IGP Routing Protocol algorithms metrics and administrative distance
 


IGP Routing Protocol Algorithms

The term routing protocol algorithm refers to the logic and processes used by different routing protocols to solve the problem of learning all routes, choosing the best route to each subnet, and converging in reaction to changes in the internetwork.  Three main branches of routing protocol algorithms exist for IGP routing protocols:

* Distance vector (sometimes called Bellman-Ford after its creators).

* Link-state.

* Balanced hybrid (sometimes called enhanced distance vector).

EIGRP is called either a balanced hybrid protocol or an advanced distance vector protocol.

Metrics

Routing protocols choose the best route to reach a subnet by choosing the route with the lowest metric.

IGP Metric Description
RIP-1, RIP-2 Hop count The number of routers (hops) between a router and the destination subnet.
OSPF Cost The sum of all interface cost settings for all links in a route, with the cost defaulting to be based on interface bandwidth.
EIGRP Composite of bandwidth and delay Calculated based on the route’s slowest link and the cumulative delay associated with each interface in the route.

The bandwidth interface subcommand does not change the actual physical speed of the interface.  It just tells the IOS what speed to assume the interface is using.

IGP Comparisons: Summary

Feature RIP-1 RIP-2 EIGRP OSPF IS-IS
Classless No Yes Yes Yes Yes
Supports VLSM No Yes Yes Yes Yes
Sends mask in update No Yes Yes Yes Yes
Distance vector Yes Yes No* No No
Link-state No No No* Yes Yes
Supports autosummarization No Yes Yes No No
Supports manual summarization No Yes Yes Yes Yes
Proprietary No No Yes No No
Routing updates are sent to a multicast IP address No Yes Yes Yes
Supports authentication No Yes Yes Yes Yes
Convergence Slow Slow Very fast Fast Fast

*EIGRP is often described as a balanced routing protocol, instead of link-state or distance vector. Some documents refer to EIGRP as an advanced distance vector protocol.
Comparing Features of IGPs: RIP-2, EIGRP, and OSPF:

224.0.0.10

Features RIP-2 OSPF EIGRP
Metric Hop count Link cost Function of bandwidth, delay
Sends periodic updates Yes (30 seconds) No No
Full or partial routing updates Full Partial Partial
Where updates are sent 224.0.0.9* 224.0.0.5, 224.0.0.6  
Metrics considered to be “infinite” 16 (2^24)-1 (2^32)-1
Supports unequal-cost load balancing No No Yes

*This table specifically refers to features of RIP-2, but the only difference with RIP-1 in this table is that RIP-1 broadcasts updates to IP address 255.255.255.255.

Administrative Distance

When a single routing protocol learns multiple routes to the same subnet, the metric tells it which route is best.  When two different routing protocols learn routes to the same subnet, because each routing protocol’s metric is based on different information, IOS cannot compare the metrics.

When IOS must choose between routes learned using different routing protocols, IOS uses a concept called administrative distance.  Administrative distance is a number that denotes how believable an entire routing protocol is on a single router.  The lower the number, the better, or more believable, the routing protocol.

Default Administrative Distances:

Route Type Administrative Distance
Connected 0
Static 1
BGP (external routes) 20
EIGRP (internal routes) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIGRP (external routes) 170
BGP (internal routes) 200
Unusable 255

IOS can be configured to change the administrative distance of a particular routing protocol, a particular route, or even a static route.  You can use the ip route command to do this by tacking on the administrative distance at the end of the command.