Routing Fundamentals

1
Routing Fundamentals

• One of a router's primary jobs is to make
  decisions about which path is the best path to a
  given destination. A router learns paths, called
  routes, from an administrator's configuration or
  from other routers via routing protocols. Routers
  keep a routing table in RAM. A routing table is a
  list of the best available routes. Routers use
  this table to make decisions about how to forward
  a packet. You can issue the show ip route command
  to view the TCP/IP routing table.

2
Routing Fundamentals

• A routing table maps network prefixes to an
  outbound interface.
• A router drops any packet destined for a network
  that is not listed in the routing table.

3
Routing Fundamentals

• New routes can be added to the routing table via
  one of two methods
• Static routing - An administrator manually
  defines routes to one or more destination
  networks.

4
Routing Fundamentals

• Dynamic routing - Routers follow rules defined by
  a routing protocol to exchange routing
  information and independently select the best
  path.
• What are some of the advantages and disadvantages
  of static and dynamic routing?

5
Static Routes

• Static routing is useful in very simple networks
  that do not have multiple paths to any
  destination network. Static routing reduces the
  memory and processing burdens on a router. Even
  on large internetworks, administrators often
  configure static routes on access routers that
  connect stub networks, or networks that have only
  one way in and one way out.

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Static Routes

• To configure static routing on a Cisco router,
  you must use the ip route command. This command
  uses the following syntax
• Router(config)ip route destination-prefix
  destination-prefix-mask address
  interface distance tag tag permanent

7
Static Routes

• You can manually add an entry to a routing table
  using one of two variations on the ip route
  command
• RTA(config)ip route 10.6.0.0 255.255.0.0 s1
• RTA(config)ip route 10.7.0.0 255.255.0.0
  10.4.0.2

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Static Routes

• The first example maps a network prefix
  (10.6.0.0/16) to a local physical interface (S1)
  on the router the same way that a directly
  connected network is mapped to an interface.
• The second example maps the network prefix
  (10.7.0.0/16) to the next-hop address (10.4.0.2).

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Static Routes

• When using a routing protocol such as RIP or
  IGRP, static routes that show as directly
  connected will automatically be advertised to
  other routers as long as the appropriate network
  command has been issued.
• Static routes can be included in updates if they
  are injected, or redistributed into the dynamic
  routing protocol.

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Static Routes

• When an interface goes down, all static routes
  mapped to that interface are removed from the IP
  routing table. In addition, when the router can
  no longer find a valid next hop for the address
  specified in a static route, the static route is
  likewise removed from the table. An alternative
  method is to map a static IP address to a
  loopback interface.

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Static Routes

• Note As a rule, you should always use the
  next-hop address when defining a static route on
  a multi-access network such as Ethernet. A router
  interface on a multi-access network could have
  several link partners, so you must use the
  next-hop address to specify which neighbor should
  receive traffic for a given network.

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Static Routes

• Static routes are also good to use when having
  trouble with a routing protocol.
• Static routing does not suit large, complex
  networks that include redundant links, multiple
  protocols, and meshed topologies.
• Dynamic routing is the best choice for complex
  networks.

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Dynamic Routes

• Dynamic routing of TCP/IP can be implemented
  using one or more protocols.
• Routing protocols designed to work inside an
  autonomous system are categorized as interior
  gateway protocols (IGPs), and protocols that work
  between autonomous systems are classified as
  exterior gateway protocols (EGPs).
• What are some examples of IGP and EGP?

14
Dynamic Routes

• You can further categorize these protocols as
  either distance-vector routing protocols or
  link-state routing protocols, depending on their
  method of operation.
• The Cisco IOS commands to enable dynamic routing
  vary depending on the routing protocol used.

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Dynamic Routes

• Although Cisco's EIGRP offers comprehensive
  support for both IPX and AppleTalk, it is
  important to be familiar with the names of the
  following proprietary routing protocols IPX RIP
  (or Novell RIP), NetWare Link Services Protocol
  (NLSP), and AppleTalk's Routing Table Maintenance
  Protocol (RTMP).

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IP Routing Protocols and the Routing Table

• Routers use administrative distance and metrics
  to evaluate, or measure, routes. When multiple
  routes to the same network exist and the routes
  are from the same routing protocol, the route
  with the lowest metric is considered the best.
• Each routing protocol calculates its metrics
  differently.
• What are some examples? RIP, IGRP, EIGRP?

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IP Routing Protocols and the Routing Table

• With default settings, EIGRP's metric for the
  route to 192.168.1.0 is 3,219,456 and RIPs is 3!
  If RTA receives a RIP update and an EIGRP update
  for this same network, how can the router compare
  what is, in effect, three apples against more
  than 3 million oranges? That is where
  administrative distance comes in.

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Administrative Distance

• When a router receives updates from different
  routing protocols about the same network, it can
  not use dissimilar metrics to evaluate a route.
  It uses administrative distance to decide which
  protocol to believe. The Cisco IOS assigns a
  default administrative distance to every routing
  protocol the lower the value, the more
  trustworthy the routing protocol.

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Distance Vector

• Distance-vector routing protocols are based on
  the Bellman-Ford algorithm (also known as a
  distance-vector algorithm).
• Routers configured to use a distance-vector
  routing protocol typically send their complete
  routing table at regular intervals to neighbor
  routers.

20
Distance Vector

• In fact, simple distance-vector protocols, such
  as RIP and IGRP, broadcast (or multicast) their
  routing table out all configured interfaces, by
  default. Routers that use these protocols do not
  actually identify their neighbors for direct
  communication.

21
Distance Vector

• A neighbor router receiving the broadcast update
  examines it and compares the information to its
  current routing table. Routes to new networks, or
  routes to known networks with better metrics, are
  inserted in the table. The neighbor then
  broadcasts its routing table, which includes any
  updated routes.

22
Distance Vector

• Distance-vector routing protocols are concerned
  with the distance and vector (direction) of
  destination networks.
• Before sending an update, each router adds its
  own distance value to the route's metric.
• Simple distance-vector routing protocols enjoy
  two major benefits over link-state protocols.
  They are relatively easy to configure, and they
  generally use less memory and processing power.

23
Distance Vector

• Simple distance-vector routing protocols do not
  scale as well as their link-state counterparts.
  RIPv1 and IGRP are classful routing protocols
  (they do not send subnet information in updates),
  so they can not support scalability features such
  as Variable Length Subnet Masking (VLSM) or
  supernetting.

24
Distance Vector

• In general, simple distance-vector routing
  protocols converge more slowly than link-state
  protocols. They do not work as well in a complex
  network.
• RIP has a 15 hop limitation and IGRP is a Cisco
  proprietary protocol and therefore it cannot
  support a multi-vendor routing environment.

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Link State Routing

• Link-state routing protocols offer greater
  scalability and faster convergence than
  distance-vector protocols such as RIP and IGRP.
  Unfortunately, these advantages come at a price.
  Link-state protocols require more memory and
  processing power from the router, and more
  knowledge and expertise from the administrator
  than do distance-vector protocols.

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Link State Routing

• Link-state protocols are based on the Dijkstra
  algorithm, sometimes referred to as the Shortest
  Path First (SPF) algorithm.

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Link State Routing

• Routers running a link-state protocol, such as
  OSPF, are concerned with the states (for example,
  up or down) of links (interfaces on other
  routers) in the network. A link-state router
  builds a complete database of all the link states
  of every router in its area. In other words, a
  link-state router gathers enough information to
  create its own map of the network.

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Link State Routing

• Each router then individually runs the SPF
  algorithm on its own map, or link-state database,
  to identify the best paths to be installed in the
  routing table. These paths to other networks form
  a tree with the local router as its root.

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Link State Routing

• Link-state routers advertise the states of their
  links to all other routers in the area so that
  each router can build a complete link-state
  database. These advertisements are called
  link-state advertisements (LSAs). Unlike
  distance-vector routers, link-state routers can
  form special relationships with their neighbors
  and other link-state routers, to ensure that the
  LSA information is properly and efficiently
  exchanged.

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Link State Routing

• After an initial flood of LSAs provides routers
  with the information that they need to build a
  link-state database, routing updates occur only
  when a link-state changes, or, if no changes have
  occurred, after a specific interval. If a link
  state changes, a partial update is sent
  immediately. The partial update contains only
  link states that have changed, not a complete
  routing table.

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Link State Routing

• Link-state protocols generally outperform
  distance-vector protocols on any size network. So
  why are not link-state protocols used exclusively
  for routing? Link-state protocols have two major
  disadvantages

32
Link State Routing

• Link-state routing may overtax low-end hardware.
  Link-state routers require more memory and
  processing power than distance-vector routers,
  which potentially makes link-state routing
  cost-prohibitive for organizations with tight
  budgets and legacy hardware.

33
Link State Routing

• Link-state protocols require complex
  administration. Configuring link-state routing
  can be a daunting task, and many administrators
  prefer to avoid its complexity and stick to
  distance-vector routing. Even capable
  administrators may opt for a straightforward
  distance-vector protocol on simple networks.  .

34
Hybrid Routing

• Cisco's proprietary EIGRP is an advanced
  distance-vector protocol that also employs the
  best features of link-state routing. For the most
  part, EIGRP configuration is similar to
  configuring a simple distance-vector protocol
  such as IGRP. EIGRP routers use partial updates,
  special neighbor relationships, and topological
  databases to provide optimal convergence. Rapid
  convergence, event driven updates, loop free
  routing, and multi-protocol support are EIGRPs
  trademark.

35
Default Routing aka Gateway of Last Resort

• Default routes are used when the router can not
  match a destination network with a more specific
  entry in the routing table thus, the gateway of
  last resort.

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Default Routing aka Gateway of Last Resort

• In effect, the router uses the default route to
  hand off to another router. The other router must
  have either a route to that destination or its
  own default route to a third router. If it is a
  default route to a third router, that router must
  have either the route to the destination or
  another default route, and so on.

37
Default Routing aka Gateway of Last Resort

• Before routers can dynamically exchange default
  information, an administrator must configure at
  least one router with a default route. An
  administrator can use two very different commands
  to statically configure default routes ip route
  0.0.0.0 0.0.0.0 and ip default-network.

38
Default Routing aka Gateway of Last Resort

• Creating an ip route to 0.0.0.0/0 is the simplest
  way to configure a default route. This is done
  using the following syntax Router(config) ip
  route 0.0.0.0 0.0.0.0 next-hop-ip-address
  exit-interface.
• To the Cisco IOS, network 0.0.0.0 /0 has special
  meaning as the gateway of last resort. All
  destination addresses match this route because a
  mask of all 0s requires none of the 32 bits in an
  address to be an exact match.

39
Default Routing aka Gateway of Last Resort

• Manually configuring 0.0.0.0/0 routes on every
  router might suffice in a simple network. You may
  want routers to dynamically exchange default
  routes in more complex situations. The exchange
  of default information works differently
  depending on the routing protocol being used and
  can create severe problems when improperly
  configured.

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Default Routing aka Gateway of Last Resort

• In IOS release 12.1, RIP does not propagate a
  static default route automatically. If you are
  using RIP and IOS 12.1, you must manually
  configure the RIP process to advertise the static
  default by issuing the network 0.0.0.0 command.

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Default Routing aka Gateway of Last Resort

• Alternately, you can use either the
  default-information originate command or the
  redistribute static command to configure static
  default route propagation. OSPF (regardless of
  the IOS version) requires the default-information
  originate command if you want to propagate static
  default routes. The following example illustrates
  this configuration for RIP (see the figure).

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Default Routing aka Gateway of Last Resort

• RTY(config) ip route 0.0.0.0 0.0.0.0
  172.16.1.2RTY(config) router ripRTY(config-rout
  er) default-information originate.

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Default Routing aka Gateway of Last Resort

• Using the default-information originate command,
  an administrator can statically configure a
  single RIP router with a 0.0.0.0/0 route, and
  that default route will be propagated to other
  routers. The default-information originate
  command can also be used with OSPF to achieve the
  same effect.

44
Default Routing W/ IGRP

• IGRP does not recognize the network 0.0.0.0/0 and
  will not include it in updates. To configure a
  dynamic exchange of default information in an
  IGRP network, you must use the ip default-network
  command.
• The ip default-network command can be used to
  flag a route to any IP network, not just
  0.0.0.0/0, as a candidate default route, using
  the following command syntax

45
Default Routing W/ IGRP

• Router(config)ip default-network
  ip-network-address.
• In complex topologies, several networks can be
  flagged as candidate defaults. Routers can then
  choose from among the available candidates to
  pick the lowest-cost route.

46
Floating Static Routes

• Floating static routes are static routes
  configured with an administrative distance value
  that is greater than that of the primary route
  (or routes). Essentially, floating static routes
  are fallback routes, or backup routes, that do
  not appear in the routing table until another
  route to the same destination fails.

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Floating Static Routes

• RTB(config)ip route 10.0.0.0 255.0.0.0 1.1.1.1
  130
• This ip route command includes an administrative
  distance of 130. Recall that static routes have a
  default administrative distance of 1

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Floating Static Routes

• To create a static route that will float (that
  is, wait for another route to fail before
  entering the routing table) you must manually set
  an administrative distance value. This value must
  be greater than the primary route's
  administrative distance value.

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Floating Static Routes

• In this example, the primary route is learned by
  RIP and thus has an administrative distance of
  120. By configuring the static route with an
  administrative distance of 130, the static route
  will be less desirable than the primary route.

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Convergence

• When all routers in a network agree on the
  topology, they have converged. Rapid convergence
  means rapid recovery from link failure or other
  network changes. Routing protocols and network
  designs are ultimately judged by how quickly they
  converge.

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Multiple Routes to a Single Destination

• Routing protocols permit the router to store
  multiple routes to each destination.
• One advantage of multiple routes is that
  equal-cost load balancing or unequal-cost load
  balancing can be used. Another advantage is that
  maintaining multiple routes to a single
  destination reduces a network's vulnerability to
  routing loops and dropped packets when a link
  fails.

52
Timed Updates Vs. Event Driven Updates

• Routing protocols that are exclusively
  time-driven react poorly to topology changes. If
  a router detects a change but has to wait 30
  seconds before alerting neighbors, routing in
  that network could break down. It could take
  several minutes before such a network's routers
  converge.

53
Timed Updates Vs. Event Driven Updates

• Routing protocols that are exclusively
  event-driven theoretically could go months
  without sending updates. If there is no other
  mechanism to ensure that routers regularly
  communicate (such as a Hello protocol), routers
  could base their routing decisions on dangerously
  outdated information.

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Timed Updates Vs. Event Driven Updates

• For these reasons, most routing protocols use a
  combination of time-driven and event-driven
  updates. RIP is time-driven, but Cisco's
  implementation of RIP sends triggered updates
  whenever a change is detected. Likewise, topology
  changes trigger immediate updates in IGRP
  routers, regardless of the update timer. Without
  triggered updates, RIP and IGRP would perform
  miserably.

55
Timed Updates Vs. Event Driven Updates

• Protocols that are primarily event-driven
  typically use timers as well. For instance, OSPF
  routers typically assign a MaxAge to routing
  information. Once information has reached its
  MaxAge, it can no longer be used in the routing
  table, and a new update must be requested.

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Metrics

• A routing metric is a value that measures the
  desirability. Some routing protocols use only one
  factor to calculate a metric such as RIP.
• Other protocols base their metric on two, three,
  or even five different factors, such as hop
  count, bandwidth, delay, load, and reliability.