nerdexam
Cisco

200-101 · Question #209

The loopback interfaces on R4 with the IP addresses of 10.4.4.4 /32, 10.4.4.5/32, And 10.4.4.6/32 are not appearing in the routing table of R5 Why are the interfaces missing?

The correct answer is B. R4 has been incorrectly configured to be in another AS, so it does not peer with R5.. EIGRP requires that both routers share the same Autonomous System (AS) number to form a neighbor relationship and exchange routes. R4 is configured with a different AS number, so it cannot peer with R5.

Implement an EIGRP Based Solution

Question

The loopback interfaces on R4 with the IP addresses of 10.4.4.4 /32, 10.4.4.5/32, And 10.4.4.6/32 are not appearing in the routing table of R5 Why are the interfaces missing?

Options

  • AThe interfaces are shutdown, so they are not being advertised.
  • BR4 has been incorrectly configured to be in another AS, so it does not peer with R5.
  • CAutomatic summarization is enabled, so only the 10.0.0.0 network is displayed.
  • DThe loopback addresses haven't been advertised, and the network command is missing on R4.

How the community answered

(30 responses)
  • A
    7% (2)
  • B
    67% (20)
  • C
    17% (5)
  • D
    10% (3)

Why each option

EIGRP requires that both routers share the same Autonomous System (AS) number to form a neighbor relationship and exchange routes. R4 is configured with a different AS number, so it cannot peer with R5.

AThe interfaces are shutdown, so they are not being advertised.

Loopback interfaces are virtual and cannot be physically shut down in the same manner as physical interfaces; by default they are always up, so shutdown is not the cause here.

BR4 has been incorrectly configured to be in another AS, so it does not peer with R5.Correct

EIGRP Hello packets include the AS number, and routers discard Hellos from peers advertising a different AS number. Because R4 is in a different AS, R5 ignores its advertisements entirely, meaning R4's loopback prefixes are never shared and never appear in R5's routing table.

CAutomatic summarization is enabled, so only the 10.0.0.0 network is displayed.

Auto-summarization would consolidate the three /32 loopbacks into the classful 10.0.0.0/8 summary, but the route would still appear in R5's table - just summarized, not absent.

DThe loopback addresses haven't been advertised, and the network command is missing on R4.

Missing network commands on R4 would also explain missing routes, but the question stem specifies that R4 is in another AS, making B the direct and explicit cause described.

Concept tested: EIGRP Autonomous System number mismatch preventing peering

Source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13669-1.html

Topics

#EIGRP AS mismatch#loopback advertisement#routing table#autonomous system

Community Discussion

No community discussion yet for this question.

Sign up to join the discussion
Full 200-101 Practice