(Exam: 642-801)
11. Distance vector protocol depends only on Hop count to determine the nearest next hop for forwarding a packet. One major disadvantage is that this may not always represent the best route. For example, if you have a destination connected through two hops via T1 lines, and if the same destination is also connected through a single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!
12. There are broadly three types of routing protocols:
Distance Vector (Number of hops) - Distance vector routing determines the direction (vector) and distance to any link in the internetwork. Typically, the smaller the metric, the better the path. EX: Examples of distance vector protocols are RIP and IGRP. Distance vector routing is useful for smaller networks. The limitation is that any route which is greater than 15 hops is considered unreachable. One important thing that differentiates distance vector with Link state is that distance vector listens to second hand information to learn routing tables whereas, Link state builds its routing tables from first hand information. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors.
Link State Routing: Link State algorithms are also known as Shortest Path First (SPF) algorithms. SPF recreates the exact topology of the entire network for route computation by listening at the first hand information. Link State takes bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more attractive for larger networks. Bandwidth and delay are the most heavily weighed parts of the metric when using Link-State protocols. EX: OSPF and NLSP.
Benefits of Link State protocols:
1. Allows for a larger scalable network
2. Reduces convergence
time
3. Allows "super netting"
3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. EX: EIGRP.
13. The default administrative distances are as below:
| Type of protocol | Administrative distance |
|---|---|
| Directly connected | 0 |
| Static route | 1 |
| EIGRP Summary | 5 |
| External BGP | 20 |
| EIGRP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| ISIS | 115 |
| RIP | 120 |
| Unreachable | 255 |
14. IGRP, EIGRP: IGRP and EIGRP are proprietary of Cisco. These two protocols use composite metric to determine the best path to a remote network.
IGRP (as well as EIGRP) use the following components as metrics:
1. Delay: Calculated by adding up the delay along the path to
the next router.
2. Reliability: This is representative of how
many errors are occurring on the interface. The best reliability
value is 255. A value of 128 represents only 50% reliability.
3. Load: Load metric also has a range from 1 to 255. If a serial
link is being operated at 50% capacity, the load value is 255X0.5
or 12.5. Lower load value is better.
4. MTU: Stands for Maximum
Transmit Unit size, in bytes. Ethernet and serial interface has
a default MTU of 1500. Larger MTU size means that the link is more
efficient.
5. Bandwidth: The bandwidth is specified in Kbps.
Larger the bandwidth, better the link.
EIGRP (as well as IGRP) uses Bandwidth and Delay as default criteria to determine the best path.
show ip route eigrp: Displays the current EIGRP entries in the routing table.
Show ip eigrp traffic: This command can be used to learn the number of EIGRP packets sent and received.
The neighbor table in EIGRP include the following fields:
1.Neighbor address: This is the network layer address of the
neighbor router.
2.Queue: This represents the number of packets
waiting in queue to be sent.
3.Smooth Round Trip Time (SRTT):
This represents the average time it takes to send and receive packets
from a neighbor. This timer is used to determine the retransmit
interval (RTO).
4.Hold Time: This is the period of time that
a router will wait for a response from a neighbor. If there is no
response at the end of this time period, the link is considered
unavailable.
15. Hello packets: The types of router protocols that use "Hello" packets are EIGRP, IS-IS, and OSPF.
16. Cisco IOS commands:
Show IP protocol: This command will show information on RIP timers including routing update timer (30sec default), hold-down timer (default 180sec). It also displays the number of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.
2.Show IP route: This command will display the IP routing table entries. In addition, it displays the Gateway of last resort (if one is assigned). It also displays the codes used for various types of routes. Some of the important codes are:
C: directly connected;
S: Statically connected
I : IGRP
R : RIP
3.show IP interface: This command shows you interface-wise information such as IP address assigned to each interface, whether the interface is up, MTU etc.
4.Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous list of routing updates as they are sent and received. This leads to lot of overhead, which is the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with debugging.
5.The command "no router rip" is used for removing all rip entries from the router.
6.The command
i.clear ip bgp *
clears all the entries from the BGP routing
table and reset BGP sessions. This command is used after every configuration
change to ensure that the change is activated and that peer routers
are informed.
ii.Another command,
clear ip bgp <address>
ex: clear
ip bgp 172.31.0.0 removes the specified network from the BGP table
17. For IGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information.
18. IGRP:
IGRP update packet is sent every 90 seconds by default. This is 30 Sec for RIP.
By giving the command "show ip route igrp", we can see the routes found by IGRP. A route discovered by IGRP is denoted by letter "I" before start of the entry.
The following three types of routes are recognized by IGRP:
19. Private Internet addresses:
The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space your use for private networks:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0
- 192.168.255.255
20. There are three ways a router learns how to forward a packet:
1. Static Routes - Configured by the administrator manually.
The administrator must also update the table manually every time
a change to the network takes place. Static routes are commonly
used when routing from a network to a stub (a network with a single
route) network.
The command is
ip route network mask address/interface
[distance]
ex: ip route 165.44.34.0 255.255.255.0 165.44.56.5
Here, 165.44.34.0 is the destination network or subnet
255.255.255.0
is the subnet mask
165.44.56.5 is the default gateway.
2. Default Routes - The default route (gateway of last resort)
is used when a route is not known or is infeasible. The command
is
ip route 0.0.0.0 0.0.0.0 165.44.56.5
The default gateway
is set to 165.44.56.5
3. Dynamic Routes - As soon as dynamic
routing is enabled, the routing tables are automatically updated.
Dynamic routing uses broadcasts and multicasts to communicate with
other routers. Each route entry includes a subnet number, the interface
out to that subnet, and the IP address of the next router that should
receive the packet. The commands to enable rip are:
router rip
network <major network number>.
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