CCIE - BGP Exam with complete solution
2024/2025
What does the ORIGIN attribute tell you? - ANSWER-Whether the router was
originated via redistribution or the network statement
what is the private ASN range? - ANSWER-64512-65535
what is the new ASN type byte number? - ANSWER-4 (4 OCTET)
a 4 byte number, as an example, is denoted by what? - ANSWER-1.1 (=256)
What port does BGP use for transport? - ANSWER-TCP 179
what tells you what address a BGP router is listening for BGP requests on? -
ANSWER-show control-plane host open-ports
where there is a multiple connections to a neighbour, what interface would be the
easiest to peer against? - ANSWER-the loopback
what is the TTL value of a eBGP peering? - ANSWER-1
what is the TTL value of an iBGP peering? - ANSWER-255
what determines becomes the TCP client? - ANSWER-The higher router-id
Do you have to set the update-source on both sides? - ANSWER-No, but you
should. If you dont you are forcing what is the TCP client
What is the driving motivation to use a loopback for iBGP peering? - ANSWER-To
be able to route around interface failures
,What is the iBGP loop prevention mechanism? - ANSWER-iBGP learned routes
are not advertised to another iBGP peer
What is the eBGP llop prevention mechanism? - ANSWER-It checks the AS_PATH
and if its AS_PATH is in the path it drops it
For iBGP peers, what are two mechnisms to allow iBGP learned routes to be
shared with other iBGP peers? - ANSWER-either route reflectors / confederations
or a full mesh of iBGP peers.
what 3 things does the BGP NLRI information contain? - ANSWER-Prefix,
Attributes, Next-hop
What is a consequence of BGP not knowing the outgoing interface to the next-
hop? - ANSWER-Its able to route to it, and therefore there is no best path
selection, and therefore the next hop cannot be installed in the routing table
Does iBGP processing updates change the next-hop value? - ANSWER-No
What does eBGP have by default for the next hop value? - ANSWER-the local
update-source for the neighbour (e.g. loopback)
what one tool can we use to make full mesh config quicker? - ANSWER-peer
groups
What is the main disadvantage of full iBGP meshing? - ANSWER-The control
plane scaling
what best path does a route reflector reflect to its peers? - ANSWER-Its own best
path
What does a route reflector do when it recieves routes in with its own cluster-id in
it? - ANSWER-It drops the routes as this is a loop prevention mechnism
What does the RR set the Originator-ID to? - ANSWER-The route reflector client
Can a reflector reflect the non-client routes from non-client to non-clients? -
ANSWER-No
What two attributes in confederations prevent loops? - ANSWER-
AS_CONFED_SEQUENCE and AS_CONFED_SET
,What is a hitless and easier to change, to RR or confed? - ANSWER-RR.
Confederations are a distruptive change
Do you have to configure reflector details on both sides or just one? - ANSWER-
Just one on the RR itself - client needs no special config
Wha are the three steps for confederation configuration? - ANSWER-Enable the
BGP process, Specify the main AS number, Speify other sub-AS that you peer
with.
Are confederation AS considered in the ASP_PATH for path selection? -
ANSWER-Yes, but only in that all the confederations in the path (in brackets) are
seen as a single AS
What is an external confed peer? - ANSWER-A poeer that is in the same global
AS, but in a different confed AS
Can you use RR in a Confed? - ANSWER-Yes and this is recommended in order
to ensure all routers in the confed are recieveing all the routes without using a
full mesh
At the end of the day, what is it that BGP tells you, or advertises to you? -
ANSWER-The IPv4 prefix, the prefix length, the attributes and the next-hop. Its up
to you HOW you get TO the next hop.
What is checked forst before a route is installed in the RIB? - ANSWER-Can we
get to the next-hop?
What is one of the three solutions to resolve next-hop reachability issues? -
ANSWER-use the next-hop self command. This done to the RR if this is in the AS
What is the second of the three solutions to resolve next-hop reachability issues?
- ANSWER-Manually change the next-hop using a route-map with neighbor x.x.x.x
route-map yy in|out
What is the third of the three solutions to resolve next-hop reachability issues? -
ANSWER-Third party next hop, when routers are on the same IP subnet (like
DMVPN). Acts like a route reflector, but for eBGP. Counfigure with "bgp next-hop
route-map XXX"
, WHy do we use next-hop tracking? - ANSWER-Next-hop tracking makes BGP
convergence a function of IGP convergence, otherwise a failure of the nexthop
would take 3 mins to converge while BGP keepalives withdraw the next-hop
What is ways to provide NLRI origination? - ANSWER-Using the network
statement, redistribution, aggregation, bgp inject-map
Can you redistribute OSPF external routes? - ANSWER-No
What weight and ORIGIN code does the network set the NLRI to? - ANSWER-
32768 and Origin of i
What ORIGIN code does BGP create if the redistribute command is used? -
ANSWER-INCOMPLETE (?)
If you use the network statement and I use the redistribute statement what path
will win? - ANSWER-I will due to the locally generated
In BGP redistrubtion what is automatically copied to BGP MED? - ANSWER-The
IGP metric
What routes are redistributed by default into the IGP? - ANSWER-External routes
- internal routes have to be specifically stated
What command does a conditional advertisement use? - ANSWER-neighbor
advertise-map
What problem is conditional advertisement was supposed to solve? - ANSWER-
control over inbound traffic flows
what does a conditional inject-map allow you to do? - ANSWER-Allows a shorter
match to be lengthened to enable traffic engineering for traffic over one link over
another
allow-as in allows what? - ANSWER-allows NLRI with out own AS number in the
AS PATH permitted in
What does bgp enforce-first-as command do? - ANSWER-check the route NLRI to
ensure the first AS in the path it the AS of my neighbour
whats preferred, a smaller med, or a larger med? - ANSWER-smaller
2024/2025
What does the ORIGIN attribute tell you? - ANSWER-Whether the router was
originated via redistribution or the network statement
what is the private ASN range? - ANSWER-64512-65535
what is the new ASN type byte number? - ANSWER-4 (4 OCTET)
a 4 byte number, as an example, is denoted by what? - ANSWER-1.1 (=256)
What port does BGP use for transport? - ANSWER-TCP 179
what tells you what address a BGP router is listening for BGP requests on? -
ANSWER-show control-plane host open-ports
where there is a multiple connections to a neighbour, what interface would be the
easiest to peer against? - ANSWER-the loopback
what is the TTL value of a eBGP peering? - ANSWER-1
what is the TTL value of an iBGP peering? - ANSWER-255
what determines becomes the TCP client? - ANSWER-The higher router-id
Do you have to set the update-source on both sides? - ANSWER-No, but you
should. If you dont you are forcing what is the TCP client
What is the driving motivation to use a loopback for iBGP peering? - ANSWER-To
be able to route around interface failures
,What is the iBGP loop prevention mechanism? - ANSWER-iBGP learned routes
are not advertised to another iBGP peer
What is the eBGP llop prevention mechanism? - ANSWER-It checks the AS_PATH
and if its AS_PATH is in the path it drops it
For iBGP peers, what are two mechnisms to allow iBGP learned routes to be
shared with other iBGP peers? - ANSWER-either route reflectors / confederations
or a full mesh of iBGP peers.
what 3 things does the BGP NLRI information contain? - ANSWER-Prefix,
Attributes, Next-hop
What is a consequence of BGP not knowing the outgoing interface to the next-
hop? - ANSWER-Its able to route to it, and therefore there is no best path
selection, and therefore the next hop cannot be installed in the routing table
Does iBGP processing updates change the next-hop value? - ANSWER-No
What does eBGP have by default for the next hop value? - ANSWER-the local
update-source for the neighbour (e.g. loopback)
what one tool can we use to make full mesh config quicker? - ANSWER-peer
groups
What is the main disadvantage of full iBGP meshing? - ANSWER-The control
plane scaling
what best path does a route reflector reflect to its peers? - ANSWER-Its own best
path
What does a route reflector do when it recieves routes in with its own cluster-id in
it? - ANSWER-It drops the routes as this is a loop prevention mechnism
What does the RR set the Originator-ID to? - ANSWER-The route reflector client
Can a reflector reflect the non-client routes from non-client to non-clients? -
ANSWER-No
What two attributes in confederations prevent loops? - ANSWER-
AS_CONFED_SEQUENCE and AS_CONFED_SET
,What is a hitless and easier to change, to RR or confed? - ANSWER-RR.
Confederations are a distruptive change
Do you have to configure reflector details on both sides or just one? - ANSWER-
Just one on the RR itself - client needs no special config
Wha are the three steps for confederation configuration? - ANSWER-Enable the
BGP process, Specify the main AS number, Speify other sub-AS that you peer
with.
Are confederation AS considered in the ASP_PATH for path selection? -
ANSWER-Yes, but only in that all the confederations in the path (in brackets) are
seen as a single AS
What is an external confed peer? - ANSWER-A poeer that is in the same global
AS, but in a different confed AS
Can you use RR in a Confed? - ANSWER-Yes and this is recommended in order
to ensure all routers in the confed are recieveing all the routes without using a
full mesh
At the end of the day, what is it that BGP tells you, or advertises to you? -
ANSWER-The IPv4 prefix, the prefix length, the attributes and the next-hop. Its up
to you HOW you get TO the next hop.
What is checked forst before a route is installed in the RIB? - ANSWER-Can we
get to the next-hop?
What is one of the three solutions to resolve next-hop reachability issues? -
ANSWER-use the next-hop self command. This done to the RR if this is in the AS
What is the second of the three solutions to resolve next-hop reachability issues?
- ANSWER-Manually change the next-hop using a route-map with neighbor x.x.x.x
route-map yy in|out
What is the third of the three solutions to resolve next-hop reachability issues? -
ANSWER-Third party next hop, when routers are on the same IP subnet (like
DMVPN). Acts like a route reflector, but for eBGP. Counfigure with "bgp next-hop
route-map XXX"
, WHy do we use next-hop tracking? - ANSWER-Next-hop tracking makes BGP
convergence a function of IGP convergence, otherwise a failure of the nexthop
would take 3 mins to converge while BGP keepalives withdraw the next-hop
What is ways to provide NLRI origination? - ANSWER-Using the network
statement, redistribution, aggregation, bgp inject-map
Can you redistribute OSPF external routes? - ANSWER-No
What weight and ORIGIN code does the network set the NLRI to? - ANSWER-
32768 and Origin of i
What ORIGIN code does BGP create if the redistribute command is used? -
ANSWER-INCOMPLETE (?)
If you use the network statement and I use the redistribute statement what path
will win? - ANSWER-I will due to the locally generated
In BGP redistrubtion what is automatically copied to BGP MED? - ANSWER-The
IGP metric
What routes are redistributed by default into the IGP? - ANSWER-External routes
- internal routes have to be specifically stated
What command does a conditional advertisement use? - ANSWER-neighbor
advertise-map
What problem is conditional advertisement was supposed to solve? - ANSWER-
control over inbound traffic flows
what does a conditional inject-map allow you to do? - ANSWER-Allows a shorter
match to be lengthened to enable traffic engineering for traffic over one link over
another
allow-as in allows what? - ANSWER-allows NLRI with out own AS number in the
AS PATH permitted in
What does bgp enforce-first-as command do? - ANSWER-check the route NLRI to
ensure the first AS in the path it the AS of my neighbour
whats preferred, a smaller med, or a larger med? - ANSWER-smaller
