Network Certification Preparation
Module - 4
 Understanding Routing Protocols and Types
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RIP
IGRP
Configure RIP and IGRP
Basics of NAT , Public and Private address
Understanding DNS
Configure, verify and troubleshoot DHCP and DNS
operation on a router
Routing Protocol and types
 Routing protocol specifies how routers
communicates with each other .
 Calculate information to select best path between
two nodes.
 This is done by using routing algorithms.
 Many types of routing protocols
 Interior Gateway Protocols (IGPs)
 Exterior Gateway Protocols (EGPs)
Routing Protocols and Types
 Interior Gateway Protocols
 Link state routing protocols i.e. OSPF, ISIS
 Path vector or distance protocols .e. RIP, IGRP and
EIGRP
 Exterior Gateway Protocols
 Example : BGP
 ISIS runs on data link layer
 OSPF, IGRP and EIGRP run directly over IP
 RIP runs over UDP
 BGP runs over TCP
RIP - Routing Information Protocol
 Distance vector routing protocol
 Employs the hop count as routing metric (lower is
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better)
Limit of number of hops, which is 15 hops
RIP transmits full updates every 30 seconds
Not Scalable for larger network
Stability features of RIP
 Split Horizon
 Route poisoning
 Hold down
RIP – Different versions
 RIP has two versions
 RIP v1
- Does not carry the subnet information hence does
not support variable length submit mask (VLSM)
- No support for authentication
 RIP v2
- Carry the subnet information hence supports VLSM
- Supports authentication
 RIPng (RIP next generation)
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Supports for IPv6
RIPv3 supports arbitrary tags to routes, RIPng does not.
RIP – stability features
 Split Horizon
According to the split-horizon rule, node A does not advertise its route for C
(namely A to B to C) back to B.
With the split-horizon rule in place, this particular loop scenario cannot happen,
improving convergence time in complex, highly-redundant environments.
 Route poisoning
Split-horizon routing with poison reverse is a variant of split-horizon route
advertising in which a router actively advertises routes as unreachable over the
interface over which they were learned. The effect of such an announcement is
to immediately remove most looping routes before they can propagate through
the network.
The main disadvantage of poison reverse is that it can significantly increase the
size of routing announcements in certain fairly common network topologies.
RIP – stability features
 Hold Down
- It works by having each router start a timer when they first receive
information about a network that is unreachable.
- Until the timer expires, the router will discard any subsequent route
messages that indicate the route is in fact reachable.
- It can solve the case where multiple routers are connected indirectly.
- The classic example would be three routers configured in a triangle. In
this situation, Split horizon and split horizon with poisoned reverse can
do nothing.
 In other words, a hold-down keeps a router from receiving route updates
until the network appears to be stable—until either an interface stops
changing state (flapping) or a better route is learned.
Configuring RIP
 Enable the RIP protocol on the router with the following command.
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Router(config)#router rip
Identify the networks to be advertised using the following command,
you need to identify only the networks that are directly connected to
the router.
Router(config-router)#network 192.168.0.0
To adjust the timers (update, invalid, holddown, and flush timers) use
the ‘timers basic’ command, all timers are in seconds
Router(config-router)#timers basic 30 180 180 240
Always keep timer ‘n’ ‘6n’ ‘6n’ ‘8n’. If, for example, you set the update
timer to 40, you need to make the other timers 240 240 320
consequently.
To prevents the interface from forwarding any RIP broadcasts, but
keeps the interface listening to what others are saying in RIP.
Router(config-router)#passive-interface Fa0/0
IGRP – Interior Gateway routingProtocol
 Distance vector routing protocol, invented by Cisco
 Created to overcome limitation of RIP
 Uses multiple metrics for each route
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- Bandwidth, Delay, MTU, Reliability
Maximum hop count is 255, default is 100
Routing updates are broadcast and every 90 seconds
Classfull routing protocol i.e. no field for subnet mask in update
Does equal cost load balancing
Metric = [K1 * Bandwidth + (K2 * Bandwidth)/(256-load) +
K3*Delay] * [K5/(reliability + K4)]
Default constant values are K1 = K3 =1 and K2=K4=K5 =0.
IGRP – Interior Gateway routingProtocol
 Find the smallest of all the bandwidths in Kbps from outgoing
interfaces and divide 10,000,000 by that number.
 In order to find the delay, add all of the delays (in
microseconds) from the outgoing interfaces and divide this
number by 10.
 Remember, the path with the smallest metric is the best path.
 The various outputs of the show commands for both the
routers are as shown here:
IGRP – Interior Gateway routingProtocol
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Saturn# show interfaces serial 1
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 172.16.10.1/24
MTU 1500 bytes, BW 224 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY, loopback not set
Keepalive set (10 sec)
LMI enq sent 167, LMI stat recvd 168, LMI upd recvd 0, DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
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Saturn# show interfaces ethernet 0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0060.5cf4.a955 (bia 0060.5cf4.a955)
Internet address is 172.17.10.1/16
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
IGRP – Interior Gateway routingProtocol
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Venus# show interfaces ethernet 0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0060.5cf4.a9a8 (bia 0060.5cf4.a9a8)
Internet address is 12.1.1.1/24
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
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Venus# show interfaces serial 0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 172.16.10.2/24
MTU 1500 bytes, BW 784 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY, loopback not set
Keepalive set (10 sec)
LMI enq sent 981, LMI stat recvd 330, LMI upd recvd 0, DTE LMI up
LMI enq recvd 340, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
IGRP – Interior Gateway routingProtocol
You are able to view the metric values calculated by IGRP with the show ip route command:
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Venus# show ip route 172.17.10.1
Routing entry for 172.17.0.0/16
Known via "igrp 100", distance 100, metric 14855
Redistributing via igrp 100
Advertised by igrp 100 (self originated)
Last update from 172.16.10.1 on serial0, 00:00:13 ago
Routing Descriptor Blocks:
* 172.16.10.1, from 172.16.10.1, 00:00:13 ago, via Serial0
Route metric is 14855, traffic share count is 1
Total delay is 21000 microseconds, minimum bandwidth is 784 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
The corresponding calculations are:
Metric = Bandwidth + Delay = 10000000/784 + (20000 + 1000)/10 = 14855
IGRP – Interior Gateway routingProtocol
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Saturn# show ip route 12.1.1.1
Routing entry for 12.0.0.0/8
Known via "igrp 100", distance 100, metric 46742
Redistributing via igrp 100
Advertised by igrp 100 (self originated)
Last update from 172.16.10.2 on serial1, 00:00:43 ago
Routing Descriptor Blocks:
* 172.16.10.2, from 172.16.10.2, 00:00:43 ago, via Serial1
Route metric is 46742, traffic share count is 1
Total delay is 21000 microseconds, minimum bandwidth is 224 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
The corresponding calculations are:
Metric = Bandwidth + Delay = 10000000/224 + (20000 + 1000)/10 = 46742
Administrative Distance and Metric
 A “metric” allows a router to choose the best path within a
routing protocol.
 Distance vector routing protocols use “distance” (usually hopcount) as their metric.
 Link state protocols utilize some sort of “cost” as their metric.
 Only routes with the best metric are added to the routing table.
Thus, even if a particular routing protocol (for example, RIP)
has four routes to the
same network, only the route with the best metric (hop-count
in this example) would make it to the routing table. If multiple
equal-metric routes
exist to a particular network, most routing protocols will loadbalance.
Administrative Distance and Metric
 If your router is running multiple routing protocols,
Administrative Distance (AD) is used to determine which
routing protocol to trust the most.
 Lowest administrative distance wins.
 If a router receives two RIP routes to the same network, it will
use the routes’ metric to determine which path to use. If the
metric is identical for both routes, the router will load balance
between both paths.
 If a router receives a RIP and an OSPF route to the same
network, it will use Administrative Distance to determine which
routing path to choose.
Administrative Distance
Type of Route
Admin distance
Connected
0
Static
1
EIGRP Summary
5
External BGP
20
Internal EIGRP
90
IGRP
100
OSPF
110
IS-IS
115
RIP
120
External EIGRP
170
Internal BGP
200
Unknown
255
NAT – Network Address Translation