CCNP
- BSCI Exam cram
(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:
1.
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:
1.
Interior: Interior routes are those that are directly
connected to a router interface.
2.
System: Routes advertised by other IGRP neighbors within
the same autonomous system (AS).
3.
Exterior: These are the routes learned from a different
Autonomous System number (ASN).
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>.
