Network Routing Protocols - IGRP, EIGRP, OSPF, ISIS, BGP
The purpose of routing protocols is to learn of available routes that exist on the
enterprise network, build routing tables and make routing decisions. Some of the most
common routing protocols include RIP, IGRP, EIGRP, OSPF, IS-IS and BGP. There are two
primary routing protocol types although many different routing protocols defined with
those two types. Link state and distance vector protocols comprise the primary types.
Distance vector protocols advertise their routing table to all directly connected
neighbors at regular frequent intervals using a lot of bandwidth and are slow to converge.
When a route becomes unavailable, all router tables must be updated with that new information.
The problem is with each router having to advertise that new information to its neighbors,
it takes a long time for all routers to have a current accurate view of the network.
Distance vector protocols use fixed length subnet masks which aren't scalable.
Link state protocols advertise routing updates only when they occur which uses bandwidth
more effectively. Routers don't advertise the routing table which makes convergence
faster. The routing protocol will flood the network with link state advertisements to all
neighbor routers per area in an attempt to converge the network with new route
information. The incremental change is all that is advertised to all routers as a
multicast LSA update. They use variable length subnet masks, which are scalable and use
addressing more efficiently.
Interior Gateway Routing Protocol (IGRP)
Interior Gateway Routing Protocol is a distance vector routing protocol developed by
Cisco systems for routing multiple protocols across small and medium sized Cisco networks.
It is proprietary which requires that you use Cisco routers. This contrasts with IP RIP
and IPX RIP, which are designed for multi-vendor networks. IGRP will route IP, IPX, Decnet
and AppleTalk which makes it very versatile for clients running many different protocols.
It is somewhat more scalable than RIP since it supports a hop count of 100, only
advertises every 90 seconds and uses a composite of five different metrics to select a
best path destination. Note that since IGRP advertises less frequently, it uses less
bandwidth than RIP but converges much slower since it is 90 seconds before IGRP routers
are aware of network topology changes. IGRP does recognize assignment of different
autonomous systems and automatically summarizes at network class boundaries. As well there
is the option to load balance traffic across equal or unequal metric cost paths.
• Distance Vector
• Routes IP, IPX, Decnet, Appletalk
• Routing Table Advertisements Every 90 Seconds
• Metric: Bandwidth, Delay, Reliability, Load, MTU Size
• Hop Count: 100
• Fixed Length Subnet Masks
• Summarization on Network Class Address
• Load Balancing Across 6 Equal or Unequal Cost Paths ( IOS 11.0 )
• Metric Calculation = destination path minimum BW * Delay (usec)
• Split Horizon
• Timers: Invalid Timer (270 sec), Flush Timer (630 sec), Holddown Timer (280 sec)
Enhanced Interior Gateway Routing Protocol (EIGRP)
Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by
Cisco systems for routing many protocols across an enterprise Cisco network. It has
characteristics of both distance vector routing protocols and link state routing
protocols. It is proprietary which requires that you use Cisco routers. EIGRP will route
the same protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the same
composite metrics as IGRP to select a best path destination.
As well there is the option to load balance traffic across equal or unequal metric
cost paths. Summarization is automatic at a network class address however it can be
configured to summarize at subnet boundaries as well. Redistribution between IGRP and
EIGRP is automatic as well. There is support for a hop count of 255 and variable length