Information Technology Reference
In-Depth Information
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 10-4, you can verify an interface's MTU using the
show interface
command.
Example 10-4
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. These schemes are
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 neigh-
boring router, thus suppressing the route. If the neighbor is already closer to the destina-
tion, it already has a better path.
In Figure 10-6, 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.
Net A = metric
Net B = metric
Router 1
Router 2
Router 3
Network A
Network B
Network C
Network D
With Split Horizon, Router 2 sends Net A
and Net B routes to Router 3; no route for
Net D is sent to Router 3.
Figure 10-6
Simple Split-Horizon Example
