The reliability parameter is the dependability of a network link. Some WAN links tend to go up and down throughout the day. These links get a small reliability rating. Reliability is measured by factors such as a link’s expected received keepalives and the number of packet drops and interface resets. If the ratio is high, the line is reliable. The best rating is 255/255, which is 100% reliability. Reliability is one of the five parameters included in the definition of the EIGRP metric. By default, it is not used to calculate the composite metric. As shown in Example 3-3, you can verify an interface’s reliability by using the show interface command.
Example 3-3 Interface Reliability
router4#
show interface serial 0
Serial0 is up, line protocol is up
Hardware is PQUICC Serial
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Maximum Transmission Unit
The MTU parameter is simply the maximum size of bytes a unit can have on an interface. If the outgoing packet is larger than the MTU, the IP protocol might need to fragment it. If a packet larger than the MTU has the Do Not Fragment flag set, the packet is dropped. As shown in Example 3-4, you can verify an interface’s MTU by using the show interface command.
Example 3-4 Interface MTU
router4#
show interface serial 0
Serial0 is up, line protocol is up
Hardware is PQUICC Serial
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Routing Loop-Prevention Schemes
Some routing protocols employ schemes to prevent the creation of routing loops in the network. The following are the commonly used loop-prevention schemes:
- Split horizon
- Poison reverse
- Counting to infinity
Split Horizon
Split horizon is a technique used by distance-vector routing protocols to prevent routing loops. Routes that are learned from a neighboring router are not sent back to that neighboring router, thus suppressing the route. If the neighbor is already closer to the destination, it already has a better path.
In Figure 3-6, Routers 1, 2, and 3 learn about Networks A, B, C, and D. Router 2 learns about Network A from Router 1 and also has Networks B and C in its routing table. Router 3 advertises Network D to Router 2. Now, Router 2 knows about all networks. Router 2 sends its routing table to Router 3 without the route for Network D because it learned that route from Router 3.
Figure 3-6 Simple Split-Horizon Example