ROUTING PROTOCOLS

ROUTING PROTOCOLS

Def.

Administrative Distance (AD) is a measure of trustworthiness, rated on a scale of 0 to 255, used by routers to select the best path, when multiple, different routing protocols provide routes to the same destinations.

Routing by rumor – distance vector  routing algorithm, that passes its complete routing table contents to neighbor router.

Round-robin load balancing – distance vector uses only hop counts to determine the best path to a network.

 

Classes of routing protocols

Distance Vector (RIP, RIP2, IGRP)

Link State (OSPF, IS-IS)

Hybrid (EIGRP, BGP)

Distance Vestor Routing Protocols

Routing Information Protocol (RIP)

  • it sends complete routing table out to all active interfaces every 30 seconds.
  • maximum allowable hop count is 15 by default
  • works well in small networks
  • only classful routing – all devices in network muss use the same subnet mask for each specific address class.

Disadvantages:

  • creates routing loops – that ist the reason for 15 hop counts
  • is slow at converging (latency in network)

RIP Version 2 (RIPv2)

  • is considered as classless routing protocol, which means that it sends subnet mask information along with route update
  • can support variable length subnet masks (VLSMs)

Variable-length subnet masks (VLSMs)

  • we can have different subnet masks for different router interfaces
  • is a networking technique that enables to devide an IP address utilization
  • requires routers to support classless routing protocols (OSPF, EIGRP, RIPv2)

 

Def.

Discontiguous network – two or more subnetworks of a classful network connected together by different classful networks.

 

Enhanced Interior Gateway Routing Protocol (EIGRP)

 

Autonomous system – set of contiguous routers that run the same routing protocol and share routing information

Hybrid routing -  sending the traditional distance vector updates containing information about networks plus the cost of reaching them from perspective of advertising router

Adventages of EIGRP:

  • synchronizes only when topology changes occure.
  • supports IP and Ipv6 via protocol dependend modules
  • is considered as classless
  • supports VLSM / CIDR (classless inter-domain routing)
  • supports route summarization and discontiguous network
  • discovers neighbor efficient
  • communicates via Reliable Transport Protocol (RTP)
  • best path selection via Diffusing Update Algorithm (DUAL)
  • creates and maintance additional tables instead of just one table
  • uses bandwidth and delay of the line (by default) as well as sending reliability, Load, and MTU information between routers

 

Neighbor table – IP (Next hop router – Interface)

  • keeps infos about about adjacent neighbors
  • when new neighbor is recorded,so addressand infos are recorded
  • RAM

 

Topology table IP (Destination 1, Destination 2 -  Successor, Feasible successor)

  • feasible successor -  backup route, EIGRP keeps 6 rates

 

Routing table (Destination 1 -  Successor)

  • successor –  best route to the network (rates in routing table)

Border Gateway Protocol (BGP)

 

  • external routing protocol used between autonomous systems
  • hybrid: can be used within AS too, path vector and distance vector
  • is alternative to default routes for controlling path selection
  • enables classless routing and summarization
  • does not broadcast entire routing table
  • is called Routing Information Base RIB
  • includes IP address to next AS – next hop attribute
  • carries informations like network prefix

 

Link-State Routing Protocols

  • OSPF
  • IS-IS

Open Shortest Path First (OSPF)

  • open standard routing protocol
  • DIJKSTRA algorithm: shortest path is constructed, then the routing table is populated with the resulting best paths
  • converges quickly
  • supports multiple equal cost routes to the same destination
  • supports IP and IPv6 – must maintain separate database and routing table for each
  • consists of areas and autonomous systems
  • supports VLMS / CIDR
  • has unlimited hop count

 

  • has hierarchical fashion – you can separate larger internetwork into smaller internetworks -> AREA

 

  • OSPF must have an AREA 0 (Backbone area) - all other areas should connect to this area
  • Routers that connect other areas to the backbone area within AS are called border routers (ABRs)
  • Router that connect ASs is called autonomous system border router (ASBR)

 

  • OSPF runs inside an autonomous system, it can also connect multiple autonomous systems together

 

Intermediate System – to – Intermediate System (IS-IS)

  • is intended for use within an administrative domain or network, not for routing between AS
  • operates by reliably flooding topology information throughout a network of routers
  • each route independly builds picture about network topology
  • data or datagrams are forwarded based on the best topological path
  • DIJKSTRA algorithm (discovers the shortest path)
  • uses connectionless network service (CLNS) to provide connectionless delivery of data packets between routers
  • does nor require an AREO 0

 

L1 - Level1 intermediate systems route within area when the destination is outside area, they route toward a Level 2 system.

L2 – Level 2 intermediate systems route between areas and toward other AS.