200-101 · Question #122
Refer to the graphic and routing table for router R1. Based on the output of the R1# show ip route command and the information presented in the graphic, which of the following is a potential routing p
The correct answer is A. The use of summarization for discontinuous networks. Auto-summarization causes routing failures when the same classful network address space is split across different parts of the topology, separated by a foreign classful network. This condition, called a discontinuous network, results in ambiguous or dropped routes.
Question
Exhibit
Options
- AThe use of summarization for discontinuous networks
- BThe use of CIDR with a routing protocol that does not support it
- CThe use of VLSM with a routing protocol that does not support it
- DThe use of the no auto-summary command with a protocol that does not support summarization
- EThe use of the ip route 0.0.0.0 0.0.0.0 command with a routing protocol that does not support it
How the community answered
(26 responses)- A65% (17)
- B19% (5)
- C8% (2)
- D4% (1)
- E4% (1)
Why each option
Auto-summarization causes routing failures when the same classful network address space is split across different parts of the topology, separated by a foreign classful network. This condition, called a discontinuous network, results in ambiguous or dropped routes.
When a classful routing protocol or a classless protocol with auto-summary enabled summarizes routes at classful boundaries, a discontinuous network causes both sides to advertise the same summary address back toward the middle. Routers in the middle receive conflicting summaries for the same major network from two different directions, creating routing loops or black holes where traffic is silently dropped. Disabling auto-summary with 'no auto-summary' is the standard corrective action.
CIDR is supported natively by classless protocols such as OSPF, EIGRP, and RIPv2, so using CIDR with a compatible protocol is not a routing problem.
VLSM is supported by classless routing protocols including OSPF, EIGRP, and RIPv2, and the routing table output showing varied prefix lengths does not indicate a VLSM incompatibility.
The 'no auto-summary' command is a valid configuration option in RIPv2 and EIGRP used to prevent the exact problem described, so its use is a solution rather than a problem.
The 'ip route 0.0.0.0 0.0.0.0' command defines a static default route, which is redistributable into any routing protocol and does not conflict with any protocol's operation.
Concept tested: Auto-summarization causing discontinuous network routing problems
Source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13920-19.html
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