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Tuesday, February 2, 2010

Day 12: Routing and Routers

Routers see networks, not hosts. There's a big difference between routed and routing protocols.
  • Protocols such as IP are routed protocols because the router uses the protocol to forward a packet from one router to another.
  • Routing protocols are used by routers to exchange routing information.
Routers decide where to forward a packet by using information stored in routing tables. They maintain a list of its interfaces and which networks are connected to those interfaces in its routing tables. Routers can dynamically learn about routes from other routers (routing protocol!) or the administrator can manually add a static route. When a packet arrives at a router, it'll look at the subnet mask, its routing table and cost.

We have four types of routes that exist in a routing table:
  1. Directly connected: a router detects configured networks connected to its interfaces and adds them to the routing table automatically (identified by prefix C). These are automatically updated when the configuration changes or an interface is shut down.
  2. Static: manually configured route, identifed by prefix S.
  3. Dynamic: these are dynamically updated by the router protocol. Prefix depends on the type of protocol, Routing Information Protocol (RIP) has prefix R.
  4. Default: static route that identifies the default gateway for packets addressed with a destination network that a router doesn't have in its routing table.
Dynamic routing protocols typically use the distance vector or link-state algorithm:

Distance Vector:
  • Periodically exchanges routing tables with neighboring routers.
  • Routes are evaluated on distance (how far) and vector (what direction).
  • Distance is expressed in a route cost or metric.
  • When a routing table is received it updates its routing information and forwards its routing table with an added hop to neighboring routers.
  • Exchanges link-state advertisements (LSA) when a change occurs in a link.
  • Maintains a topological database of the network and builds a shortest path first (SPF) tree.
  • When an LSA is received, the router will update and recalculate paths.
Routing Information Protocol (RIP):
  • Simple distance vector protocol, an interior routing protocol.
  • Exchanges complete copies of routing table.
  • Maximum 15 hop count - this to determine best path.
  • RIP version 2 (RIPv2) is preferred because it includes subnet mask information, where RIPv1 relies on classful default subnet masks. This means that RIPv2 allows VLSM and CIDR.
Enhanced Interior Gateway Routing Protocol (EIGRP):
  • Cisco-proprietary interior routing protocol.
  • Uses hop count (maximum 224), metrics and advertisements.
  • Maintains routing table, neighbor table and topology table.
Open Shortest Path First (OSPF):
  • Nonproprietary link-state interior routing protocol that sends LSA updates when there's a topology change.
Router Bootup Process, POST and loading Cisco IOS software:
  1. Router performs power-on self-test (POST) to check hardware.
  2. Loads bootstrap and initializes Cisco IOS from flash, TFTP or ROM. The location is defined in configuration register.
  3. Loads startup configuration file from nonvolatile random-access memory (NVRAM) to random-access memory (RAM) as running configuration.
  4. If NVRAM has no configuration file, the router will look for a TFTP-server. If it can't find it, it starts setup.

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