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Zorluk: Çok zorRouter Forwarding Decision Logic

A Cisco router receives an IPv4 unicast packet destined for 10.150.45.6710.150.45.67. The router's Routing Information Base (RIB) and Forwarding Information Base (FIB) contain multiple overlapping routes, static entries, and dynamic routing protocol paths. In what exact sequence does the router evaluate routing parameters and process the packet from ingress lookup to physical egress forwarding? Arrange the steps in the correct chronological order from first to last.

  1. 1Evaluate all candidate routes in the routing table to identify the entry with the longest subnet mask (Longest Prefix Match) matching the destination IP address.
  2. 2Compare Administrative Distance (AD) values among candidate routes sharing the exact same longest prefix length to select the most believable route source.
  3. 3Evaluate protocol-specific metrics (cost/distance) to break ties between candidate routes learned from the same routing protocol for the identical prefix length.
  4. 4Query the ARP cache or NDP table to resolve the Layer 2 hardware address (MAC) associated with the chosen next-hop IP address.
  5. 5Encapsulate the packet in a new Layer 2 frame header with updated MAC addresses, decrement the IPv4 Time to Live (TTL) by 11, recalculate the IP header checksum, and transmit out the egress interface.

Cevap

The correct evaluation sequence for router forwarding decision logic is: 1) Longest Prefix Match evaluation, 2) Administrative Distance tie-breaking for identical prefixes across different protocols, 3) Metric tie-breaking for identical prefixes within the same protocol, 4) ARP lookup for next-hop MAC address resolution, and 5) Layer 2 encapsulation rewrite, TTL decrement, checksum update, and frame transmission.
The router forwarding process follows a strict hierarchy: Longest Prefix Match (LPM) is evaluated first across all routes in the routing table. If multiple routes from different protocol sources share the exact same prefix length, Administrative Distance (AD) breaks the tie. If multiple routes from the same protocol share the exact same prefix length, protocol metric breaks the tie. Once the best route and next-hop are chosen, the router resolves the next-hop MAC via ARP/NDP, updates IP header fields (TTL decrement by 1 and checksum update), rewrites the Layer 2 header, and transmits the frame out the egress interface.

Adım Adım Çözüm

1
Perform destination IP lookup using Longest Prefix Match (LPM).
The router identifies all matching subnet routes and selects the route entry with the highest number of matching network bits (most specific mask length).
LPM supersedes lower Administrative Distance values or routing metrics. A /28/28 route will always be chosen over a /24/24 route regardless of whether the /24/24 is static or connected.
2
Compare Administrative Distance (AD) if multiple sources present identical prefix masks.
The route from the source with the lowest AD (e.g., Static 11 vs. OSPF 110110) is preferred.
AD measures route source believability and only arbitrates between identical prefix masks from different routing processes.
3
Compare routing protocol metrics for ties within the same protocol.
The path with the lowest metric (cost, hop count, composite metric) is selected for RIB insertion.
Metrics only compare routes of the exact same prefix length learned via the same routing protocol algorithm.
4
Perform Layer 2 address resolution for the resolved next-hop address.
The router locates the next-hop IPv4 address in the ARP cache to retrieve its target MAC address.
Layer 3 forwarding decision requires binding the IP next-hop to a physical Layer 2 destination MAC address.
5
Execute packet header modifications and physical transmission.
The IPv4 TTL is decremented by 11, header checksum recalculated, source MAC updated to egress interface MAC, destination MAC updated to next-hop MAC, and the frame is serialized onto the wire.
Routers act as Layer 3 boundaries and must rebuild the Layer 2 frame header for each routed hop.

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