Routing

Routing fundamentals

Routers forward traffic between subnets and networks on IP, and each subnet is its own broadcast domain. The exam rule: “switch” means Layer 2, “multilayer switch” or “router” means Layer 3.

Devices send off-network packets to the default gateway. Routers re-wrap Layer 2 frames into Layer 3 packets for WAN transit, and back to frames at the far side. Switches deliver on MAC, routers on IP.

Routing tables

The table maps destinations to next hops, and the longest (most specific) prefix wins. The ARP cache maps IP to MAC on the local segment.

Route sources: directly connected (a physical link), static (manual; 0.0.0.0/0 is the default route for unknown destinations), and dynamic (protocols exchange routes automatically).

Loop prevention: split horizon (don’t advertise a route back out the interface it came from) and poison reverse (advertise it back with an unusable cost).

Routing protocols

• RIP: distance vector, interior. Hop count metric (max 15), 30 s updates, UDP.
• OSPF: link state, interior. Cost from link speed, fast convergence.
• IS-IS: link state, interior. Cost from link speed, the less popular OSPF-alike.
• EIGRP: advanced distance vector (hybrid), interior. Bandwidth + delay metric, Cisco shops.
• BGP: path vector, exterior. AS-hop metric, the backbone of the internet, slow convergence.

Distance vector sends full tables to neighbors on intervals: slow convergence, with hold-down timers helping. Link state means every router knows the full topology: faster convergence.

Route redistribution lets one router translate between protocols (OSPF on one interface, EIGRP on another).

Route selection

Administrative distance ranks believability, lower wins: directly connected 0, static 1, EIGRP 90, OSPF 110, RIP 120, external EIGRP 170, unknown 255 (unreachable). Within a protocol, its metric picks the path, lower is better.

NAT and PAT

NAT translates private IPs to public to stretch IPv4 space. Dynamic NAT is one-to-one from a pool; static NAT is a manual one-to-one (doubling as a security mapping); PAT is many-to-one, with port numbers distinguishing the flows (home router behavior).

The terminology: inside local (private IP of an inside host), inside global (public IP of an inside host), outside local, outside global.

First hop redundancy

FHRPs auto-fail-over the default gateway. A virtual IP fronts the router group, and subinterfaces split one physical interface into logical ones.

• HSRP (Cisco): active + standby routers. Preempting lets a higher-priority router reclaim active.
• VRRP: the same idea, open standard.
• GLBP (Cisco): adds load balancing, with different virtual MACs per member so traffic spreads across the group.

Multicast routing

Send once to a Class D group address; only subscribed hosts process it.

IGMP lets clients join and leave groups so routers know which interfaces have receivers (v1 has noisy queries, v2 adds leave messages, v3 adds source-specific multicast).

PIM routes multicast between routers via distribution trees. Dense mode floods then prunes (heavy, avoided today); sparse mode starts on a shared tree then switches to the shortest path tree (the modern default).

GRE

Generic Routing Encapsulation wraps almost any Layer 3 protocol in a point-to-point IP tunnel: lightweight branch-to-branch connectivity without leased lines.

It has no encryption of its own (pair it with IPSec for security), and it’s configured on the routers at both ends.