You are configuring a new BGP service to your service provider. You want to ensure that BGP is fully
established and has all the routes in the route table before allowing traffic to transit the router.
Which feature achieves this requirement?
A.
BGP route reflector
B.
IS-IS mesh group
C.
BGP local preference
D.
IS-IS overload bit
Why configure is-is for a BGP connection? Answer is C.
Took exam today: 100% on BGP.
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How can you use BGP Local Preference when your peering isn’t established? I don’t see how C can be correct.
Also read: https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/overload-edit-protocols-isis.html
It says:
“To understand the reason for setting the overload bit, consider that BGP converges slowly. It is not very good at detecting that a neighbor is down because it has slow-paced keepalive timers. Once the BGP neighbor is determined to be down, it can take up to 2 minutes for a BGP router to declare the neighbor down. IS-IS is much quicker. IS-IS only takes 10-30 seconds to detect absent peers. It is the slowness of BGP, more precisely the slowness of internal BGP (IBGP), that necessitates the use of the overload bit. IS-IS and BGP routing are mutually dependent on each other. If both do not converge at the same time, traffic is dropped without notification (black holed).”
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The question isn’t clear, answer C seems like the best option:
in order for the BGP router to serves as a transit router, it should advertise the best routes for the prefixes.
local preference can make the advertised routes from the BGP protocol to be less prefered, therefore it will advertise all of the routes in the iBGP with low local-preference value which will make sure that traffic from BGP learned neighbor will not be transimet across of it, and since we are talking about an ISP, then it must already have a BGP configured for another peer which can be used as a transit router by the time we will ensure our new BGP router has full knowledge and routes.
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However, if the BGP routes are redistributed / exported into the IGP for some reason, we cannot used the local-preference as the regular preference (AD in cisco terminology) will be prefered for the IGP.
in a case we have IS-IS as the IGP protocol in the core, which it is in ISP environments, we can use the IS-IS overload bit which will advertise all of the exported routes to its neighbors with that “overload-bit” set, and therefore the neighbors routers will know that routes in their topology, but will not route the traffic to the router which advertised those routes with the overload-bit set.
https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/overload-edit-protocols-isis.html
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