An enterprise edge router receives an IP packet destined for host address 172.16.42.100. T… — CompTIA Network+

An enterprise edge router receives an IP packet destined for host address $172.16.42.100$ . The router's active routing table contains four candidate routes:

Protocol Source	Destination Subnet	Administrative Distance	Metric	Next Hop
Static	$172.16.0.0/16$	1	0	Null0
eBGP	$172.16.40.0/21$	20	100	$192.168.1.1$
OSPF	$172.16.42.0/24$	110	20	$10.0.0.2$
RIPv2	$172.16.42.96/28$	120	2	$10.0.0.6$

Which route will the router select to forward the packet, and why?

The RIPv2 route ( $172.16.42.96/28$ ), because Longest Prefix Match takes precedence over Administrative Distance.Cevap
B
The Static route ( $172.16.0.0/16$ ), because static routes have the lowest Administrative Distance.
C
The eBGP route ( $172.16.40.0/21$ ), because exterior gateway protocols are prioritized over interior gateway protocols.
D
The OSPF route ( $172.16.42.0/24$ ), because it offers the optimal balance between prefix specificity and Administrative Distance.

Cevap

The router will select the RIPv2 route (

172.16.42.96/28

) because Longest Prefix Match (LPM) is the absolute first criterion used in IP routing decisions, taking precedence over Administrative Distance and metric.

When a router receives a packet, it evaluates all matching routing table entries using the Longest Prefix Match (LPM) rule first. Because

172.16.42.96/28

has the longest subnet mask (/28 = 28 matching bits) among all matching candidate routes, it is chosen immediately. Administrative Distance (AD) and metrics are only evaluated to select a winner when two or more routes advertise identical destination prefix lengths.

Adım Adım Çözüm

Determine which routing table entries match the target IP address (

172.16.42.100

All four entries match:

172.16.0.0/16

(range

172.16.0.0 - 172.16.255.255

172.16.40.0/21

(range

172.16.40.0 - 172.16.47.255

172.16.42.0/24

(range

172.16.42.0 - 172.16.42.255

), and

172.16.42.96/28

(range

172.16.42.96 - 172.16.42.111

The router must filter candidate routes to only those subnets that encompass the destination address.

Compare the prefix lengths (subnet masks) of all matching candidate routes.

Prefix lengths are /16, /21, /24, and /28. The longest prefix match is /28.

In IP routing, the most specific route (highest prefix length) is always selected before Administrative Distance or metric are evaluated.

Identify the protocol associated with the longest prefix match (/28).

The /28 route corresponds to the RIPv2 learned entry via next-hop

10.0.0.6

Administrative Distance (120 vs 1 vs 20 vs 110) is ignored because the prefix lengths are not equal.

Anahtar Kavram

Longest Prefix Match (LPM) Rule

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

Sık Yapılan Hatalar

Konular

Bu konudan daha fazla soru