nerdexam
Cisco

352-001 · Question #233

Enterprises A and B agree to merge, but keep IGP and BGP independent of each other. They are served by a common ISP for their Internet connectivity. During the merge, A and B will provision a…

The correct answer is C. configure EBGP between the two networks and block each other's routes from the ISP. Configuring EBGP over the point-to-point link and filtering each enterprise's prefixes from the shared ISP is the simplest design because it leverages existing BGP infrastructure without merging autonomous systems or IGP domains, and the ISP filtering ensures the direct link is…

Designing Network Infrastructure

Question

Enterprises A and B agree to merge, but keep IGP and BGP independent of each other. They are served by a common ISP for their Internet connectivity. During the merge, A and B will provision a point-to-point link between the two networks. What is the simplest design option that will allow data to travel between A and B without passing through the ISP?

Options

  • Aconfigure OSPF and make OSPF routes more attractive than the same routes learned via EBGP
  • Bconfigure OSPF and make OSPF routes less attractive than the same routes learned via EBGP
  • Cconfigure EBGP between the two networks and block each other's routes from the ISP
  • Dconfigure iBGP between the two networks and block each others route's from the ISP
  • Econfigure OSPF between the two networks and block each others route's from the ISP

How the community answered

(47 responses)
  • A
    17% (8)
  • B
    4% (2)
  • C
    43% (20)
  • D
    6% (3)
  • E
    30% (14)

Why each option

Configuring EBGP over the point-to-point link and filtering each enterprise's prefixes from the shared ISP is the simplest design because it leverages existing BGP infrastructure without merging autonomous systems or IGP domains, and the ISP filtering ensures the direct link is preferred.

Aconfigure OSPF and make OSPF routes more attractive than the same routes learned via EBGP

Running OSPF across the link violates the requirement to keep IGPs independent between the two enterprises and adds unnecessary protocol complexity when BGP is already present.

Bconfigure OSPF and make OSPF routes less attractive than the same routes learned via EBGP

Making OSPF routes less attractive than EBGP routes would cause traffic to prefer the ISP path over the direct link, which is the opposite of the design goal.

Cconfigure EBGP between the two networks and block each other's routes from the ISPCorrect

Since both enterprises already operate independent BGP autonomous systems, standing up an EBGP session over the new point-to-point link is a natural extension requiring minimal new configuration. Blocking each other's prefixes from being advertised to or received from the ISP ensures the BGP best-path selection favors the direct EBGP route, preventing the ISP from acting as an unintended transit and keeping inter-enterprise traffic local.

Dconfigure iBGP between the two networks and block each others route's from the ISP

iBGP requires both routers to belong to the same autonomous system, which directly contradicts the requirement to keep BGP independent between the two enterprises.

Econfigure OSPF between the two networks and block each others route's from the ISP

Running OSPF between the two networks violates the constraint of keeping IGPs independent and is a more complex solution than simply extending the BGP peering that both enterprises already operate.

Concept tested: EBGP peering between separate ASes with route filtering to prevent ISP transit

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/irg-neighbor.html

Topics

#EBGP#inter-domain routing#route filtering#network merger

Community Discussion

No community discussion yet for this question.

Sign up to join the discussion
Full 352-001 Practice