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WAN Access Problems
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Static and Dynamic Routing
Static routing depends solely on a manual input of routes. If you do not want
to enable a routing protocol on the router, you can manually enter all the
routes that you believe will be necessary; for everything else, the default gate-
way is used. This is a very cumbersome and poor way to configure a router.
Static routes are only used locally and are not advertised to neighboring
routers unless they are redistributed into a routing protocol session.
Dynamic routing is based on active routing protocols that share route
information with one another. When a destination is no longer reachable,
the route is removed from the routing table and the change is propa-
gated throughout the network. If a new destination becomes available, the
router adds the information into the route table and propagates the change
throughout the network.
This approach is much better than static routes. If a host that is entered in
the route table via a static route fails, the route remains in the route table, but
the source interface changes to Null 0. If static routes are redistributed, other
routers would still learn the route and send traffic there. Once the packets
reach the router with the static address, the packet will be sent to Null 0--
in other words, dropped.
By issuing the show ip route command, you can tell which routes are
learned dynamically and which are learned statically. Here is an example:
Router_B>show ip route
Codes: C- connected, S- static, I- IGRP, R - RIP, M -
mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF
inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA
external type 2
E1 - OSPF external type 1, E2 - OSPF external type
2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2,
* - candidate default
U - per-user static route, o - ODR
Gateway of last resort is 172.16.50.2 to network 0.0.0.0
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