From: Kenneth Wygand (KWygand@customonline.com)
Date: Tue Jul 06 2004 - 00:14:49 GMT-3
Phillip,
Thanks for the information.
To be explicit, the IOS actually considers the default Administrative DIstance as an unchangable second-level distance within the AD comparison module that decides which routes to put into the IP Routing Table.
Internally, you can understand the logic as adding (default AD / 1000) to the user-definable AD through the distance commands for all IP routing protocols. For example, consider both OSPF and EIGRP with their distances changed from their default distance to 120:
OSPF:
Default OSPF Distance: 110
Configured OSPF Distance: 120
New Internal OSPF Distance: Configured AD + (Default AD / 1000) = 120+ (110 / 1000) = 120 + 0.110 = 120.110
New Internal OSPF Distance is now 120.110
EIGRP:
Default EIGRP Distance: 90
Configured EIGRP Distance: 120
New Internal EIGRP Distance: Configured AD + (Default AD / 1000) = 120+ (90 / 1000) = 120 + 0.090 = 120.090
New Internal EIGRP Distance is now 120.090
Now, even though both OSPF and EIGRP have their AD set to 120, EIGRP still maintains a better internal distance. This circumvents the problem of having to choose between two metrics that have no correlation to one another due to inconsistent metric derivation algorithms between both routing protocols. It also implements a level of consistency, whereas this will be consistent in all devices, as opposed to having some other criteria that may not be consistent across a routing domain.
If you perform a "debug ip routing", you will see that the EIGRP routes will bump the OSPF routes due to "closer admin distance" as follows:
<SNIP>
*Mar 1 00:07:10.131: RT: closer admin distance for 10.0.0.0, flushing 1 routes
*Mar 1 00:07:10.131: RT: add 10.0.0.0/24 via 2.3.4.7, EIGRP metric [120/46738]
*Mar 1 00:07:10.131: RT: NET-RED 10.0.0.0/24
*Mar 1 00:07:10.131: RT: NET-RED queued, Queue size 1
</SNIP>
Good job to those who got this! :)
Ken
________________________________
From: nobody@groupstudy.com on behalf of Tinis, Phillip
Sent: Mon 7/5/2004 10:08 PM
To: ccielab@groupstudy.com
Subject: RE: Quiz question of the day 20040704
Hello Ken,
It took process of elimination to get the answer. I was not able to find it
documented anywhere.
Good question. :-)
.-Phillip Tinis-.
_____
From: Kenneth Wygand [mailto:KWygand@customonline.com]
Sent: Monday, July 05, 2004 8:49 PM
To: Tinis, Phillip; ccielab@groupstudy.com
Subject: RE: Quiz question of the day 20040704
Phillip,
You are correct! :)
I didn't see this mentioned in the link you sent. Have you found this
documented by Cisco somewhere?
Thanks!
Ken
_____
From: nobody@groupstudy.com on behalf of Tinis, Phillip
Sent: Sun 7/4/2004 8:45 PM
To: ccielab@groupstudy.com
Subject: RE: Quiz question of the day 20040704
1st - BGP
2rd - EIGRP
3rd - OSPF
4th - RIP
The router will revert back to the default decision making algorithm based
on the default administrative distances.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr
<http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fip
r>
rp_r/ind_r/1rfindp1.htm#wp1039950
.-Phillip Tinis-.
-----Original Message-----
From: Kenneth Wygand [mailto:KWygand@customonline.com
<mailto:KWygand@customonline.com> ]
Sent: Sunday, July 04, 2004 11:46 AM
To: ccielab@groupstudy.com
Subject: Quiz question of the day 20040704
Hello everybody!
Happy 4th of July! It's time for your next brain-twisting quiz question of
the day!
R1(s0=.1)----12.12.12.0/24----(s0=.2)R2(lo0=.2)----2.2.2.0/24
R1 connect to R2 through a serial link with a network of 12.12.12.0/24 (R1
uses .1 and R2 uses .2).
R2 also has a loopback0 interface with 2.2.2.2/24.
R1 and R2 are running the following routing protocols between each other:
BGP (External)
RIP (Version 2)
EIGRP
OSPF
R2 advertises its 2.2.2.0/24 network into all of these routing protocols and
sends them to R1.
R1 and R2 have all of their routing protocols configured so that R1 receives
the exact same network (2.2.2.0/24) in all four routing protocols natively
(no redistribution). Routes are -NOT- summarized or entered as host routes
anywhere.
R1 has the administrative distances for each routing protocol set as
follows:
BGP (External) - AD set to 120
RIP (Version 2) - AD defaults to 120
EIGRP - AD set to 120
OSPF - AD set to 120
The clock rate of the serial interface between R1 and R2 is set to 64000,
but the interface bandwidth has not been modified from its default.
The metric derivation algorithm for each routing protocol has -NOT- been
modified.
To give all routing protocols a "fair chance", after all configuration is
set, both routers are reloaded at the same time.
Now for the questions------------------------
1) Once all routing protocols have converged, which route(s) from which
routing protocol(s) will be in the IP routing table?
2) If not all routes from all routing protocols will be active in the IP
routing table, which route(s) will be "backup(s)" for the primary route(s)
and in what order?
3) What is the determining factor for which route(s) get put into the IP
routing table as the primary route(s)?
Good luck with this one. I made up the question and I didn't get the results
I expected. Let's see if any of you can get it!
Best of luck, and enjoy your 4th of July holiday!
Ken
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