Routes and routing in GCP - Step-by-Step Execution

Process Flow - Routes and routing

Request sent to VM or service

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Check routing table for destination

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Match route with destination IP

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Route found

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Forward packet

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Packet reaches destination

A network request is checked against routing rules to decide where to send it next, either forwarding it to the right place or dropping it if no route matches.

Execution Sample

GCP

sudo ip route add 10.0.0.0/24 via 192.168.1.1
ping 10.0.0.5

Add a route for 10.0.0.0/24 via next hop 192.168.1.1, then send a ping to 10.0.0.5 to test routing.

Process Table

Step	Action	Routing Table Lookup	Result	Packet Forwarding
1	Send ping to 10.0.0.5	Check routes for 10.0.0.5	Match found: 10.0.0.0/24 via 192.168.1.1	Forward packet to 192.168.1.1
2	Packet arrives at next hop 192.168.1.1	Next hop routes packet internally	Destination reachable	Packet delivered to 10.0.0.5
3	Send ping to 10.0.1.5	Check routes for 10.0.1.5	No matching route	Packet dropped or sent to default route
4	Send ping to 8.8.8.8 (default)	Check default route	Default route found	Forward packet to default gateway

💡 Routing stops when packet reaches destination or no route matches causing drop.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Destination IP	None	10.0.0.5	10.0.0.5	10.0.1.5	8.8.8.8
Route Matched	None	10.0.0.0/24	10.0.0.0/24	None	Default route
Next Hop	None	192.168.1.1	192.168.1.1	None	Default gateway
Packet Status	Not sent	Forwarded	Delivered	Dropped	Forwarded

Key Moments - 3 Insights

Why does the packet get dropped when no route matches?

When the routing table has no entry matching the destination IP (see step 3 in execution_table), the system cannot forward the packet, so it drops it or uses a default route if available.

What happens if a default route exists?

If no specific route matches, the default route (0.0.0.0/0) is used to forward the packet (see step 4), ensuring packets still leave the network instead of being dropped.

How does the routing table decide which route to use?

The routing table matches the destination IP against routes from most specific to least specific. The first matching route is used to forward the packet (see step 1).

Visual Quiz - 3 Questions

Look at the execution_table, what is the next hop for the destination 10.0.0.5 at step 1?

A 10.0.0.5

B No next hop

C 192.168.1.1

D Default gateway

💡 Hint

Check the 'Next Hop' column in variable_tracker after Step 1.

At which step does the packet get dropped due to no matching route?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Result' and 'Packet Forwarding' columns in execution_table for step 3.

If a default route was removed, what would happen at step 4?

A Packet would be dropped

B Packet would reach destination directly

C Packet would be forwarded to 192.168.1.1

D Packet would loop indefinitely

💡 Hint

Refer to the explanation in key_moments about default routes and step 4 in execution_table.

Concept Snapshot

Routes and routing decide where network packets go.
Routing table matches destination IP to routes.
If match found, packet sent to next hop.
If no match, packet dropped or sent via default route.
Default route handles all unmatched destinations.
Routing ensures packets reach correct destination or fail safely.

Full Transcript

Routes and routing in cloud infrastructure work by checking each network packet's destination IP against a routing table. The routing table contains rules that say where to send packets based on their destination. When a packet is sent, the system looks for the most specific route matching the destination IP. If found, it forwards the packet to the next hop IP address. If no route matches, the packet is either dropped or sent to a default route if one exists. This process repeats until the packet reaches its destination or cannot be forwarded further. Understanding routing helps ensure network traffic flows correctly and troubleshooting connectivity issues becomes easier.