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Distance Vector Routing
Protocols
PJC CCNA Semester 2 Ver. 3.0
by
William Kelly
Distance Vector Routing Protocols
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Intro to Distance Vector Protocols
Load Balancing
Routing Loops and their solution
Examining Routing Tables
Administrative Distance
Gateway of Last Resort
Integrating Static Routes with RIP
Basics of RIP and IGRP
Troubleshooting Routing Protocols
Distance Vector Protocols Intro.
 The two important Distance
vector routing protocols are RIP
and IGRP
 The ENTIRE routing table is
sent periodically to neighboring
routers
 A topology change or a periodic
update sends routing tables to
neighbors
Load Balancing
 The maximum paths range from 1 to 6
 By default most IP routing protocols install 4
routes in parallel
 Static Routes always install 6 routes
 Rip can only load balance paths that
have same number of hops
 IGRP can load balance up to 6 unequal
paths
 The maximum-paths maximum
command allows the number of parallel
paths used to load balance to be
changed in configuration mode
Load Balancing (switching)
 process switching (packets)
 The router alternates paths on a
per packet basis
 fast switching (per destination)
 All packets in the packet stream
bound for a specific host take the
same path
 Packets bound for a second host
on the same destination network
would all take an alternate path
What is a Routing Loop?
Router B
Edge of Update
Spreads
Router A
Router E
Router C
Routing
Loop
Router C has
old Route to
Network 1
that it sends
to D
Network 1
Router D
How to Solve Routing Loops
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Count to Infinity
Split Horizon
Route Poisoning
Triggered Updates
Hold Down Timers
Count to Infinity
 Distance Vector Protocols define Infinity as
a specific number
 Looping continues until Infinity (16 for RIP)
is reached
 When infinity is reached the Network is
considered unreachable
 In our example the loop would continue
until the county reached 16 and then
network 1 would be marked unreachable
Split Horizon
 A routing loop occurs when incorrect
information is sent to a router that just
sent out correct information
 The solution is to avoid sending information
back in the direction it came
 In our example Routers A and E would
never have received C’s bad info and thus
no routing loop would have occurred.
Route Poisoning
 Route poisoning is used to overcome loops
in large networks by setting the hop count
to one more than the maximum
 The poison reverse rule states “Once you
learn of a route through an interface,
advertise it as unreachable back through
that same interface”.
 Route poisoning is essentially the same as
poison reverse + split horizon
Triggered Updates
 By sending updates faster than the
default update convergence occurs
must faster
 A topology change quickly propagates
through a network
 Triggered updates used with route
poisoning marks a “down” network as
unreachable quickly through the
network
Hold Down Timers
1.
2.
3.
4.
When a router receives information that a
previously accessible route is inaccessible it starts
a hold down timer and marks the route as
inaccessible
If an update is received indicating the route is back
up before the hold down timer expires then the
route is marked accessible again
If an update is received from a different neighbor
with a better metric then the route is marked
accessible and the hold down time is removed
If an update is received from a different neighbor
with a poorer metric before the hold down timer
expires then the update is IGNORED
Examining Routing Tables
 Show ip route command
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How did I receive a route?
What is the metric?
What is the administrative distance?
Is the route directly connected?
What is the output interface to get to a
route?
Examining Routing Tables
Vista#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E – EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o – ODR
P - periodic downloaded static route
Gateway of last resort is not set
C
R
C
172.16.0.0/24 is subnetted, 3 subnets
172.16.8.0 is directly connected, FastEthernet0/0
172.16.5.0 [120/1] via 172.16.7.1, 00:00:02, Serial0/0
172.16.7.0 is directly connected, Serial0/0
Administrative Distance
 Administrative distance is a default value
assigned to each routing protocol that will
favor the best route from one protocol over
another one
 Distances for common protocols are:
Directly Connected
0
Static Routes
1
IGRP
100
OSPF
110
RIP
120
Gateway of Last Resort
 Routers try to keep routing tables as small as possible
 A router may not be able to match a destination
network with an entry in it’s routing table
 Default routes can be entered statically or learned
dynamically
 ip default network x.x.x.x establishes a default route
in networks using dynamic routing
 Any router set with default network x.x.x.x that has a
route to x.x.x.x flags x.x.x.x as a candidate default
route
 A default route can be statically set by saying:
ip route 0.0.0.0 0.0.0.0 {next hop ip or exit interface}
 If no path to the destination network is found in the
routing table then the quad zero default is used
Integrating Static Routes with RIP
 If your static route was not defined with a
network command then it is not distributed
unless you use the redistribute static
command
ip route dest. mask ip/interface admin_distance
 Packets bound for specific destination
networks can be forced to follow a certain
path
 Using a higher administrative distance can
provide a backup path in case of main link
failure
RIP Basics
 To configure use route rip, then network
x.x.x.x
 The metric is hop count
 A hop count of 16 is infinity
 Period updates are sent every 30 seconds
 It is a distance vector protocol
 The entire routing table is sent during
updates
 The administrative distance is 120
 The hold down timer default is 180 seconds
IGRP Basics
 To configure use route igrp as-number, then
network x.x.x.x
 The metrics are bandwidth, delay, load, and
reliability
 Period updates are sent every 90 seconds
 The hold down time is 3x the update timer or
90 secs. x 3 + 10 seconds = 280 seconds.
 It is a distance vector protocol
 The entire routing table is sent during updates
 The administrative distance is 100
 Scalable for very large networks
Troubleshooting Routing Protocols
 Is the routing protocol set?
 show ip protocols
 Is the route in the routing table?
 show ip route
 Are the interfaces configured?
 show running-config
 Am I using the same version of RIP
throughout my network?
 ping, traceroute, debug