Exam 300-420 topic 1 question 30 discussion

Answer A makes more sense in this case. The question clearly states that this is a remote branch site. So I would assume the ISP A in this case is some kind of WAN link and we would want the ISP A to propagate the internal routes within our Branch A location so that our HQ can send traffic out (destined for the restricted Branch A prefixes) and be routed through the ISP A network to reach the Branch location. Answer B does make sense if it the picture did not show ISP A, but rather a direct connection to Global Internet with an ISP A router sitting in the Internet cloud.

Guys i am sorry, the answer is actually A. Thank god i had to double check my work. Here i will put it easy for you to understand and you can backup what i am saying by looking at Cisco documentation No-Adverstise: It restricts a BGP Router to not advertise prefixes to iBGP AND eBGP No-Export: It restricts a BGP Router to not advertise prefixes to eBGP ONLY, iBGP prefixes WILL BE ADVERTISED. This is the less restrictive option. Well since this is an Internet Service Provider we can very easly assume this is a Public Network where eBGP lives, so we can use the no-export here instead of the no-advertise, even though both would work they are testing your knowledge.

The No-Export and No-Advertise communities in BGP are both used to control the advertisement of routes to external BGP (eBGP) peers. However, there are some key differences between the two communities. The No-Export community prevents a router from advertising a route to eBGP peers outside of a specific autonomous system (AS). This can be useful for preventing routes from being leaked to other ASes. The No-Advertise community prevents a router from advertising a route to any eBGP peers. This can be useful for preventing routes from being advertised to the public internet.

B makes more sense to me to not send it to Internet. There`s a risk of ISP A backbone being huge, it`s going to propagate to all other routers inside the same AS if we use No-Export community, others customers from ISP A connected to Internet are going to use the ISP A backbone to reach out the Customer networks. Using No-Adversite, makes more sense and avoid that because it will be installed on the ISP router and won`t be advertised to anyone.

Both A and B will get the job done, as they both will keep Branch A prefixes from being advertised to the internet, however there is a fundamental difference in what happens after the prefix is advertised to ISP A. If the No-Export community is used, then the prefixes will only be advertised to other BGP speakers with the same AS as ISP A. Which means the prefix will not be advertised to the internet but will be advertised within ISP A’s network which would be ideal. If the No-Advertise community is used, then only the BGP router that is peered with Branch A will have the routes. And while it won’t advertise these prefixes to the internet, it also won’t advertise them to any other routers with the same AS as it (ISP A). This would keep all other routers inside of ISP A from learning the routes, which is not ideal from a failover perspective.

A - this answer is good. When a No-Advertise community is attached to a route, the BGP speaker won’t advertise the route to any internal or external BGP peers. When a No-Export community is attached to a route, the router won’t advertise the route to external peers--only to internal peers. reference: https://www.catchpoint.com/network-admin-guide/bgp-communities

no advertise community makes more sense