Refer to the exhibit. An engineer is trying to implement BGP configuration on a router. Which configuration error prevents the ASBR from establishing a BGP neighborship to a directly connected BGP speaker?
A.
The routing policy is absent for this Cisco IOS XR eBGP instance.
B.
The TCP session parameters are not specified.
C.
The VPNv4 address family interferes with the BGP IPv4 address family negotiations.
D.
The IPv4 address family configuration under neighbor configuration-mode must be removed.
The question says "Establishing BGP Neighborship" so if the RPL is not configured the 2 eBGP Peers still have to form an adjacency but routes will not be propagated between them due to the default drop policy !! IDK this is weird question
Yes, I agree. If question was: VPNv4 address family interferes with the BGP IPv4 address family negotiations ***in the neighbor subcommand***, C could be the answer. Really it is weird question.
I have literally no idea why people say "C", the question is clear, without activating address-family vpnv4, they are supposed to build up neighborship over address-family ipv4. The routing policy is missing here, so the right answer is "A" for sure.
Correct Answer is B: Neighbor x.x.x.x Activate: Initiates a TCP connection with neighbor allowing the exchange of BGP messages. The TCP Session parameter is missing under address-family.
It's B
"BGP neighborship to a directly connected BGP speaker"
TTL-security parameter is a TCP parameter
neighbor x.x.x.x ttl-security hops 2 , This don't cover the question because the TTL arrived from the neighbor could be equal o lower than 2 .
But using "neighbor ebgp-multihop"
Accepts or initiates BGP connections to external peers residing on networks that are not directly connected.
https://www.cisco.com/c/en/us/td/docs/ios/12_2sx/feature/guide/fsxebtsh.html#wp1059212
The ttl-security not really a TCP parameter, the TTL is actually a field in the IP packet which by default is set to 1.
For a "DIRECTLY CONNECTED" eBGP neighbor to come up you do not need "ebgp-multihop" OR the "ttl-security",only if the peer is further hops away.
"The neighbor ttl-security command cannot be configured for a peer that is already configured with the neighbor ebgp-multihop command. The configuration of these commands is mutually exclusive, and only one of these commands is needed to enable a multihop eBGP peering session. An error message will be displayed in the console if you attempt to configure both commands for the same peering session."
Missing routing policy IS a problem because you will not exchange any routes but it does NOT stop the establishing of the BGP neighborship. I changed my mind, it cannot be A. I am not sure about this one.
A because
Route policies are mandatory for E-BGP peers, at least a "pass-all" like RPL is required in order to import and export routes.
https://community.cisco.com/t5/service-providers-knowledge-base/asr9000-xr-understanding-and-using-rpl-route-policy-language/ta-p/3117050
DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
This is not a good or clearly formulated question. By process of elimination options C and D make no sense, option B also has no relevance, we know that in order for an eBGP process to exchange routes an RPL is needed in ios XR, even if the neighbor is already established, so A makes the most sense.
More or less. If question was: VPNv4 address family interferes with the BGP IPv4 address family negotiations ***in the neighbor subcommand***, C could be the answer. Really it is weird question. 'A' would just indicate the prefixes are not received, but there would be establishing a BGP neighborship
There ins't TCP session parameters in a BGP configuration. B is not answer.
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