Day 11 Routing Fundamentals & Static Routing

What is Routing? Routing is the process that routers use to determine the path that IP packets should take over a network to reach their destination. Routers store routes to all known destinations in a routing table, which functions similarly to a switch's MAC address table but operates at Layer 3 with IP addresses instead of MAC addresses.

The Routing Table A routing table is essentially a set of instructions that tells the router how to forward packets. Each route provides guidance: to reach destination network X, send the packet to next-hop Y (the next router in the path), or if the destination is directly connected, send the packet directly to that destination.

Connected Routes (Code: C) Connected routes are automatically added to the routing table when you configure an IP address on an interface and enable it with the NO SHUTDOWN command. These routes point to the network that the interface is directly connected to, using the actual netmask configured on the interface.

Local Routes (Code: L) Local routes are also automatically added when you configure and enable an interface. These routes point to the exact IP address configured on the interface and always use a /32 prefix length, which specifies only that single IP address. Local routes tell the router that packets destined for this address are meant for the router itself.

Static Routes (Code: S) Static routes are manually configured by a network administrator to tell the router how to reach remote networks that are not directly connected. Unlike connected and local routes, static routes are not added automatically and must be explicitly configured.

Default Routes A default route is a route to 0.0.0.0/0, which matches all possible IP addresses. It serves as the "route of last resort" - if no more specific route matches a packet's destination, the default route is used. This is commonly used to direct traffic toward the Internet.

Key Commands:

show ip route - Display routing table

show ip interface brief - Display interface status and IPs

ip route [net] [mask] [next-hop] - Configure static route with next-hop

ip route [net] [mask] [exit-int] - Configure static route with exit interface

ip route [net] [mask] [exit-int] [next-hop] - Configure with both

ip route 0.0.0.0 0.0.0.0 [next-hop] - Configure default route

1. Confusing Connected vs Local Routes

   • Connected = route to the NETWORK (uses configured mask like /24)

   • Local = route to the EXACT IP ADDRESS (always /32)

2. Forgetting Two-Way Reachability

   • Just because Host A can reach Host B doesn't mean B can reply

   • ALL routers in the path need routes to BOTH networks

3. Router vs Switch Behavior

   • Routers DROP packets with no matching route

   • Switches FLOOD frames with unknown destinations

   • This is a common exam question!

4. Static Route Syntax

   • You CANNOT use slash notation (/24) in static route commands

   • Must write full netmask: 255.255.255.0, NOT /24

5. Default Route Netmask

   • Default route netmask is 0.0.0.0 (all zeros)

   • NOT 255.255.255.255 (all ones)

6. Interface Must Be Enabled

   • Connected and Local routes only appear if interface is UP (no shutdown)

   • Shutdown interfaces = no routes in table

7. "Directly Connected" Static Routes

   • When you specify only exit-interface in a static route, it shows as "directly connected"

   • Don't confuse this with actual Connected (C) routes

8. Most Specific Route Selection

   • /32 is MORE specific than /24

   • /24 is MORE specific than /16

   • /0 is the LEAST specific (default route)

"CL-32" - Connected and Local routes are added automatically; Local routes use /32

"Routers are Rude, Switches are Social"

• Routers DROP unknown packets (rude)

• Switches FLOOD unknown frames (social)

"Longer is Stronger"

• Longer prefix length = more specific = wins route selection

• /32 beats /24 beats /16 beats /0

"Two Routes, One Interface"

• Every enabled interface = 2 routes (Connected + Local)

• 3 interfaces = 6 routes automatically

"Zero Zero for Default"

• Default route: 0.0.0.0 0.0.0.0 (network AND mask are zeros)

"S-C-L" for Static, Connected, Local

• Remember the codes alphabetically

"Layer 3 stays, Layer 2 changes"

• Source/Destination IP = unchanged through network

• Source/Destination MAC = changes at each hop

Example 1: Reading a Routing Table

R1# show ip route

C 192.168.1.0/24 is directly connected, GigabitEthernet0/0

L 192.168.1.1/32 is directly connected, GigabitEthernet0/0

S 192.168.4.0/24 [1/0] via 192.168.13.3

S* 0.0.0.0/0 [1/0] via 203.0.113.2

Analysis:

• Interface G0/0 has IP 192.168.1.1/24 (we know from the L route)

• To reach 192.168.4.0/24, send to next-hop 192.168.13.3

• Default route sends all other traffic to 203.0.113.2

• The asterisk (*) indicates candidate default

Example 2: Route Selection

If R1 receives a packet destined for 192.168.1.1:

• Matches 192.168.1.0/24 (Connected) ✓

• Matches 192.168.1.1/32 (Local) ✓

• Winner: 192.168.1.1/32 because /32 > /24 (more specific)

• Result: R1 receives packet for itself

Example 3: Static Route Configuration

Scenario: Configure R1 to reach network 10.0.0.0/8 via next-hop 192.168.1.2

R1(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.2

Example 4: Packet Journey Through Network

PC1 (192.168.1.10) → PC4 (192.168.4.10)

Hop 1: PC1 → R1

- L3 Dest IP: 192.168.4.10 (PC4)

- L2 Dest MAC: R1's G0/2 MAC (default gateway)

Hop 2: R1 → R3

- L3 Dest IP: 192.168.4.10 (unchanged)

- L2 Dest MAC: R3's G0/0 MAC (next-hop)

Hop 3: R3 → R4

- L3 Dest IP: 192.168.4.10 (unchanged)

- L2 Dest MAC: R4's G0/1 MAC (next-hop)

Hop 4: R4 → PC4

- L3 Dest IP: 192.168.4.10 (unchanged)

- L2 Dest MAC: PC4's MAC (final destination)

Routing is the process that routers use to determine the path that IP packets should take over a network to reach their destination.

Connected route (C) and Local route (L)

• Connected route: Route to the network the interface is connected to (uses configured mask, e.g., /24)

• Local route: Route to the exact IP address configured on the interface (always /32)

/32 - because it specifies exactly one IP address (all 32 bits are fixed)

• Connected: C

• Local: L

• Static: S

The router DROPS the packet. (Unlike switches, which flood unknown frames)

The router selects the MOST SPECIFIC matching route, which is the route with the LONGEST PREFIX LENGTH.

/32 is more specific because it matches only 1 IP address, while /24 matches 256 addresses.

A default route is a route that matches ALL destinations. Its network/mask is 0.0.0.0/0 (also written as 0.0.0.0 0.0.0.0)

show ip route

ip route [destination-network] [subnet-mask] [next-hop-ip]

Example: ip route 192.168.4.0 255.255.255.0 192.168.13.3

ip route 0.0.0.0 0.0.0.0 10.1.1.1

It refers to the default route - the route used when no more specific route matches the destination.

1. Next-hop only: ip route 10.0.0.0 255.0.0.0 192.168.1.2

2. Exit-interface only: ip route 10.0.0.0 255.0.0.0 g0/0

3. Both: ip route 10.0.0.0 255.0.0.0 g0/0 192.168.1.2

The IP address of the router interface that the host should send packets to when the destination is outside its local network.

• Connected: 10.0.0.0/16

• Local: 10.0.0.1/32

It indicates the route is a "candidate default" - a potential default route for the router.

FALSE - The Layer 3 (IP) addresses remain the same. Only the Layer 2 (MAC) addresses change at each hop.

Two-way reachability means traffic can flow in both directions between source and destination. It's important because all routers in the path need routes to BOTH the source and destination networks for communication to work (including replies).

192.168.1.0/24 - It's the most specific match (longest prefix length: /24 > /16 > /0)

The interface remains administratively down, and the Connected and Local routes will NOT appear in the routing table.

• /32 = 1 address

• /24 = 256 addresses

• /16 = 65,536 addresses

• /0 = ~4.3 billion addresses (all IPv4)

• Routers: DROP packets with no matching route

• Switches: FLOOD frames with unknown destination MAC (unknown unicast)

NO - You must write the full dotted-decimal mask (e.g., 255.255.255.0)

The static route was configured with only an exit-interface (no next-hop IP). The route uses Proxy ARP to function. Note: This is different from an actual Connected (C) route.

Administrative Distance / Metric (These concepts are covered in more detail with dynamic routing protocols)

show ip interface brief

To tell the router: "Packets destined for this IP address are for ME - receive them, don't forward them."

The next router in the path toward the destination. It's the IP address where packets should be forwarded to reach a remote network.

LEAST; MOST

1. Dynamic Routing: Routers use protocols (like OSPF) to automatically share routing information

2. Static Routing: Network admin manually configures routes

(Note: Connected and Local routes are neither - they're added automatically when interfaces are configured)

The MAC address of PC1's default gateway (the router interface connected to PC1's network) - NOT PC4's MAC address.

The default route (0.0.0.0/0 via 203.0.113.2) - it's the only route that matches 8.8.8.8

FALSE - Routers only need routes to the source and destination networks, not intermediate networks. Each router just needs to know the next-hop to reach the destination.

Wide Area Network - a network that extends over a large geographical area (e.g., connecting offices in different cities or countries)