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

Process Flow - Routes and routing

Request sent to VM or service

↓

Check routing table for destination

↓

Match route with destination IP

↓

Route found

↓

Forward packet

↓

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.

Practice

1. What is the main purpose of a route in Google Cloud Platform networking?

easy

A. To create virtual machines

B. To store data in the cloud

C. To direct network traffic from one place to another

D. To monitor network usage

Solution

1. Step 1: Understand what routes do in networking

   Routes tell network traffic where to go, like a map for data packets.

2. Step 2: Identify the correct purpose in GCP context

   In GCP, routes guide traffic between subnets, VMs, and external networks.

3. Final Answer:

   To direct network traffic from one place to another -> Option C

4. Quick Check:

   Routes guide traffic = C [OK]

Hint: Routes always guide traffic flow in networks [OK]

Common Mistakes:

• Confusing routes with storage or compute services
• Thinking routes monitor traffic instead of directing it
• Mixing routes with firewall rules

2. Which of the following is the correct way to specify a next hop in a GCP route configuration?

easy

A. nextHopAddress: "192.168.1.1"

B. nextHop: "192.168.1.1"

C. nextHopGateway: "192.168.1.1"

D. nextHopIp: "192.168.1.1"

Solution

1. Step 1: Recall GCP route next hop syntax

   GCP routes use specific fields like nextHopIp to define the next hop IP address.

2. Step 2: Match the correct field name

   Among options, only nextHopIp is valid for specifying an IP address as next hop.

3. Final Answer:

   nextHopIp: "192.168.1.1" -> Option D

4. Quick Check:

   Correct field for IP next hop = nextHopIp [OK]

Hint: Use nextHopIp to specify IP address next hop [OK]

Common Mistakes:

• Using incorrect field names like nextHop or nextHopAddress
• Confusing next hop IP with gateway name
• Omitting quotes around IP address

3. Given the following route configuration snippet in GCP, what destination IP range will this route apply to?

{"destRange": "10.0.0.0/16", "nextHopIp": "192.168.1.1"}

medium

A. All IP addresses in 10.0.0.0 to 10.0.255.255

B. Only the IP 10.0.0.0

C. All IP addresses in 192.168.1.0/24

D. All IP addresses in 0.0.0.0/0

Solution

1. Step 1: Understand CIDR notation 10.0.0.0/16

   The /16 means the first 16 bits are fixed, covering IPs from 10.0.0.0 to 10.0.255.255.

2. Step 2: Identify the destination range

   The destRange field defines the IP range this route applies to, which is 10.0.0.0/16 here.

3. Final Answer:

   All IP addresses in 10.0.0.0 to 10.0.255.255 -> Option A

4. Quick Check:

   10.0.0.0/16 covers 10.0.0.0-10.0.255.255 [OK]

Hint: CIDR /16 covers 65,536 IPs starting at base address [OK]

Common Mistakes:

• Thinking /16 means only one IP
• Confusing nextHopIp with destination range
• Assuming 0.0.0.0/0 means local subnet

4. You created a route with destination range 0.0.0.0/0 but forgot to specify a next hop. What will happen when you try to deploy this route?

medium

A. The route will fail to create due to missing next hop

B. The route will be created and direct traffic to the internet automatically

C. The route will create but traffic will be dropped silently

D. The route will create and send traffic to the default gateway

Solution

1. Step 1: Recall route requirements in GCP

   Every route must have a destination and a next hop to know where to send traffic.

2. Step 2: Understand deployment validation

   Without a next hop, GCP rejects the route creation because it cannot route traffic properly.

3. Final Answer:

   The route will fail to create due to missing next hop -> Option A

4. Quick Check:

   Missing next hop causes creation failure [OK]

Hint: Routes need next hop or gateway to deploy [OK]

Common Mistakes:

• Assuming route auto-assigns next hop
• Thinking route silently drops traffic
• Confusing route creation with firewall rules

5. You want to route all traffic destined for 10.1.0.0/24 subnet through a VM instance with IP 192.168.5.10. Which route configuration is correct?

hard

A. {"destRange": "192.168.5.10/24", "nextHopIp": "10.1.0.0"}

B. {"destRange": "10.1.0.0/24", "nextHopIp": "192.168.5.10"}

C. {"destRange": "10.1.0.0/24", "nextHopGateway": "192.168.5.10"}

D. {"destRange": "0.0.0.0/0", "nextHopIp": "10.1.0.0"}

Solution

1. Step 1: Identify destination subnet to route

   The destination subnet is 10.1.0.0/24, so destRange must be this value.

2. Step 2: Specify next hop as VM IP

   The next hop should be the VM's IP 192.168.5.10, using nextHopIp field.

3. Step 3: Validate correct JSON structure

   {"destRange": "10.1.0.0/24", "nextHopIp": "192.168.5.10"} correctly sets destRange and nextHopIp with proper values and syntax.

4. Final Answer:

   {"destRange": "10.1.0.0/24", "nextHopIp": "192.168.5.10"} -> Option B

5. Quick Check:

   Destination subnet + VM IP next hop = {"destRange": "10.1.0.0/24", "nextHopIp": "192.168.5.10"} [OK]

Hint: Destination subnet in destRange, VM IP in nextHopIp [OK]

Common Mistakes:

• Swapping destination and next hop IPs
• Using nextHopGateway instead of nextHopIp for VM IP
• Setting wrong destination range