concepts and Configuration


ip route[address][subnet mask][next hop IP]

Routers can only forward packets to subnets in its routing table

ip route[address][subnet mask][next hop IP]

the new routes can be verified with

ip route [address][subnet mask][interface]

show ip route (displays all of router’s ip routes)

-> Disadvantage to doing this is that static routing works one-way

(add route on Router A to Router B, but Router B still doesn’t have route to Router A)

Extended Ping Command: Simulates a ping from ethernet host, but actually comes from router itself. When a ping from a router works, but a ping from a host does not, the extended ping could help you re-create the problem without needing to work with the end user on the phone.

1. ping

2. Target Address (IP)

3. Extended Commands = y

4. Source Address (IP)


Distance Vector Concepts

Advertise entire routing table (subnet number and metric) to directly connected neighbors.

Key Points:

  • Updates are sent to all neighboring routers through active interfaces, once every so many seconds.
  • If a router learns multiple routes to the same subnet, it chooses the best route based on the metric (number of hops).
  • Failure to receive updates after x amount of time results in removal of that route
  • Routers add directly connected subnets to their routing tables, even without a routing protocol

Distance Vector Loop Avoidance Features eliminate roundabout "ghost" routes to failed or cyclically-linked routers.

Route Poisoning: Router notices link is down, continues to advertise the route, but with very large metric (view as infinite and invalid). Other routers remove their routes to the downed subnet.

RIP uses 16 as the infinite metric


:

Split Horizon:If 2 routers advertise tables at about the same time, with one link down, they would continually exchange incorrect routing metrics (counting to infinity). Split horizon doesn't allow this because all routes with outgoing interface x are not included in updates sent out that same interface x.

In other words, if route to subnet comes in through interface 1, don’t send update of the same route out interface 1

Ex) Router A’s Ethernet goes down, set metric to 16 and send to neighbor. At the same time, Router B sends update to Router A, using the old metric that the cost is 2. Now Router A = 2, Router B = 16. After x amount of time, the two routers exchange routing table and in turn switch the metrics. This process would repeat indefinitely.


*Split Horizon with Poison Reverse (or Poison Reverse):Cisco’s proprietary distance vector routing protocols use this (used by default). Spit horizon used when network links up, but when link fails, allows broadcast of infinite metric (including previously blocked port from Split Horizon).


*Hold-Down Timer:Defeats the counting-to-infinity problem when multiple/redundant links exist between routers. Routers must wait the duration of the hold-down timer before believing any "good" information about that route.


Triggered/Flash Updates: Sends new update as soon as route fails.


Summary

Issue Solution
Multiple routes to the same subnet have equal metrics
Either use the first route learned or put multiple routes to the same subnet in the
routing table.
Routing loops occur due to updates passing each other over a single link
Split horizon—The routing protocol advertises routes out an
interface only if they were not learned from updates entering
that interface.

Split horizon with poison reverse—The routing protocol uses
split-horizon rules unless a route fails. In that case, the route is
advertised out all interfaces, including the interface in which
the route was learned, but with an infinite-distance metric.
Routing loops occur because routing information loops
through alternative paths
Route poisoning—When a route to a subnet fails, the subnet is
advertised with an infinite-distance metric. This term
specifically applies to routes that are advertised when the
route is valid. Poison reverse refers to routes that normally are
not advertised because of split horizon but that are advertised
with an infinite metric when the route fails.
Counting to infinity
Hold-down timer—After finding out that a route to a subnet
has failed, a router waits a certain period of time before
believing any other routing information about that subnet.

Triggered updates—When a route fails, an update is sent
immediately rather than waiting on the update timer to expire.
Used in conjunction with route poisoning, this ensures that all
routers know of failed routes before any hold-down timers
can expire.



Comparing RIP and IGRP

Feature RIP IGRP
Update Timer 30 seconds 90 seconds
Metric Hop count Bandwidth/delay (also, reliability, MTU, and load)
Hold-Down Timer 180 280
Flash Updates Yes Yes
VLSM No No
Infinite-Metric Value 16 4,294,967,295


Configuring RIP and IGRP

Command Configuration Mode
router rip Global
router igrp [as number] Global
router igrp [as number] Global enable IGRP for the AS and enter (config-router) mode.
network [net number] Router subcommand advertise to other routers that this router has a path to the given network.
passive-interface [default]
{interface type interface number}
Router subcommand don't send routing updates out through the given interface.
maximum-pathsnumber Router subcommand keep at most number'''different routes to the same destination in the routing table.
traffic-share {balanced | min} Router subcommand
traffic-share {balanced | min} Router subcommand share traffic between routes, either proportionally or just between those of the minimum metric value.


EXEC Commands

Command Description
show ip route Shows the entire routing table, or a subset if parameters are entered.
show ip protocols Shows routing protocol parameters and current timer values.
debug ip rip Issues log messages for each RIP update.
debug ip igrp transactions [ip address] Issues log messages with details of the IGRP updates.
debug ip igrp transactions [ip address] Issues log messages with details of the IGRP updates.
debug ip igrp events [ip address] Issues log messages for each IGRP packet.
trace Sends a series of ICMP echoes with increasing TTL values to verify the current route to a host.


RIP Configuration

router rip

network [network address1] -> Use network number (address w/ normal class address)
network [network address2]


IGRP ConfigurationIGRP Configuration

router igrp [as number] -> Note: All routers should use the same AS number

network [network address1]
network [network address2]

show running-config -> I = address found by IGRP, C = directly connected

Example

I 10.1.4.0 [100/8539] via 10.1.2.14, 00:00:50, Ethernet0

-> The [100/8359] can be broken into two separate parts:

  • 100 = administrative distance
  • 8539 = metric (function of bandwidth and delay)
  • The higher the bandwidth, the lower the metric
  • The lower the cumulative delay, the lower the metric


Bandwidth Defaults

LAN Interfaces = default reflects the correct bandwidth

Serial Interfaces = defaults to 1544 kbps (T1 speed)

-> Configure using the bandwidth [kbps] interface command

To migrate from RIP to IGRP (Commands)

  • no router rip
  • router igrp [as number]
  • network [network ID]


Debug/show commands include

  • debug ip rip
  • show ip route
  • debug ip igrp transactions (detailed info on updates)
  • debug ip igrp events (summary that states updates received)
  • show ip protocol (Update timer, elapsed time since update received)


Additional Notes

- If multiple route exist, router chooses best metric route
- If routes tie, keep the first/pre-existing route
    Command: maximum-paths 1 (default is maximum-paths 4)
   
  - When RIP places multiple routes, router balances traffic
    Command to use lowest-cost: traffic-share min
 

variance allows metrics to be considered equal, since IGRP/EIGRP metrics are calculated through formula and often won’t be exactly the same.

Example: metric = 100, variance = 2, If value > (lowest metric * variance), add route

Administrative Distance:In order to compare metrics between different routing protocols, use administrative distance to denote how believable an entire routing protocol is on a single router.

-> The lower the number, the better


Default Administrative Distances

Route Type Administrative Distance
Connected 0
Static 1
EIGRP summary route 5
EBGP 20
EIGRP (internal) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIGRP (external) 170
iBGP (external) 200
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