- Read bootstrap (ROMmon) from ROM
- Run POST
- Load IOS from Flash (into RAM)
- Load startup-config from NVRAM (into RAM) as running-config
Route traffic between devices (serial, ethernet, atm).
Formatted as [type][module number(optional)]/[interface number], e.g. Ethernet0/1.
Allow to connect into and configure Cisco devices (console, auxiliary, and telnet/SSH ports).
OSI Model and Operation
Routers operate from Layer 3 and down. Forwarding happens at level 3, where the destination IP address is examined and the routing table is consulted. The Layer 3 IP packet is encapsulated into the data portion of a Layer 2 data-link frame for the exit interface.This is then encoded into Layer 1 physical signals to represent bits.
Contain:
- Destination network and subnet mask
- Next hop router to the Destination
- Routing metrics and Administrative Distance
- Routed (layer 3) apply logical addresses and route between networks e.g. IP
- Routing build the network, topology, and next hop
- Prefix-Length (# of bits identifying network)
- Metric (distance for DV, cost for LS)
- AD (between protocols)
Classful protocols:
- Divide address into Network/Subnet/Host
- Do not send packets containing subnet mask info
- Require more bandwidth
- Regular/periodic updates
Classless protocols:
- Divide into Subnet/Host
- Send packets with subnet mask info
- Less bandwidth, hello packets
- Triggered updates
- Periodic updates of the full routing table
- Advertise direct networks to neighbors
- Neighbors take these as learned networks and add them to their own knowledge
- Known + learned connections are all sent to own neighbors
- Blind trust to others' information, aka "routing by rumor"
- Distance used as metric
- RIP: hopcount, IGRP: bandwidth + delay
- Slow convergence, susceptible to loops
- Side-effect of the periodic update system
- Bellman-Ford algorithm to determine shortest path
Routing table contains distance (hopcount) and direction (neighbor) to reach destination, not actual topology
For use on smaller networks.
- 30sec routing updates
- Full routing table
- Hopcount distance metric
- IP and IPX
- AD of 120
- 15 hops max
- Round-robin load balancing when equal hops
- Bandwidth-agnostic, which can cause congestion
Version 1
- Classful (no VLSM)
- Subnets must be contiguous and under identical masks
- Broadcasts to 255.255.255.255
Version 2
- Classless (supports VLSM)
- Multicasts updates
- Encrypted auth between routers
- Classless
- Cisco proprietary
- Reliable Transport Protocol (RTP)
- Keeps a topology table besides the regular routing table, including backup paths to destinations that DUAL (Diffusing Update Algorithm) considers loop-free
- Faster convergence than traditional DV
- Hello packets, not unlike LS
Routers know about their own links by checking for interfaces in the up state, and neighbors "meet" each other by sending Hello packets. Tables constructed with this information are flooded to all neighbors, ending up with all routers having full knowledge of the network topology.
- Neighbor table: list of neighbors and relevant interface. Formed by Hello packets
- Topology/link-state table: map of all links in an area, and link status
- Shortest-path/routing table: best routes to each destination
- Any changes are directly advertised to routers in the area
Dijkstra formula calculates shortest path using cost metric
- DV route by rumor, LS send updates advertising the state of their direct networks
- These LS updates are less bandwidth-intensive
- But, the three LS tables require more system resources
- LS feature faster convergence and no routing loops
- DV only consider hops, LS thinks speed
- LS have hierarchical design by using areas, allowing better summarization and isolating issues within these areas
Designed to scale efficiently with larger networks.
- Hierarchical design using areas
- Forms neighbor relationships with adjacent routers in area
- Traffic multicast to 224.0.0.5 or .6
- Classless (supports VLSM)
- Only IP routing
- Cost metric, based on bandwidth
Unique router ID determined either by manual specification, highest address on a loopback interface, or highest address on any physical interface.
Neighbor table constructed from Hello packets, which include per entry:
- Neighbor's Router ID
- Current state
- Interface
- IP address
Routers may elect a Designated Router, which they will send link-state advertisements to, and the DR will hold the responsibility of multicasting the advertisement to the other nodes in the network.
- Security, with entirely separated parts of the network
- Cost reduction, since it reduces the need for extra hardware
- Higher performance and broadcast storm mitigation by dividing Layer 2 networks into smaller logical groups
- Simpler management, from aggregating users and network devices
VLANs are designated IDs, to denote separate networks. 1-1005 is the normal range, 1006-4904 extended
- Data VLAN, carrying user-generated traffic
- Default VLAN
- Native VLAN, a trunk port that supports traffic from many VLANS and not from a VLAN
- Management VLAN, configured to access management of a switch
- Voice VLANs, for VoIP