CS6250 BUNDLE CERTIFICATION
EVALUATION EXAMS COMPLETE
QUESTIONS AND ANSWERS PRACTICE
SOLUTION
⩥ BGP Basics.
Answer: BGP session. A pair of routers, known as BGP peers, exchange
routing information over a semi-permanent TCP port connection called a
BGP session. To begin a BGP session a router will send an OPEN
message to another router. Then the sending and receiving router will
send each other announcements from their individual routing tables.
Depending on the number of routes being exchanged, this can take from
seconds up to several minutes.
A BGP session between a pair of routers in two different ASes is called
external BGP (eBGP) session, and a BGP session between routers that
belong to the same AS is called internal BGP (iBGP) session.
BGP messages. After a session is established between BGP peers, the
peers can exchange BGP messages to provide reachability information
and enforce routing policies. We have two types of BGP messages:
UPDATE
Announcements: These messages advertise new routes and updates to
existing routes. They include several standardized attributes.
Withdrawals: These messages are sent when a previously announced
route is removed. This could be due to some failure or due to a change in
the routing policy.
,2. KEEPALIVE: These messages are exchanged to keep a current
session going.
BGP prefix reachability. In the BGP protocol, destinations are
represented by IP Prefixes. Each prefix represents a subnet or a
collection of subnets that an AS can reach. Gateway routers running
eBGP advertise the IP Prefixes they can reach according to the AS's
specific export policy to routers in neighboring ASes. Then, using
separate iBGP sessions, the gateway routers disseminate routes to
internal routers according to the AS's import policy. Internal routers run
iBGP to propagate the routes to other internal iBGP speaking routers.
Path Attributes and BGP Routes. In addition to the reachable IP prefix
field, advertised BGP routes c
⩥ iBGP vs eBGP.
Answer: eBGP (for sessions between border routers of neighboring
ASes)
iBGP (for sessions between internal routers of the same AS).
Both protocols are used to disseminate routes for external destinations.
The eBGP speaking routers learn routes to external prefixes and they
disseminate them to all routers within the AS.
This dissemination is happening with iBGP sessions.
,⩥ iBGP vs IGP-like protocols (RIP or OSPF).
Answer: IGP-like protocols are used to establish paths between the
internal routers of an AS based on specific costs within the AS.
In contrast, iBGP is only used to disseminate external routes within the
AS.
⩥ ● How does a router use the BGP decision process to choose which
routes to import?.
Answer: The router compares routes by going through a list of attributes
and chooses the path with the lowest amount of hops, in the simplest
scenario.
The router uses a decision tool called LocalPref attribute which is the
preferred route learned through a specific AS over other ASes.
An AS ranks routes by first preferring the routes learned from its
customers, then the routes learned from its peers, and finally from the
routes learned from its providers.
Another attribute that can affect routing decisions is the MED (Multi-
Exit Discriminator) attribute.
, This is used by ASes connected by multiple links to designate which of
those links are preferred for inbound traffic.
⩥ Main Challenges with BGP.
Answer: Misconfigurations and faults.
A misconfiguration can result in an excessively large number of updates
which in turn can result in route instability, router processor and memory
overloading, outages, and router failures.
IXP stands for Internet Exchange Point.
They provide physical infrastructure for ASes to interconnect and
directly exchange traffic with each other.
The physical infrastructure of an IXP is usually a network of switches
that are located either in the same physical location, or they can be
distributed over a region or even at a global scale.
Typically, the infrastructure has fully redundant switching fabric that
provides fault-tolerance, and the equipment is usually located in
EVALUATION EXAMS COMPLETE
QUESTIONS AND ANSWERS PRACTICE
SOLUTION
⩥ BGP Basics.
Answer: BGP session. A pair of routers, known as BGP peers, exchange
routing information over a semi-permanent TCP port connection called a
BGP session. To begin a BGP session a router will send an OPEN
message to another router. Then the sending and receiving router will
send each other announcements from their individual routing tables.
Depending on the number of routes being exchanged, this can take from
seconds up to several minutes.
A BGP session between a pair of routers in two different ASes is called
external BGP (eBGP) session, and a BGP session between routers that
belong to the same AS is called internal BGP (iBGP) session.
BGP messages. After a session is established between BGP peers, the
peers can exchange BGP messages to provide reachability information
and enforce routing policies. We have two types of BGP messages:
UPDATE
Announcements: These messages advertise new routes and updates to
existing routes. They include several standardized attributes.
Withdrawals: These messages are sent when a previously announced
route is removed. This could be due to some failure or due to a change in
the routing policy.
,2. KEEPALIVE: These messages are exchanged to keep a current
session going.
BGP prefix reachability. In the BGP protocol, destinations are
represented by IP Prefixes. Each prefix represents a subnet or a
collection of subnets that an AS can reach. Gateway routers running
eBGP advertise the IP Prefixes they can reach according to the AS's
specific export policy to routers in neighboring ASes. Then, using
separate iBGP sessions, the gateway routers disseminate routes to
internal routers according to the AS's import policy. Internal routers run
iBGP to propagate the routes to other internal iBGP speaking routers.
Path Attributes and BGP Routes. In addition to the reachable IP prefix
field, advertised BGP routes c
⩥ iBGP vs eBGP.
Answer: eBGP (for sessions between border routers of neighboring
ASes)
iBGP (for sessions between internal routers of the same AS).
Both protocols are used to disseminate routes for external destinations.
The eBGP speaking routers learn routes to external prefixes and they
disseminate them to all routers within the AS.
This dissemination is happening with iBGP sessions.
,⩥ iBGP vs IGP-like protocols (RIP or OSPF).
Answer: IGP-like protocols are used to establish paths between the
internal routers of an AS based on specific costs within the AS.
In contrast, iBGP is only used to disseminate external routes within the
AS.
⩥ ● How does a router use the BGP decision process to choose which
routes to import?.
Answer: The router compares routes by going through a list of attributes
and chooses the path with the lowest amount of hops, in the simplest
scenario.
The router uses a decision tool called LocalPref attribute which is the
preferred route learned through a specific AS over other ASes.
An AS ranks routes by first preferring the routes learned from its
customers, then the routes learned from its peers, and finally from the
routes learned from its providers.
Another attribute that can affect routing decisions is the MED (Multi-
Exit Discriminator) attribute.
, This is used by ASes connected by multiple links to designate which of
those links are preferred for inbound traffic.
⩥ Main Challenges with BGP.
Answer: Misconfigurations and faults.
A misconfiguration can result in an excessively large number of updates
which in turn can result in route instability, router processor and memory
overloading, outages, and router failures.
IXP stands for Internet Exchange Point.
They provide physical infrastructure for ASes to interconnect and
directly exchange traffic with each other.
The physical infrastructure of an IXP is usually a network of switches
that are located either in the same physical location, or they can be
distributed over a region or even at a global scale.
Typically, the infrastructure has fully redundant switching fabric that
provides fault-tolerance, and the equipment is usually located in
