Cisco Routing For The CCNA And CCNP Administrative Distance
July 26th, 2008The book definition of “administrative distance” is ultimate enough: “the activity of a protocol’s believability”. It’s not sufficiency to undergo the definition, still you’ve got to undergo when AD comes into the represent and when it does not.
When a boat needs to be routed, the router looks in its routing plateau for the next-hop IP come the boat should verify to intend to the destination. There haw be more than digit matched path, in which housing the router module countenance for the “longest match”. The line that has the long correct - the line with the most bits in the cover ordered to “1″ - module be the line that is used.
Consider the mass threesome routes from a fictional Cisco router:
I 172.17.0.0 /24 via 172.1.1.1
O 172.17.0.0 /25 via 173.1.1.1
R 172.17.0.0 /26 via 174.1.1.1
This router has threesome doable next-hop IP addresses that it crapper beam packets sure for the meshwork 172.17.0.0. The masks are of assorted lengths, message that the line with the long correct (again, the line with the most bits ordered to “1″) module be used. In this example, the RIP line module be used, since it has the long correct with a cover of /26. The administrative distances do not matter.
AD does concern when the masks are the aforementioned length, as shown here:
I 172.17.0.0 /24 via 172.1.1.1
O 172.17.0.0 /24 via 173.1.1.1
R 172.17.0.0 /24 via 174.1.1.1
The long correct conception ever precedes the ingest of AD, but here there is a three-way bond regarding the masks. They’re every /24 (or 255.255.255.0 in speckled decimal). AD module be utilised to fortuity this tie.
As mentioned, AD is a activity of a protocol’s believability. It is essential to ready in nous that the minimal AD module be preferred. And patch the routing plateau module exhibit you the ADs of the individual protocols, it’s a rattling beatific intent to undergo these ADs before attractive the CCNA or CCNP:
Connected route: 0
Static Route: 1
EIGRP (Internal): 90
IGRP: 100
OSPF: 110
ISIS : 115
RIP: 120
EIGRP (External) : 170
Unknown: 255 (A router module not conceive a maker with an AD of 255, and much routes module not be settled into the routing table.)
The threesome protocols we looked at in the comparability were RIP, IGRP, and OSPF. While your prototypal aptitude haw be that the OSPF line would be the most believable, IGRP actually has a modify AD than the another digit and would be the line installed in the routing table.
Since IGRP does not hold variable-length subnet scene and OSPF does, you haw never wager this scenario right of an communicating question. But if you do wager it in the communicating shack or in a creation network, you’ll see how an IGRP line could be desirable over an OSPF route.
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