CCNP
- BSCI Exam cram
(Exam:
642-801)
CCNP-BSCI (Building Scalable Cisco Internetworks) exam is a requirement towards obtaining CCNP
certification. Skills measured are: Designing and
implementing complex routed WANs including EIGRP, OSPF,
BGP, and IS-IS. Valid CCNA certification is a pre-requisite for obtaining
CCNP certification.
To
be CCNP certified, the following exams need to be
successfully completed:
|
Exam
|
Exam Code
|
Study material covering exam
objectives
|
|
BSCI Exam
|
642-801
|
Building Scalable
Cisco Internetworks or BSCI
|
|
Switching
Exam
|
642-811
|
Building Cisco
Multi-layer Switched Network or BCMSN
|
|
Remote Access
Exam
|
642-821
|
Building Cisco
Remote Access Networks
|
|
Support Exam
|
642-831
|
Cisco Internetwork
Troubleshooting
|
Alternatively, one can take the following exams
to obtain CCNP certification:
|
Exam
|
Exam Code
|
Study material covering exam
objectives
|
|
Foundations
Exam
|
640-841
*retired"
|
Building Scalable
Cisco InterNetworks
(Corresponds to 640-901 exam)
|
|
Building Cisco
Multi-layer Switched Network.
(Corresponds to 640-604 exam)
|
|
Building Cisco
Remote Access Networks.
(Corresponds to 640-605 exam)
|
|
Support
Exam
|
640-606
"retired"
|
Cisco Internetwork
Troubleshooting (CIT).
|
The BSCI exam is of 75min duration and there will be
approximately 65 questions. You need to score 690 or
more to pass the exam. Visit
the official website here.
1. Scalable networks:
The key 5 characteristics of Scalable
Internetworks are:
- Reliable and available: An internetwork is usually up
for 24 hours a day and seven days a week.
- Efficient: Efficiency means optimization of resources
keeping in view available bandwidth. An internetwork
should have less amount of overhead traffic, such as
broadcasts, routing updates etc.
- Responsive: It is necessary that the internetwork
meet QoS
requirements for different protocols. Cisco IOS has
been developed keeping in view the QoS demands.
Different protocols may require different QoS
standards.
- Adaptable: An internetwork should be able to
accommodate variety of
networks and protocols. The available
protocols may include for example, TCP/IP, IPX, and
SNA. An adaptable internet should be able to
accommodate legacy as well as more recent
technologies such as VOIP.
- Accessible and Secure: An internet should be
accessible by using different access methods, such
as dial-up, dedicated, switched connections. At the
same time, it should provide secure environment.
2. The typical three-layer hierarchical
internetworking model consists of the following:
- Core
layer: Core layer is responsible to provide an
optimal and reliable transport structure. The core
layer is the backbone network of the entire
internetwork and may include LAN and WAN backbones.
Core layer usually consists of fully redundant paths
with technologies such as FDDI, Fast Ethernet, and/
or ATM.
- Distribution
layer: Distribution layer is responsible to provide
access to the internetwork as well as to the
servers. Distribution layer sits between the Core
layer and the Access layer. The policies such as
ACLs are implemented at the distribution layer.
Distribution layer is also known as workgroup layer.
- Access
layer, provides the users, access to the resources
on internetwork.
In real world, a single
device may be functioning at both Access layer as well
as distribution layer. This is true for even Core layer.
3. Network segmentation:
Network congestion can be addressed by
segmentation of the network. Network segmentation, also
called micro segmentation, can be done by using:
- Bridges,
- Routers, and
- Switches.
The primary purpose of segmentation is to
reduce congestion in the network.
4. Bridges and switches forwards all
broadcasts, which puts extra load on the network. In
other words, though bridges divide the network into
different collision domains, the broadcast domain remain
only one. This increases the overhead on the network.
5. The Cisco IOS features that allow reduction
in bandwidth are:
- Access Control Lists: ACLs are used to permit or deny
protocol update traffic, data traffic, and broadcast
traffic. Cisco access lists are available for IP,
IPX, and AppleTalk protocols.
- Snapshot routing: Snapshot routing can reduce WAN
costs, by exchanging the routing table at predefined
intervals. The routing tables for the distance
vector protocols are kept frozen until the next
update occurs. Snapshot routing is used only on
distance vector protocols such as IP RIP. Snapshot
routing is widely used on ISDN lines.
- Compression over WANs: Cisco IOS supports TCP/IP
packet header, as well as data compression. Link
compression is also supported, that compresses both
header and data information in packets across point
to point connections.
- DDR (Dial on Demand Routing): DDR are useful when the
traffic flow is not continuous in nature. In DDR,
channel is created only after intended traffic is
detected by the router, by dialing the destination.
- Switched network access: Switched networks, such as
Frame Relay, X.25 can share the bandwidth by
establishing virtual circuits.
- Optimization of routing table size: Routing table
entries consume bandwidth and processing power.
These entries can be reduced by techniques such as
route summarization, and incremental updates.
6. Snapshot routing builds routing table based
on a snapshot of a dynamic routing table available when
the network is active. The snapshot routing table is
used until another activity occurs on the network, at
which time the routing table is rebuilt. No routing
information is exchanged when the network is quiet.
Snapshot routing can be applied to distance vector
protocols such as IP RIP, IGRP, IPX RIP, and RTMP.
7. Cisco IOS supports the following queuing
methods:
- Weighted fair-queuing: This is an automatic queuing
method that provides fair bandwidth to all network
traffic.
- Priority queuing: Here, one particular type of traffic
is given priority over all other types of traffic.
Thus this particular traffic, for which priority is
given, is assured of bandwidth. All other types of
traffic do not have assured bandwidth.
- Custom queuing: Here, each traffic type gets a
pre-allocated bandwidth. Certain types of traffic
can be allocated higher bandwidth depending on the
requirement.
8. RIP
-
RIP (and IGRP) always summarizes routing
information by major network numbers. This is called
classful routing.
-
IP RIP based networks send the complete routing
table during update. The default update interval is 30
seconds.
-
RIP version 2 is a classless routing protocol,
where as RIP version 1 (RIP 1) is a classful routing
protocol. The disadvantage of classfull routing is that
some address space may be wasted. In classless routing,
routing protocols exchange the subnet mask information
during periodic routing updates. This allows variable
subnet masks to be used in the network, allowing better
use of address space. For example, a WAN link may need
only two IP addresses. If you use classless routing
protocol with, say 6 bits for subnetting (62-2 subnets),
only 2 subnet addresses are utilized and the remaining
become wasted. On the other hand, if you use classless
routing protocol, Variable Length Subnet Mask (VLSM) can
be used within the network, giving only 2 valid
addresses for the WAN link, thus saving valuable address
space. (If you are using IP addresses, address space
involves IP addresses).
9. Metric limit for link-state protocols is
65,533.
10. Convergence is the term used to describe
the state at which all the internetworking devices, running specific routing protocol, are having the same
information about the internetwork in their routing
tables. The time it takes to arrive at common view of
the internetwork is called Convergence Time.
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>.
21.
OSPF:
1.
An OSPF area is a collection of networks and routers
that has the same area identification.
2.
The following are the types of OSPF routers:
i.
Internal router: An internal router has all the
interfaces in the same area. All internal routers
maintain same link state databases.
ii.
Backbone router: Backbone routers reside on the
perimeter of Area 0, with at least one interface
connected to backbone (Area 0).
iii.
Area Border Router (ABR): ABRs are routers that have
interfaces attached to multiple areas. It may be noted
that these routers maintain separate link-state
databases for each area that they are connected. They
are capable of routing traffic destined for or arriving
from other areas.
iv.
Autonomous System Boundary Router (ASBR): This router
has at least one interface to the external network
(another autonomous system). This autonomous network can
be non-OSPF. ASBRs are capable of route redistribution.
Redistribution is the ability of a router to import
routing information from non-OSPF networks, and
distribute the same in OSPF network for which it is
responsible and visa versa.
3.
LSA Types:
i.
LSA Type 1: Router link entry, generated by all routers
for each area to which it belongs. These are flooded
within a particular area.
ii.
LSA Type 2: Network link entry, generated by designated
router (DRs). Type 2 LSAs are advertised only to routers
that are in the area containing the specific network.
iii.
LSA Type 3 and Type 4: Summary link entry, these LSAs
are generated by area border routers (ABRs). These are
sent to all routers within an area. These entries
describe the links between the ABR and the internal
routers of an area. These entries are flooded throughout
the backbone area and to the other ABRs.
iv.
LSA Type 5: Autonomous System External Link Entry, these
are originated by ASBR. These entries describe routes to
destinations external to the autonomous system. These
LSAs are flooded throughout the OSPF autonomous system
except for stubby and totally stubby areas.
4.
The sequence of steps followed in OSPF operation are as
below:
1.
Establish router adjacencies
2.
Elect DR and BDR
3.
Discover Routes
4.
Choose appropriate routes for use
5.
Maintain routing information.
5.
The command "show ip ospf database" displays
the contents of the topological database maintained by
the router. This command also displays router id and the
ospf process id.
6.
show ip ospf interface can be used to check whether the
interfaces have been configured properly. The command
also gives the timer intervals, including hello
intervals, and neighbor adjacencies.
7.
OSPF keeps up to six equal-cost route entries in the
routing table for load balancing.
8.
OSPF uses Dijkstra algorithm to calculate lowest cost
route. The algorithm adds up the total costs between the
local router and the each destination network. The
lowest cost route is the preferred route when there are
multiple paths to a given destination.
9.
OSPF has the following advantages over Distance Vector
protocols such as RIP:
1.
Faster convergence: OSPF network converges faster
because routing changes are flooded immediately and
computed in parallel.
2.
Support for VLSM: OSPF supports VLSM. However, please
note that RIP version2 also supports VLSM.
3.
Network Reachability: RIP networks are limited to 15
hops. On the other hand, OSPF has practically no
reachability limitation.
4.
Metric: RIP uses only hop count for making routing
decisions. This may lead to poor efficiency in some
cases. For example, that a route is nearer but is very
slow compared to another route with plenty of bandwidth
available but few more hops away. OSPF uses
"cost" metric to choose best path. Cisco uses
"bandwidth" as metric to choose best route.
5.
Efficiency: RIP uses routing updates every 30 seconds.
OSPF multicasts link-state updates and sends the updates
only when there is a change in the network status
10.
The path cost in OSPF network is calculated using
bandwidth. The formula used is [10 <8> divided by
Bandwidth]. For example, the cost of a 56kbps serial
link is 1785. The default cost of a 10mbps Ethernet is
10.
22.
When a serial line is configured on a Cisco router, the
default bandwidth is 1.544Mbps. If the line is slower
speed, "bandwidth" command can be used to
specify the real link speed. The cost of the link will
then automatically correspond to the changed value.
23.
You must manually configure a static route to configure
DDR (Dial on Demand Routing). DDR is widely used as a
backup route, in case of failure of primary link.
24.
Route Summarization:
Route summarization is
calculated as below:
Step 1:
1.
Take the first IP: 172.24.54.0/24: 172.24. 0 0 1 1 0 1 1
0.0
2.
Take the second IP: 172.24.53.0/24: 172.24. 0 0 1 1 0 1
0 1.0
Note
that we are not really concerned about the octets that
have equal decimal values. This is because they don’t
come into play while calculating summarization route, in
this case.
Step 2:
Count
the number of bits in the third octet that are aligned
(or lined up) with same values. In this case 6 bits are
lined up in the third octet. The summarization route is
calculated by adding this number (6) to the octets
preceding the third (first and second octets).
Therefore,
the number of bits in the summarized route is 8+8+6 = 22
Step 3:
Calculate
the decimal equivalent for third octet with 6 bits as
given in the matching binary. That is
0 0 1 1 0 1 x x. Note x is because it corresponds
to non matching binary number. It is equal to 128*0 +
64*0 + 32*1 + 16*1 + 8*0 + 4*1 or 32+16+4 or 52.
Therefore,
the summarized route is:
172.24.52.0/22
25.
While evolving a network addressing scheme for an
organization, you need to assign a different network
number for each subnet. Also, you need to set aside one
network number for each WAN connection.
26.
Representing a subnet mask with / notation:
Consider an IP subnet mask
of 255.255.255.128. The same be represented as /25. This
is arrived at, by taking the binary equivalent of
255.255.255.128 (= 11111111.11111111.11111111.10000000).
Count the number of ones’, there are 25 of them.
Therefore, the same can be written as /25.
27.
The following are link state routing protocols:
IPX NLSP
IS-IS
IP-OSPF
28.
OSPF - LSA, LSR, and LSUs:
1.
LSA (Link State Advertisement): LSAs are included in the
database description packets (DDPs or DBDs).
LSA entries include link-state type, the address
of the advertising router, the cost of the link, and the
sequence number.
2.
LSR ( Link State Request): When a slave router receives
a DDP (Database Description Packet), it sends an LSAck
packet. Then it compares the received information with
its own information. If the DDP has more recent
information, the slave router sends a link-state request
(LSR) to the master router.
3.
LSU ( Link State Update): LSU packet is sent in response
to LSR (Link-State Request) packet that is sent from a
slave router to a master router. LSU contains complete
information about the requested entry.
4.
In an OSPF environment,
1.
A DDP (Data Description Packet) is used during the
exchange protocol and includes summary information about
link-state entries.
2.
A hello packet is used during the hello process and
includes information that enables routers to establish
neighbor relationship.
3.
An internal router is a router that resides within an
area.
29.
Important features of stub area are:
1.
A stub area reduces the size of the link-state database
to be maintained in an area, which in turn result in
less overhead in terms of memory capacity, computational
power, and convergence time.
2.
The routing in Stub and totally Stubby areas is based on
default gateway. A default route (0.0.0.0)
need to be configured to route traffic outside
the area.
3.
The stub areas suited for Hub-Spoke topology.
4.
Area 0 is not configured as Stubby or totally Stubby.
This is because stub areas are configured mainly to
avoid carrying external routes, whereas Area 0 carries
external routes.
30.
EIGRP:
Some of the important
terms used in Enhanced IGRP are:
1.
Successor: A route (or routes) selected as the primary
route(s) used to transport packets to reach destination.
Note that successor entries are kept in the routing
table of
the router.
2.
Feasible successor: A route (or routes) selected as
backup route(s) used to transport packets to reach
destination. Note that feasible successor entries are
kept in the topology table of a router. There can be up
to 6 (six) feasible successors
for IOS version 11.0 or later. The default is 4
feasible successors.
3.
DUAL (Diffusing Update Algorithm): Enhanced IGRP uses
DUAL algorithm to calculate the best route to a
destination.
31.
BGP:
-
Internet Assigned Numbers Authority (IANA) is
responsible for assigning BGP autonomous system numbers.
1.
The assignable BGP autonomous system numbers are from 1
to 65,535 (I.e. 65,535 in total). Autonomous system
numbers are of 16 bit length. There are 2 ^ 16 = 65536
-1 possible ASNs. ASN of all 0s is not assigned. Out of
this, the Internet Assigned Numbers Authority (IANA) has
reserved the following block of AS numbers for private
use: 64512 through 65535.
2.
External BGP (eBGP) is used to establish session and
exchange route information between two or more
autonomous systems. Internal BGP (iBGP) is used by
routers that belong to the same Autonomous System (AS).
3.
Routers running BGP in an AS use network Policy to
choose the best path. Metrics are not used in BGP.
Remember that Internet is made of autonomous systems
(AS) that are connected together based on Policies
specific to each AS. Also, AS numbers (ASN) are assigned
by AINA and are unique over the Internet. In an internet
(not big I) the ASNs can be assigned by the corporation
itself that is implementing internet.
4.
The following are the four possible message types in a
BGP header:
Type
1: OPEN message - This is the first message sent after
TCP session is established.
Type
2: UPDATE message - An UPDATE message contains a new
route or a route to be withdrawn or both. Note that only
one new route can be advertised with one UPDATE message.
Type
3: NOTIFICATION message - this message is sent if an
error occurs during a BGP session. This message can be
used to troubleshoot the problem.
Type
4: KEEPALIVE message - KEEPALIVE message is used to
confirm that the connection between the neighboring
routers is still active.
5.
Command to set the router RouterA to autonomous system
number 1340:
The
correct syntax for the command is:
RouterA(config)#router
bgp 1340
where
1340 is the AS number which can have a value between 1
and 65535 in an internetwork.
6.
Port number 179 is used to establish a session between
two routers running BGP.
7.
Well-Known mandatory attributes must appear in all BGP
update messages. The well-known mandatory messages are:
1.
AS_PATH: BGP messages carry the sequence of AS numbers
indicating the complete path a message has traversed.
2.
NEXT_HOP: This attribute indicates the IP address of the
next-hop destination router.
3.
ORIGIN: This attribute tells the receiving BGP router,
the BGP type of the original source of the NLRI
information.
8.
Any two routers that have formed a TCP connection in
order to exchange BGP routing information are called
peers, or neighbors. BGP peers initially exchange their
full BGP routing tables. After this exchange, routing
table changes are sent as incremental updates. BGP keeps
a version number of the BGP table, which should be the
same for all of its BGP peers. The version number
changes whenever BGP updates the table, likely due to
routing information changes. Keep alive packets are sent
to ensure that the connection is alive between the BGP
peers.
9.
show ip bgp neighbors
This
is a very useful command in troubleshooting BGP
connections. When the connection is established, the
peer/ neighbor router exchanges BGP information. If a
TCP connection (BGP session) is not established, a BGP
router can not exchange any BGP routing information with
the adjacent router.
10.
Few recommended
scenarios, where you use BGP are:
1.
Connect two or more ISPs
2.
The traffic flow out of your network need to be managed
to suit the requirements of your organization.
3.
The traffic need to be sent through one AS to get to
another AS.
10.
The weight attribute in BGP has a range from 0 to 65535.
This attribute can be set using "neighbor"
command. The default value is 32,768.
11.
Various debug commands useful in troubleshooting bgp
are:
1.
Debug ip bgp events: Displays all bgp events as they
occur.
2.
Debug ip bgp dampening: Displays bgp dampening events as
they occur.
3.
Debug ip bgp keepalives: Displays all events related to
bgp keepalive packets.
4.
Debug ip bgp updates: Displays information on all bgp
update packets.
12.
Prefix lists (filtering)
are available only in Cisco IOS versions 12.0 and
later.
-
Characteristics of Prefix lists:
1.
These are used for filtering BGP routing updates, so
that certain path policy is applied.
2.
Prefix lists put less load on the processor compared to
Access lists.
3.
Prefix lists are easier to configure and implement.
4.
Prefix lists are read one line at a time.
5.
There is an implicit deny all at the bottom of the
Prefix list. However, if the prefix list is empty, there
will be an implicit permit any.
6.
The statement with the smallest sequence numbers is read
first.
7.
Sequence values are generated in increments of 5. The
first sequence value generated in a prefix list would be
5, then 10, then 15, and so on.
-
The following are a few examples of how a prefix list
can be used (while configuring BGP policies to filter
route updates):
To
deny the default route 0.0.0.0/0:
ip
prefix-list mylist1 deny 0.0.0.0/0
To
permit the prefix 20.0.0.0/8:
ip
prefix-list mylist1 permit 20.0.0.0/8
32.
1.
A stub AS is a single-homed network with only one entry
and exit point. This type of AS can be connected to the
external world through the use of a statically
configured route.
2.
Transit AS: Data from one AS need to reach a remote AS,
then it has to travel through intermediate AS. The AS or
Autonomous Systems which carry the data from one AS to
another AS is (are) called Transit AS (es).
3.
eBGP: External BGP is used between two or more
Autonomous Systems.
4.
iBGP: Internal BGP is used within an AS.
33.
In BGP, to disable automatic summarization of subnet
routes into network level routes use the command:
no auto-summary
To enable automatic
summarization of subnet routes into network level routes
use the command:
auto-summary
Note that by default,
auto-summary is enabled.
34.
BGP is an exterior routing protocol, whereas RIP, IGRP,
and OSPF are all Interior routing protocols (IRP).
Interior routing protocols run inside a company's
network and can't run on the
Internet. The Internet consists of numerous
autonomous systems (AS) which are connected by Exterior
Routing protocols like BGP.
35.
BGP commands:
-
Suppose, RouterA and RouterB are running iBGP. The
correct syntax for establishing neighbor relationship
is:
router
bgp 100
neighbor
175.23.1.2 remote-as 100
iBGP
routers don't have to be directly connected, as long as
there is some IGP running, that allows the two neighbors
to reach one another. If two routers belong to the same
AS, then they run iBGP, whereas, if they belong to
different ASs, they need to run eBGP.
-
The output is that of "show ip bgp summary".
It contains the following among other details:
1.
BGP router identifier: Router identifier specified by
the bgp router-id command, loop back address, or lowest
IP address.
2.
BGP table version:
Internal version number of BGP database.
3.
Main routing table version: Last version of BGP database
that was injected into main routing table.
4.
Neighbor: IP
address of a neighbor.
5.
V: BGP
version number spoken to that neighbor.
6.
AS: Autonomous system.
-
To specify the networks to be advertised by the Border
Gateway Protocol (BGP) use the network command.
To remove an entry, use the no network form of
this command.
network
network-number [mask network-mask]
To
remove,
no
network network-number [mask network-mask]
-
To distribute Border Gateway Protocol (BGP) neighbor
information as specified in a prefix list, use the
neighbor prefix-list command in address family or router
configuration mode.
The
following router configuration mode example applies the
prefix list named mylist1 to outgoing advertisements
from the neighbor 192.10.0.0:
!
router
bgp 100
network
120.101.0.0
neighbor
192.10.0.0 prefix-list mylist1 out
-
To distribute Border Gateway Protocol (BGP) neighbor
information as specified in an access list, use the
neighbor distribute-list command in address family or
router configuration mode.
36.
Route maps are used with BGP to control and modify
routing information and to define the conditions by
which routes are redistributed between Autonomous
Systems. The format of a route map is as follows:
route-map map-name
[[permit | deny] | [sequence-number]]
The
map-name is a name that identifies the route map, and
the sequence number indicates the position that an
instance of the route map is to have in relation to
other instances of the same route map.
37.
Some of the terms used commonly with route reflectors in
BGP are:
1.
Route reflector: It is a router that is configured to
advertise the routes that are learned from iBGP
neighbors.
2.
Client: A router that shares information with the router
configured as route reflector.
3.
Cluster: The set of all routers configured as route
reflectors and clients.
4.
Cluster ID: If there are one route reflector in a
cluster, then, cluster ID is used to identify the route
reflectors uniquely in the specified cluster.
38.
Do not apply both a neighbor distribute-list and a
neighbor prefix-list command to a neighbor in any given
direction (inbound or outbound) on a BGP router.
These two commands are mutually exclusive, and
only one command (neighbor prefix-list or neighbor
distribute-list) can be applied to each inbound or
outbound direction.
39.
BGP peer groups:
1.
A BGP peer group significantly reduces the overhead of
configuring policies
on every individual BGP neighbor in an AS. When a peer
group is created, policies are assigned to the name of
the peer group itself and not to the individual
neighbors.
2.
Route maps, distribution lists, and filter lists usually
set update policies.
3.
Members of the peer group can be configured to override
the configuration options for incoming updates, but not
to the outgoing updates.
40.
The command (BGP)
neighbor
<ip-address> peer-group
<peer group name>
is used to add a neighbor
to a peer-group.
The complete commands to
add a neighbor are:
!
RouterA(config)#router bgp
100
RouterA(config-router)#neighbor
mygroup peer-group
RouterA(config-router)#neighbor
1.1.1.1 peer-group
mygroup
!
41.
When a route reflector in a BGP AS
receives an update, it takes the following
actions, depending on the type of peer that sent the
update:
1.
If the update is from a non-client peer : It sends the
update to all clients in the cluster.
2.
If the update is from a client peer: It sends the update
to all nonclient peers and to all client peers.
3.
If the update is from eBGP peer: It sends the update to
all nonclient peers and to all client peers.
42.
The following are well
known communities in BGP:
1.
Internet: All routers belong to this community by
default. Advertises the route to internet community.
2.
No-export: This indicates not to advertise a route to
eBGP
3.
No-advertise: This indicates not to advertise a router
to peers.
The
community attribute in BGP can contain a value in the
range 0 to 4294967200.
43. The correct syntax to
configure a router as a BGP route reflector is:
RouterA(config-router)#neighbor
<ip-address>
route-reflector-client
Here, it is:
RouterA(config-router)#neighbor
route-reflector-client 144.44.44.1
The
above command will configure RouterA as a route
reflector with the specified neighbor 144.44.44.1 as the
route reflector's client.
44.
Methods available for filtering BGP updates:
1.
Distribute lists: To restrict the routing information
can be filtered based on routing updates to/from a
particular neighbor. An access list that is applied to
updates to/from a neighbor serves as a filter.
2.
AS_Path filtering: Here, you specify an access list on
both incoming and outgoing updates based on the value of
the AS_path attribute.
3.
Route Map Filtering: Here, the "neighbor
route-map" command is used to apply a route map to
incoming and outgoing routes.
4.
Community Filtering: You can filter by setting the
community attribute on router updates.
45.
External and summary routes are not injected into a
totally stubby area in an OSPF network. The advantages
of totally stubby areas are reduced routing tables,
faster convergence, and stability.
46.
To enable the synchronization between Border Gateway
Protocol (BGP) and Interior Gateway Protocol (IGP)
system, synchronization command is used. To
advertise a network route without waiting for the IGP,
use the no synchronization command. By default,
synchronization is enabled.
The following router
configuration mode is an example that enables a router
to advertise a network route without waiting for the IGP:
!
router bgp 160
no synchronization
47.
Show ip bgp neighbors is a command most often used to
see neighbor details, which include the following:
1.
AS number,
2.
Uptime
3.
BGP messages received / sent
4.
Hold time, Keepalive intervals
5.
Remote router ID etc.
48.
BGP AS-PATH length:
You can increase the
AS-PATH length by adding dummy AS numbers.
The route map
configuration command:
set as-path prepend 100
causes a router to prepend
100 once to
the value of the AS_path attribute before it sends
updates to the specified neighbor.
If you want to prepend 100
twice, use the command
set as-path prepend 100
100
This will increase the
AS-PATH length in the updates being sent to the neighbor
and hence the path selection.
49.
BGP Distribute lists are created using IP standard
access lists and IP extended access lists. The range of
numbers for standard access list is 1 to 99 and extended
access list is 100 to 199. Therefore, the allowed range
of numbers is 1 to 199.
50.
EIGRP uses multicasts to send queries to neighbor
routers.
51. The three multi-homing
classifications are:
1.
Basic: Here the ISP will offer only the default route to
the AS. This kind of connection is least processor
intensive and recommended for simple networks with only
one ISP connection.
2.
Medium: This uses default routes and BGP. Internal AS
can select the best ISP to use depending on the
preferences.
3.
Full: Full multi-homing uses only BGP. Here the routes
are learned using the AS_PATH attribute information to
make routing decisions.
52.
BGP can load balance up to six links. You can have up to
six links to ISPs and use those links for Internet
traffic. This arrangement provides redundancy as well as
load balancing.
53.
BGP version 4 supports CIDR (Classless InterDomain
Routing).
54.
In BGP, the term Multihoming is used when one AS is
connected to two or more ASP. The purpose of multihoming
is:
1.
To improve the reliability of connectivity to the
Internet, that even if one connection fails, the other
connection will be available.
2.
To share the traffic load, resulting in the performance
improvement.
55.
While selecting best route in BGP, the order of
preferences are as below:
1.
Weight - If multiple routes exist, the route with the
highest weight is preferred.
2.
Local preference - If multiple routes have the same
weight, the route with the highest local preference is
preferred.
3.
Local router - If multiple routes have same local
preference, prefer the route originated by the local
router.
4.
AS path - If multiple routes have the same local
preference, prefer the route with shortest AS path.
56.
On an OSPF network, when a packet need to traverse from
one area to another area to reach its destination, it is
routed as below:
Source
Area -> Source ABR
-> Backbone Area -> Destination ABR ->
Destination Area Routers
57. IS-IS:
1.
Some of the OSI terms that are used in OSI routing environment are given below:
ES: End System, refers to any node that does not take part in the routing process, such as a work station.
IS: Intermediate System, refers to any network node that takes part in the routing process.
IS-IS: Intermediate System-to-Intermediate System, a routing protocol defined for OSI environment.
CSNP: ConnectionLess Network Protocol, protocol used by IS-IS for routing in OSI environment.
2.
The following are the notable features of IS-IS ( Intermediate System to Intermediate System) routing protocol:
1. IS-IS routing protocol is a link state protocol.
2. IS-IS uses different types of Hello packets to form adjacencies.
3. IS-IS protocol can be used in dual (IP and OSI) environment. The term given for IS-IS
implementation for mixed environment is Integrated IS-IS or Dual IS-IS.
4. IS-IS has the following metrics:
Cost metric - This is the only required metric.
The optional metrics are Delay, Expense, and Error.
IS-IS uses a single default metric with a maximum path value of 1024. The metric is typically assigned by a network administrator. Any single link value can be up to a maximum of 64.
Cisco IOS running IS-IS supports load balancing up to six equal-cost paths.
58.
IS-IS LSPs:
Most routers participating in IS-IS flood LSPs (Link State Packets) to adjacent neighbors, except for the interface on which the LSP was received.
Note that in a shared media like LAN, a DIS (Designated Intermediate System) is elected, and DIS floods the media with
LSPs.
The LSPs are used for constructing link state database. The LSPs have a life time of 20 minutes. The LSPs are refreshed by the originator periodically.
LSPs maintain a checksum and a sequence number.
59.
The following are the features common to both OSPF and IS-IS:
1. SPF (Shortest Path First) algorithm is used by both OSPF and IS-IS for computing shortest route to the destination.
2. They both elect designated router in multi access environment. The term used for designated router in IS-IS environment is Designated Intermediate System (DIS). However, the DIS election in IS-IS is pre-emptive. If a new router boots on the LAN with a higher priority, it becomes the DIS replacing the old DIS.
3. Both OSPF and IS-IS have authentication capability.
4. Both use Hello packets to establish adjacencies.
5. Both use SPF (Shortest Path First) algorithm to compute the shortest path to the destination.
6. Both use Areas, however there is no ABR (Area Border Router) in IS-IS. The L2 router is analogous to ABR.
Some differences between OSPF and IS-IS are:
OSPF uses IP whereas IS-IS uses CLNS in pure OSI environment.
IS-IS uses hierarchical architecture with two level (L1,L2) hierarchy, where as OSPF uses Area 0 with ABRs. Contiguous L2 or L1/L2 routers (ISes) form the backbone in IS-IS environment.
-Vijay
