Routing Fundamentals And Static Routes: Distance Vector Fundamentals, RIP – EIGRP Comparison, and AD

Typically, distance vector protocols are used on Local Area Networks (LANs). While DV protocols work well in smaller and more stable environments, they have several drawbacks that prevent them from being used as Wide Area Network (WAN) protocols.

Both these protocols will broadcast full routing tables as part of their updates, regardless of whether anything has actually changed since the last update. This is a waste of bandwidth and resources, since your average LAN’s subnets aren’t going to change every minute. One such drawback is the default behavior of the DV protocols:

  • RIP version 1 (“RIPv1”). RIP broadcasts routing updates every 30 seconds
  • RIPv2 multicasts updates to 224.0.0.9 rather than broadcasting them, but the updates are still sent out every 30 seconds.
  • RIP routing updates can hold a maximum of 25 routes in a single RIP packet
  • RIP is generally a poor choice for a WAN protocol.
  • RIPv1 is a classful routing protocol, and therefore does not support VLSM. The only masks RIPv1 understands are the classful masks for Class A (255.0.0.0), Class B (255.255.0.0), and Class C (255.255.255.0).
  • RIP’s routing algorithm, the Bellman-Ford algorithm, considers only hop count in computing its metric.

Hop count is a poor decider of how to determine the best route to a destination, since WAN links quite often vary in speed. A smaller hop count to a destination, may in fact contain a slower overall path to a destination resulting in an inefficient route.

Default DV Protocol Behaviors

Split Horizon simply means that a routing protocol cannot advertise a route via the same interface upon which it was learned.

Poison Reverse allows a router to advertise a network with an metric of “unreachable” when that network becomes unavailable. This allows the other routers to learn that the network is unreachable much faster than if it were left up to the normal DV protocol behaviors.

High Points of Of RIPv1, RIPv2, and EIGRP.

RIPv1:

  • Broadcasts updates every 30 seconds
  • Classful, does not recognize VLSM, update carries entire routing table
  • Uses Bellman-Ford algorithm
  • Equal-cost load shares by default, max hop count is 15
  • No routing update authentication available
  • Updates carry 25 routes max

RIPv2:

  • Multicasts updates every 30 seconds to 224.0.0.9
  • Classless, supports VLSM, update carries entire routing table
  • Uses Bellman-Ford and default equal-cost load sharing, max hop count is 15, updates carry 25 routes max
  • Supports routing update authentication (clear-text and MD5)

EIGRP:

  • Multicasts to 224.0.0.10
  • Sends entire routing table only when the adjacency is first formed
  • Sends only routing update after that when necessary, update reflects only the changes
  • Uses DUAL routing algorithm
  • Equal-cost load sharing by default, unequal-cost load sharing configured with the variance command

Administrative Distance

4 Step process to determine the best route to a chosen destination:

  • 1. If there are multiple routes to a destination, the route with the longest prefix length is used.
  • 2. If there are multiple routes to a destination and they have the same prefix length, the route with the lowest administrative distance is used.
  • 3. If there are multiple routes with the same prefix length and AD, the route with the lowest metric is used.
  • 4. If there are multiple routes with the same prefix length, AD, and metric, all of these routes will be used in load balancing as allowed by the protocol.

More High Points with AD Values

  • Directly connected route / Static route using exit interface: 0
  • Static route with next-hop IP address: 1
  • EIGRP Summary: 5 (if you know where to look)
  • External BGP: 20
  • Internal EIGRP: 90
  • OSPF: 110
  • RIP: 120
  • External EIGRP: 170
  • Internal BGP: 200
  • Unknown network: 255
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