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Zorluk: OrtaIPv4 Static Routing

A Cisco IOS router receives an IPv4 packet destined for 10.150.20.510.150.20.5. The routing table contains a static route `ip route 10.150.20.0 255.255.255.0 172.16.1.2` pointing to a next-hop address. Place the steps describing how the router recursively resolves the next-hop and forwards the packet in the correct sequential order from first to last.

  1. 1The router receives the packet destined for 10.150.20.510.150.20.5 and inspects its IPv4 destination header.
  2. 2The router performs an initial lookup in the IPv4 routing table and matches the static route entry for 10.150.20.0/2410.150.20.0/24, identifying 172.16.1.2172.16.1.2 as the next-hop IP address.
  3. 3The router performs a second (recursive) routing table lookup for the next-hop IP 172.16.1.2172.16.1.2 to determine the outbound interface.
  4. 4The router identifies connected subnet 172.16.1.0/24172.16.1.0/24 on interface GigabitEthernet0/1 and queries the ARP cache for the MAC address of 172.16.1.2172.16.1.2.
  5. 5The router encapsulates the IP packet into a Layer 2 frame with the destination MAC address of 172.16.1.2172.16.1.2 and transmits it out GigabitEthernet0/1.

Cevap

The correct sequence for recursive next-hop resolution is: receiving the packet and inspecting the destination header, performing the primary routing table lookup for the destination network, executing the recursive lookup for the next-hop IP address, resolving the Layer 2 MAC address on the egress interface, and encapsulating/transmitting the frame.
When a static route is configured with a next-hop IP address rather than a directly connected exit interface, Cisco IOS must perform a recursive lookup. The router first inspects the packet header and matches the destination subnet against the routing table. Next, it identifies the next-hop IP address associated with that static route entry. Because an IP address cannot directly dictate physical egress, a second (recursive) lookup is performed to find which local connected subnet and physical interface reach that next-hop IP. Once GigabitEthernet0/1 is identified, the router checks its ARP table to map the next-hop IP to a destination MAC address, encapsulating and transmitting the frame out the resolved egress interface.

Adım Adım Çözüm

1
Packet Ingress & Parsing
Destination IP 10.150.20.510.150.20.5 is extracted from the IPv4 header.
The router must inspect the destination IP address of an incoming packet to make a forwarding decision.
2
Primary Routing Lookup
Matches static route 10.150.20.0/2410.150.20.0/24 with next-hop IP 172.16.1.2172.16.1.2.
The routing table is searched using Longest Prefix Match (LPM) logic.
3
Recursive Lookup
Resolves next-hop IP 172.16.1.2172.16.1.2 to connected interface GigabitEthernet0/1.
When a static route specifies only a next-hop IP address, Cisco IOS requires a secondary lookup to resolve that IP address to a local exit interface.
4
Layer 2 Address Resolution
ARP table is queried/resolved for next-hop 172.16.1.2172.16.1.2 MAC address.
Ethernet interfaces require a destination MAC address to build the Layer 2 frame header.
5
Frame Encapsulation & Egress
Packet is sent out GigabitEthernet0/1 toward the next-hop router.
The router rewrites the Layer 2 source and destination MAC addresses and pushes the frame out the physical interface.

Anahtar Kavram

IPv4 Static Route Recursive Lookup & Packet Forwarding Logic
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Konular

IPv4 Static Routing

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