Router Routing and Redundancy

Overview

This lecture explains how routers make decisions using routing tables, the criteria routers use to select routes, and methods for redundancy and handling VLANs using sub-interfaces.

Router Functions and Routing Tables

• Routers evaluate incoming traffic, determine the destination, and send packets out the correct interface.
• Routers use routing tables to decide the best direction for packets.
• All network devices (routers, workstations, servers) maintain routing tables.
• Troubleshooting routers generally starts with examining the routing table.
• Routing tables contain both directly connected routes and routes learned dynamically (e.g., via RIP).

Interpreting Routing Table Entries

• Each routing table entry includes route code, destination subnet/prefix, administrative distance, metric, next hop, timestamp, and outgoing interface.
• 'C' means directly connected; 'R' means learned via RIP version 2.
• The destination subnet and prefix length define the specific network for that route.
• Administrative distance indicates the trustworthiness of the route (lower is better).
• Metric is used by the routing protocol to choose between multiple routes.

Route Selection and Tiebreakers

• Routers compare the destination IP address to routing table entries, using subnet ID and prefix length.
• Most specific prefix length (largest number, e.g., /32) is chosen if multiple entries match.
• If multiple matching entries have the same prefix, administrative distance is used to select the best route.
• Static routes (admin distance 1) are preferred over dynamic protocols like EIGRP (90), OSPF (110), RIP (120).
• If administrative distance is equal, the routing protocol metric acts as the tiebreaker.
• Routing metrics differ between protocols and are not comparable across different routing protocols.

Redundancy and Virtual IP Addresses

• Devices typically support only one default gateway IP.
• Redundancy is achieved using a virtual IP (VIP) address, allowing failover between routers.
• First Hop Redundancy Protocol (FHRP) ensures seamless transition to a backup router if the primary fails.

Router Sub-Interfaces and VLANs

• Routers can have multiple virtual (sub) interfaces on a single physical interface.
• Each sub-interface can be linked to a different VLAN, with its own IP address and configuration.
• Trunk connections allow multiple VLANs over one physical link, managed by sub-interfaces.

Key Terms & Definitions

• Routing Table — a list of routes used by a router to determine packet forwarding.
• Administrative Distance — a value representing the trustworthiness of a routing source.
• Metric — a protocol-specific value used to determine the best path when multiple exist.
• Virtual IP (VIP) — a shared IP address used for redundancy between routers.
• Sub-interface — a virtual interface on a physical router port, typically used for VLANs.
• FHRP — First Hop Redundancy Protocol, enables router redundancy through VIPs.

Action Items / Next Steps

• Review the format and fields of routing table entries.
• Study administrative distances and metrics for common routing protocols.
• Understand the setup and purpose of sub-interfaces for VLAN routing.