From Network Security Wiki

Here we will discuss important questions from routing concepts.

Routing Concepts

  • In Classful addressing, Routing Table has 3 columns While in Classless addressing, at least 4 Columns are needed:- CIDR Mask, Network Address, Next hop address, Interface.
  • There is no MTU for Serial Interfaces
  • Applying Route Maps:-
 int fe 0/0
 ip policy route map                 #for transit traffic
 ip local policy route map           #for local generated traffic (e.g self telnet,ssh)
  • Redistribution configuration is done in Destination protocol. To Redistribute OSPF routes to BGP, Configure BGP.
  • The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table:
  • The main considerations while building the routing table are:
Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance.
Metrics - This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.
Prefix length
  • To determine the best route to a destination, a router considers three elements (in this order):
1st: Prefix-Length - Choose the route that has the longest prefix match.
2nd: Administrative Distance(between separate routing protocols) - Choose the route that has the lowest administrative distance. Administrative distance is a value a router uses to trust a route, if the route has multiple exits to the destined network.
3rd: Metric(within a routing protocol) - Choose the route that has the lowest metric.
  • AD of common routing protocols (lowest wins):
Protocol AD
Connected 0
Static 1
EIGRP Summary 5
External BGP 20
Internal EIGRP 90
OSPF 110
RIP 120
External EIGRP 170
Internal BGP 200
Unknown 255
  • If there is a more specific route available, How can you make the L3 device use the other route?[1]
Ans) By using Static ARP entry.

  • Split horizon
Method of preventing a routing loop in a network.
Information about the routing for a particular packet is never sent back in the direction from which it was received.

Routing Protocol Classification

Classification based on deployment location

Interior Gateway Protocol:


Exterior Gateway Protocol:


Distance Vector

  • Protocols:
BGP (Adv Distance Vector or Path Vector)
  • Also called Routing by Rumour
  • Processor Efficient as they do not have full Topology information
  • Best Route calculation might be less accurate

Link State Routing Protocol

  • Protocols:
  • Maintain 3 tables:
  1. Neighbor Table
  2. Topology Table
  3. Routing Table
  • Processor Intensive as they have full topology information about each and every network
  • Better Route calculation
  • EIGRP also have topology table but they contain a list of the best routes calculated by neighbors only.
  • Areas should not become too big.
  • LS Routing protocols send only triggered updates i.e. to announce network changes only(RIP sends full routing table every 30 seconds).
  • Sends Periodic updates - LS Refresh on long intervals (OSPF 30 minutes) to avoid any missed routes.

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