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.