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Home > Archive > alt.certification.cisco > June 2002 > Re: Which has higher priority, administrative distance or longest
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Re: Which has higher priority, administrative distance or longest
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I think the longest match has higher priority. You have to think of
routing as a two step process: building the routing table and selecting
a route in the routing table. In building the routing table, a router
will select a route with the best administrative distance, e.g. OSPF
is better than RIP. Since different prefix-lengths are considered as
different routes, they will all be installed in the routing table
regardless of what routing protocol (ad. distance) they were learned
from. In selecting a route from the routing table, the longest match
is the rule, administrative distance is no longer an issue here.
Doan
On Sun, 23 Jun 2002, Jason Thomas wrote:
> Another question, if you all don't mind.
>
> Can anyone tell me which has higher priority administrative distance or
> longest prefix match.
>
> Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> knows a route and matches 24 bits would the router pick the EIGRP route or
> the OSPF route.
>
> I just need a bit of clarification
>
> Thanks,
> Jason
>
>
>
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On Mon, 24 Jun 2002, Hansang Bae wrote:
> In article <vSsR8.78541$Na1.2573488@news4.srv.hcvlny.cv.net>,
> jthomas@optonline.net says...
> > Another question, if you all don't mind.
> > Can anyone tell me which has higher priority administrative distance or
> > longest prefix match.
> > Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> > knows a route and matches 24 bits would the router pick the EIGRP route or
> > the OSPF route.
> > I just need a bit of clarification
>
>
> Administrative distance determines which routes will end up in the routing
> table. OSPF route will not be in the table if EIGRP has the same route
> (regardless of the mask)
>
But if the mask is different, then it is a different route, i.e.
192.168.32.0/24 and 192.168.32.0/28 are two routes.
Doan
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I don't think you can have 2 same routes (same # of bits) in the routing
table.
Doan
On Mon, 24 Jun 2002, Thomas Kuborn wrote:
> Yooo,
>
> If you have 2 routes (matching the same # bits) in your routing table,
> then AD is first used as a differentiator, then metric is used.
> 1. # bits matched
> 2. AD
> 3. metric
> In your example, the OSPF route will be used
>
> Tom
>
> doan wrote:
>
> > I think the longest match has higher priority. You have to think of
> > routing as a two step process: building the routing table and selecting
> > a route in the routing table. In building the routing table, a router
> > will select a route with the best administrative distance, e.g. OSPF
> > is better than RIP. Since different prefix-lengths are considered as
> > different routes, they will all be installed in the routing table
> > regardless of what routing protocol (ad. distance) they were learned
> > from. In selecting a route from the routing table, the longest match
> > is the rule, administrative distance is no longer an issue here.
> >
> > Doan
> >
> > On Sun, 23 Jun 2002, Jason Thomas wrote:
> >
> >
> >>Another question, if you all don't mind.
> >>
> >>Can anyone tell me which has higher priority administrative distance or
> >>longest prefix match.
> >>
> >>Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> >>knows a route and matches 24 bits would the router pick the EIGRP route or
> >>the OSPF route.
> >>
> >>I just need a bit of clarification
> >>
> >>Thanks,
> >>Jason
> >>
> >>
> >>
> >>
> >
>
>
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I think the longest match has higher priority. You have to think of
routing as a two step process: building the routing table and selecting
a route in the routing table. In building the routing table, a router
will select a route with the best administrative distance, e.g. OSPF
is better than RIP. Since different prefix-lengths are considered as
different routes, they will all be installed in the routing table
regardless of what routing protocol (ad. distance) they were learned
from. In selecting a route from the routing table, the longest match
is the rule, administrative distance is no longer an issue here.
Doan
On Sun, 23 Jun 2002, Jason Thomas wrote:
> Another question, if you all don't mind.
>
> Can anyone tell me which has higher priority administrative distance or
> longest prefix match.
>
> Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> knows a route and matches 24 bits would the router pick the EIGRP route or
> the OSPF route.
>
> I just need a bit of clarification
>
> Thanks,
> Jason
>
>
>
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On Mon, 24 Jun 2002, Hansang Bae wrote:
> In article <vSsR8.78541$Na1.2573488@news4.srv.hcvlny.cv.net>,
> jthomas@optonline.net says...
> > Another question, if you all don't mind.
> > Can anyone tell me which has higher priority administrative distance or
> > longest prefix match.
> > Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> > knows a route and matches 24 bits would the router pick the EIGRP route or
> > the OSPF route.
> > I just need a bit of clarification
>
>
> Administrative distance determines which routes will end up in the routing
> table. OSPF route will not be in the table if EIGRP has the same route
> (regardless of the mask)
>
But if the mask is different, then it is a different route, i.e.
192.168.32.0/24 and 192.168.32.0/28 are two routes.
Doan
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I don't think you can have 2 same routes (same # of bits) in the routing
table.
Doan
On Mon, 24 Jun 2002, Thomas Kuborn wrote:
> Yooo,
>
> If you have 2 routes (matching the same # bits) in your routing table,
> then AD is first used as a differentiator, then metric is used.
> 1. # bits matched
> 2. AD
> 3. metric
> In your example, the OSPF route will be used
>
> Tom
>
> doan wrote:
>
> > I think the longest match has higher priority. You have to think of
> > routing as a two step process: building the routing table and selecting
> > a route in the routing table. In building the routing table, a router
> > will select a route with the best administrative distance, e.g. OSPF
> > is better than RIP. Since different prefix-lengths are considered as
> > different routes, they will all be installed in the routing table
> > regardless of what routing protocol (ad. distance) they were learned
> > from. In selecting a route from the routing table, the longest match
> > is the rule, administrative distance is no longer an issue here.
> >
> > Doan
> >
> > On Sun, 23 Jun 2002, Jason Thomas wrote:
> >
> >
> >>Another question, if you all don't mind.
> >>
> >>Can anyone tell me which has higher priority administrative distance or
> >>longest prefix match.
> >>
> >>Meaning is EIGRP knows of a route to a host and matches 22 bits and OSPF
> >>knows a route and matches 24 bits would the router pick the EIGRP route or
> >>the OSPF route.
> >>
> >>I just need a bit of clarification
> >>
> >>Thanks,
> >>Jason
> >>
> >>
> >>
> >>
> >
>
>
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| Category 5 2002-06-24, 11:25 pm |
| "Jason Thomas" <jthomas@optonline.net> writes:
> Can anyone tell me which has higher priority administrative distance
> or longest prefix match.
It's interesting how much confusion there seems to be on this point.
Administrative distance only comes into play as a tie-breaker when
building the routing table in the event that a particular route
(address and mask) is available from multiple routing information
sources. If different masks are involved there's no tie to break: all
routes end up in the routing table. Packet forwarding always follows
the longest-match rule.
The process of route selection is described here, among other places:
http://www.cisco.com/warp/public/105/21.html
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