Default Route
Default routing is a routing technique used in computer networks where a router is configured with a special route, known as a default route. This route specifies a next-hop gateway for all packets that have a destination network not explicitly listed in the router’s routing table.
This is just the “route of last resort.” When a router receives a packet, it forwards it to the next-hop provided by the default route if it cannot locate a more precise or better match for the destination address in its database. This is frequently an upstream router, such one that is connected to the internet, that has more in-depth knowledge of the network architecture.
Default Route Purpose and Function
A default route serves as a catch-all method for packets that don’t fit into any other route. In the absence of it, a router can return an ICMP “Destination Unreachable” message to the source host and delete packets with unknown origins. Routers may effectively forward packets to external networks, such the internet, by using default routes, which eliminate the need to maintain unique routes for each potential destination.
How Default Route works
A routing table containing particular entries for recognized networks is kept up to date by a router. The router looks at this table whenever a packet comes in. Upon receiving a packet, a router initially looks for a specific route to the target IP address in its routing database. The router determines the most precise path by applying the “longest prefix match” technique.
When no specific route fits the target address, the router searches for and employs the default route that has been defined, sending the packet to the next hop that the route specifies. This procedure is repeated by subsequent routers until the packet reaches its destination or, in the event that a router in the path is unable to route the packet using its default route, is dropped.
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Default Route Characteristics and Use Cases
Simplifies Configuration and Reduces Routing Table Size: Because default routing does not require defining routes for every conceivable destination, it makes network configuration easier, particularly in smaller networks or at the periphery of larger networks. By doing this, the routing table’s size is greatly decreased.
Internet Connectivity: All traffic intended for the internet is often routed to the router that is connected to an Internet service provider (ISP).
Stub Networks: To make routing setting easier, a default route is frequently used to send all traffic to the single exit point in “stub networks,” or networks with only one exit point.
Backup Routes (Floating Static Routes): Floating static routes serve as a backup to a primary route by utilizing the administrative distance idea; they only activate in the event that the primary path fails. Unlike dynamic routing protocols, a floating static route is set up with a larger administrative distance, guaranteeing that it will only be utilized in the event that the dynamic route is unavailable.
Traffic Originating on the Device: For a non-directly linked network, a default route or static routes are required for traffic that originates on the Firewall Threat Defense device (such as messages to a syslog or SNMP server) and is headed through a bridge group member interface.
Management vs. Data Traffic: For data traffic and management traffic, the Firewall Threat Defense employs different routing tables, enabling distinct default paths for each. In the event that a route cannot be determined, traffic coming from the device may fall back to either the management-only or data routing table.
Route to null0 Interface: Access rules can be supplemented by a static route to the null0 interface, which drops undesired or undesirable traffic. In addition to having a favorable performance profile, these routes can avoid routing loops.
IPv4 Default Route Configuration and Characteristics
Syntax: An IPv4 default route is commonly called a quad-zero route because it uses 0.0.0.0 for both the network address and subnet mask parameters. The command syntax is ip route 0.0.0.0 0.0.0.0 {ip-address | exit-interface}.
Show Command Output:In show ip route output, a default route is often represented as 0.0.0.0/0. It also shows a “Gateway of last resort” line. A static default route is indicated by an S* code, where S means static and * means it is a candidate for the default route.
Dynamic Learning via DHCP: Using DHCP, routers that connect to the Internet may dynamically learn a default gateway IP address, which they can then set as a default route in their routing table. The average administrative distance for this route is 254.
Stub Networks: Because stub networks only have one exit channel to other networks, default routing is very helpful in these situations. All traffic is sent to the central site via a single default route, rather than several static routes.
ip classless Command: Because Cisco routers are classful by default, each interface should have a default subnet mask. A router may discard a packet for an unknown subnet of an associated network, even if there is a default route. This is fixed using the ip classless command, which tells the router to use the default route and disregard class boundaries.
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IPv6 Default Route Configuration and Characteristics
Syntax: IPv6 default routes use ::/0 as the destination, which is the abbreviation for “all IPv6 addresses”. The command syntax is ipv6 route ::/0 {next-hop | exit-interface}.
Show Command Output: The show ipv6 route command directly lists the default route as ::/0 and does not typically include a “Gateway of Last Resort” line like IPv4.
Dynamic Learning via SLAAC/NDP RA Messages: Hosts can learn a default IPv6 route from Neighbor Discovery Protocol (NDP) Router Advertisement (RA) messages. The ipv6 address autoconfig default interface subcommand can configure a router to add a default route with the next-hop of the router responding with the RA message. The address used for default routers is typically the Link-Local address (FE80::/10).
Advertising/Redistributing Default Routes with Dynamic Protocols
OSPF:
The default-information originate command is used to advertise a default route into an OSPF routing domain.
- By default,
default-information originateonly advertises a default route if one already exists in the router’s routing table (e.g., a static default route). - The
alwayskeyword forces the router to advertise a default route even if it doesn’t have one, but this should be used with caution to avoid black-holing traffic.
Equal-Cost Multi-Path (ECMP) Routing
ECMP routing is supported by the Firewall Threat Defense, enabling up to eight equal-cost static or dynamic routes per interface. This hashes the source/destination IP addresses, incoming interface, protocol, and ports to allow load balancing of traffic over several gateways. By utilizing traffic zones, ECMP may also be set up over multiple interfaces, permitting up to eight equal-cost routes spanning up to eight interfaces in each zone. The gadget effortlessly switches the flow to a new path in the event that a route is lost. For VPN communication that is tunneled, ECMP is not supported.
Static vs dynamic vs default routing
| Feature | Static Routing | Dynamic Routing | Default Routing |
|---|---|---|---|
| Configuration | Manually by admin | Automatic (routers exchange info) | Manually set to a single next-hop |
| Adaptability | Does not adapt to changes | Automatically adapts to changes | Used only when no specific route is found |
| Complexity | Simple in small networks | More complex, requires protocols | Very simple |
| Overhead | No CPU or bandwidth overhead | Consumes CPU and bandwidth for updates | Very low overhead |
| Scalability | Poor (hard to manage in large networks) | Excellent for large networks | Suitable for small/edge networks |
| Failure Handling | Manual intervention needed | Automatically finds alternate paths | Still needs admin if default path fails |
| Use Case | Small, stable networks | Large, dynamic, complex networks | When there is only one exit point (e.g., Internet connection) |
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