BGP EXAM STUDY GUIDE A+ GRADED
What methods can be used to route to or from a service provider? - ANSWER Static
routes, MPLS VPNs, circuit emulation, or BGP
Explanation:
Routing information can be exchanged with a service provider using a variety of
methods: static routing, MPLS VPNs, circuit emulation, or BGP. IGPs are typically not
used for routing Internet traffic with a service provider, although they are sometimes
used to exchange MPLS VPN routing information between a PE and CE router.
What protocol does BGP use to communicate at layer 4? - ANSWER TCP
Explanation:
Unlike other routing protocols, BGP uses TCP to exchange reliable updates at layer 4.
BGP uses TCP port 179 for neighbor sessions.
What is displayed by the show ip bgp command? - ANSWER The router's BGP table
Explanation:
The show ip bgp command displays the router's BGP table. This lists all BGP paths
available for each prefix, along with the characteristics of each path.
What is the difference between a router's BGP table and its routing table? - ANSWER
The routing table displays the best path to each prefix, regardless of protocol, whereas
the BGP table lists all BGP paths known to the router.
Explanation:
The BGP table lists all unique BGP paths for each destination prefix. The router then
selects the best BGP path, according to the BGP path selection process. The routing
table accepts candidate paths from the BGP table and uses them only if no other
protocols (such as static routes, IGPs, and so on) offer a path to the same prefix with a
better administrative distance.
What is the function of a BGP open message? - ANSWER It begins a BGP neighbor
session.
Explanation:
The four BGP message types are open, keepalive, update, and notification. The open
message is used to establish a BGP neighbor relationship and includes the BGP router
ID and the BGP session hold time.
,How does BGP detect that a neighbor is down, by default? - ANSWER By default, BGP
uses keepalive messages and a hold timer for neighbor-down detection.
Explanation:
Routing protocols need a way to determine whether a neighbor has gone down when the
physical link to the neighbor stays up. In BGP, routers send keepalive messages every
60 seconds by default. If a router sends keepalives for a period exceeding the hold time,
it declares the neighbor down and purges path information from that neighbor from its
BGP table.
What is contained in a BGP update message? - ANSWER Information about an AS path.
Explanation:
A BGP update method contains information about the attributes of an AS path. It can
contain information about multiple networks that can be reached via that path.
What are external BGP neighbors? - ANSWER External BGP neighbors are located in
different autonomous systems.
Explanation:
The two types of BGP neighbors are external and internal. Internal BGP neighbors are
located in the same autonomous system, whereas external BGP neighbors are located
in different autonomous systems.
What are internal BGP neighbors? - ANSWER Internal BGP neighbors are located in the
same autonomous system.
Explanation:
The two types of BGP neighbors are external and internal. Internal BGP neighbors are
located in the same autonomous system, whereas external BGP neighbors are located
in different autonomous systems.
Can an external BGP session between two routers pass through multiple intermediate
router hops? - ANSWER By default, eBGP neighbors must be directly connected.
Explanation:
By default, eBGP routers send routing protocol packets with an IP TTL of 1, which
means that they cannot pass through more than a single router hop; in other words,
default eBGP neighbors must be directly connected. The default behavior can be
overridden with the ebgp-multihop command.
Can an internal BGP session between two routers pass through multiple intermediate
router hops? - ANSWER Yes
Explanation:
, iBGP is the only routing protocol that allows sessions to pass through multiple
intermediate routers by default. The purpose of this is to allow routers at disparate
locations in an AS to share BGP routing information without the need to redistribute
routes into the IGP.
Why does BGP usually require a full mesh of iBGP neighbors? - ANSWER With iBGP,
routing information learned from one iBGP neighbor is not propagated to another iBGP
neighbor.
Explanation:
iBGP implements a version of split horizon as part of the loop protection mechanism
inside a BGP AS. Routing information learned from one iBGP neighbor is not propagated
to another iBGP neighbor. The implication of this is that iBGP routers must be fully
meshed, with certain exceptions (such as route reflectors).
What topology is required for BGP routers inside an autonomous system? - ANSWER
BGP routers inside an autonomous system must be fully meshed unless using advanced
features, such as route reflectors or confederations.
Explanation:
Because of the iBGP split-horizon behavior, a BGP router will not propagate routing
information from one iBGP neighbor to another iBGP neighbor. This implies that iBGP
routers inside an AS must have a full mesh of iBGP neighbor relationships. Note that this
does not mean that the routers must be physically meshed; the full mesh requirement
applies only to iBGP neighborship, not physical links.
What problems could occur if the iBGP full-mesh requirement is ignored inside an AS? -
ANSWER Inconsistent routing information or routing black holes could occur.
Explanation:
All iBGP neighbors inside an AS must be fully meshed, unless route reflectors or
confederations are used. If iBGP routers are not fully meshed, routers in the AS could
have different routing information, causing some destinations to be unreachable.
How does a router know if a BGP session is internal or external? - ANSWER By
comparing the AS number in the BGP neighbor command with its own AS number
Explanation:
If the neighbor command points to the local AS, an iBGP session is established. If the
neighbor command points to another AS, an eBGP session is established.
What is the source IP address of BGP protocol packets sent to a neighbor? - ANSWER
By default, the IP address of the interface on which the packets are sent is used as the
source address for BGP packets.
What methods can be used to route to or from a service provider? - ANSWER Static
routes, MPLS VPNs, circuit emulation, or BGP
Explanation:
Routing information can be exchanged with a service provider using a variety of
methods: static routing, MPLS VPNs, circuit emulation, or BGP. IGPs are typically not
used for routing Internet traffic with a service provider, although they are sometimes
used to exchange MPLS VPN routing information between a PE and CE router.
What protocol does BGP use to communicate at layer 4? - ANSWER TCP
Explanation:
Unlike other routing protocols, BGP uses TCP to exchange reliable updates at layer 4.
BGP uses TCP port 179 for neighbor sessions.
What is displayed by the show ip bgp command? - ANSWER The router's BGP table
Explanation:
The show ip bgp command displays the router's BGP table. This lists all BGP paths
available for each prefix, along with the characteristics of each path.
What is the difference between a router's BGP table and its routing table? - ANSWER
The routing table displays the best path to each prefix, regardless of protocol, whereas
the BGP table lists all BGP paths known to the router.
Explanation:
The BGP table lists all unique BGP paths for each destination prefix. The router then
selects the best BGP path, according to the BGP path selection process. The routing
table accepts candidate paths from the BGP table and uses them only if no other
protocols (such as static routes, IGPs, and so on) offer a path to the same prefix with a
better administrative distance.
What is the function of a BGP open message? - ANSWER It begins a BGP neighbor
session.
Explanation:
The four BGP message types are open, keepalive, update, and notification. The open
message is used to establish a BGP neighbor relationship and includes the BGP router
ID and the BGP session hold time.
,How does BGP detect that a neighbor is down, by default? - ANSWER By default, BGP
uses keepalive messages and a hold timer for neighbor-down detection.
Explanation:
Routing protocols need a way to determine whether a neighbor has gone down when the
physical link to the neighbor stays up. In BGP, routers send keepalive messages every
60 seconds by default. If a router sends keepalives for a period exceeding the hold time,
it declares the neighbor down and purges path information from that neighbor from its
BGP table.
What is contained in a BGP update message? - ANSWER Information about an AS path.
Explanation:
A BGP update method contains information about the attributes of an AS path. It can
contain information about multiple networks that can be reached via that path.
What are external BGP neighbors? - ANSWER External BGP neighbors are located in
different autonomous systems.
Explanation:
The two types of BGP neighbors are external and internal. Internal BGP neighbors are
located in the same autonomous system, whereas external BGP neighbors are located
in different autonomous systems.
What are internal BGP neighbors? - ANSWER Internal BGP neighbors are located in the
same autonomous system.
Explanation:
The two types of BGP neighbors are external and internal. Internal BGP neighbors are
located in the same autonomous system, whereas external BGP neighbors are located
in different autonomous systems.
Can an external BGP session between two routers pass through multiple intermediate
router hops? - ANSWER By default, eBGP neighbors must be directly connected.
Explanation:
By default, eBGP routers send routing protocol packets with an IP TTL of 1, which
means that they cannot pass through more than a single router hop; in other words,
default eBGP neighbors must be directly connected. The default behavior can be
overridden with the ebgp-multihop command.
Can an internal BGP session between two routers pass through multiple intermediate
router hops? - ANSWER Yes
Explanation:
, iBGP is the only routing protocol that allows sessions to pass through multiple
intermediate routers by default. The purpose of this is to allow routers at disparate
locations in an AS to share BGP routing information without the need to redistribute
routes into the IGP.
Why does BGP usually require a full mesh of iBGP neighbors? - ANSWER With iBGP,
routing information learned from one iBGP neighbor is not propagated to another iBGP
neighbor.
Explanation:
iBGP implements a version of split horizon as part of the loop protection mechanism
inside a BGP AS. Routing information learned from one iBGP neighbor is not propagated
to another iBGP neighbor. The implication of this is that iBGP routers must be fully
meshed, with certain exceptions (such as route reflectors).
What topology is required for BGP routers inside an autonomous system? - ANSWER
BGP routers inside an autonomous system must be fully meshed unless using advanced
features, such as route reflectors or confederations.
Explanation:
Because of the iBGP split-horizon behavior, a BGP router will not propagate routing
information from one iBGP neighbor to another iBGP neighbor. This implies that iBGP
routers inside an AS must have a full mesh of iBGP neighbor relationships. Note that this
does not mean that the routers must be physically meshed; the full mesh requirement
applies only to iBGP neighborship, not physical links.
What problems could occur if the iBGP full-mesh requirement is ignored inside an AS? -
ANSWER Inconsistent routing information or routing black holes could occur.
Explanation:
All iBGP neighbors inside an AS must be fully meshed, unless route reflectors or
confederations are used. If iBGP routers are not fully meshed, routers in the AS could
have different routing information, causing some destinations to be unreachable.
How does a router know if a BGP session is internal or external? - ANSWER By
comparing the AS number in the BGP neighbor command with its own AS number
Explanation:
If the neighbor command points to the local AS, an iBGP session is established. If the
neighbor command points to another AS, an eBGP session is established.
What is the source IP address of BGP protocol packets sent to a neighbor? - ANSWER
By default, the IP address of the interface on which the packets are sent is used as the
source address for BGP packets.
