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Routing administrative distance
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| I am going to attempt this exam on Monday and while reviewing I have managed to really confuse myself, I think it all has to do with nerves. Anyways using RIP would the router place a directly connected device on Ethernet0 in it's routing table verse another device with a distance of say 15 on Serial1. I know the connected device would show up as 0 and the other as 15 but which one would stand the greater chance of being applied?
I was looking through the ICND book and it was saying the lower the administrative distance the higher the likelihood of being used. So I guess my question is the directly connected device would appear in the routing table before a device with a distance of 15, is this safe to say? Thanks | |
| cyscoguru 2002-07-27, 6:55 pm |
| i don't get what you mean another device as administrative distance of 15. this is how adminitrative distance goes. and directly connected interfaces are always in the routing table.
0 directly connected
1 static routes
20 ebgp
90 eigrp
100 igrp
110 ospf
115 isis
120 rip
160 ipmobile <-- i believe that is right
170 external eigrp
200 ibgp
255 unreachable
you can change the administrative distance of the routing protocol under the routing process. ex:
router igrp 100
distance 150 <-- this sets igrp to adminstrative distance of 150 | |
| zee_shan99 2002-07-27, 8:15 pm |
| I agreed with the book. The greater the AD the less reliable the packet would become. Directly connected network has AD "zero", but when you assign the AD to any route that is the route's AD. And if you used the same network in the route as directly connected offcourse your route is useless b/c router will look for the network with the less AD which is zero or directly connected. Using RIP you can't have static routes, i think this is because RIP AD is 120 and static are 1, it wont work.
Directly connected would stay there...
what do you think...... | |
| cyscoguru 2002-07-27, 8:40 pm |
| ok, here is what happens when a routers recieves two routes. ok here is an example. you have a router that has two routing processes, rip and igrp for simplicity, both send there whole routing table for there updates. the router recieves a route for example 192.168.1.0/24 from both routing processes. all the router will do is compare the administrative distance of both routing processes and pick igrp becuase it has a lower administrative distance. all administrative distance is the believability of the route. who/what is the router going to chose. now with static routes. say for example again you make a static route to the network 192.168.1.0. and you are still running both of those routing processes. its now going to choose the static entry and place it in the routing table becuase of AD of 1. so no matter what if you are not running any routing protocols your directly connected network are inserted in the routing table. becuase if it has two interfaces and you have two networks 192.168.1.0 & 192.168.2.0. it will insert as follows
C 192.168.1.0/24 [0|1] directly connected e1
C 192.168.2.0/24 [0|1] directly connected e0
i don't know exactly what it says but thats pretty close. that way if the router gets a packet from a host 192.168.1.13 to a destenation of 192.168.2.65 it will send it to its default gateway and the router will know where to pass it (e0).
hope this helps. | |
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| quote:
Originally posted by cyscoguru
ok, here is what happens when a routers recieves two routes. ok here is an example. you have a router that has two routing processes, rip and igrp for simplicity, both send there whole routing table for there updates. the router recieves a route for example 192.168.1.0/24 from both routing processes. all the router will do is compare the administrative distance of both routing processes and pick igrp becuase it has a lower administrative distance. all administrative distance is the believability of the route. who/what is the router going to chose. now with static routes. say for example again you make a static route to the network 192.168.1.0. and you are still running both of those routing processes. its now going to choose the static entry and place it in the routing table becuase of AD of 1. so no matter what if you are not running any routing protocols your directly connected network are inserted in the routing table. becuase if it has two interfaces and you have two networks 192.168.1.0 & 192.168.2.0. it will insert as follows
C 192.168.1.0/24 [0|1] directly connected e1
C 192.168.2.0/24 [0|1] directly connected e0
i don't know exactly what it says but thats pretty close. that way if the router gets a packet from a host 192.168.1.13 to a destenation of 192.168.2.65 it will send it to its default gateway and the router will know where to pass it (e0).
hope this helps.
Ok I got that part, but what happens if you have RIP routes as follows:
R 192.168.11.0 [120/7] via 192.168.2.0, S0
R 192.168.3.0 [120/15] via 192.168.2.2, S1
C 192.168.2.0 [0/1] directly connected e0
of course there would be more in the "show ip route" but would the directly connected route show up in the receiving routers routing table or would the RIP route [120/15]sshow up? | |
| cyscoguru 2002-07-27, 9:17 pm |
| there is a mistake 192.168.11.0 can't take 192.168.2.0, that 2.0 is a network address. i am guessing its 192.168.2.2. but you said 192.168.2.0 its directly connected to e0 but you are also saying to get to 192.168.3.0 take 192.168.2.2 via s1. its e0. i'll revise that. ok i am guessing you added 192.168.2.0 in the rip routing process. so that would be in other routers routing table. the router won't send 192.168.3.0 out e0 becuase of split horizon issues. but if you had another rip router attached to like a e1 or any other serial interface ther router will recive that route but not put in its routing table becuase of rip hop limitation of 16. so here is what i think you thought.
R 192.168.11.0 [120/7] via 192.168.2.2
R 192.168.3.0 [120/15] via 192.168.2.2
C 192.168.2.0/24 is directly connected, E1
C 192.168.4.0/24 is directly connected, E0
so think there is another router attached to E0 so that router will have 192.168.4.0 as directly connected and a R of 192.168.11.0 of [120/8]. but 192.168.3.0 won't be in there. you can actually see when you recieve and update with DEBUG IP RIP. you will see it recieve the route of 3.0 and after that it will say inaccessable.
hope that clears everything up. | |
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| quote:
Originally posted by cyscoguru
there is a mistake 192.168.11.0 can't take 192.168.2.0, that 2.0 is a network address. i am guessing its 192.168.2.2. but you said 192.168.2.0 its directly connected to e0 but you are also saying to get to 192.168.3.0 take 192.168.2.2 via s1. its e0. i'll revise that. ok i am guessing you added 192.168.2.0 in the rip routing process. so that would be in other routers routing table. the router won't send 192.168.3.0 out e0 becuase of split horizon issues. but if you had another rip router attached to like a e1 or any other serial interface ther router will recive that route but not put in its routing table becuase of rip hop limitation of 16. so here is what i think you thought.
R 192.168.11.0 [120/7] via 192.168.2.2
R 192.168.3.0 [120/15] via 192.168.2.2
C 192.168.2.0/24 is directly connected, E1
C 192.168.4.0/24 is directly connected, E0
so think there is another router attached to E0 so that router will have 192.168.4.0 as directly connected and a R of 192.168.11.0 of [120/8]. but 192.168.3.0 won't be in there. you can actually see when you recieve and update with DEBUG IP RIP. you will see it recieve the route of 3.0 and after that it will say inaccessable.
hope that clears everything up.
Yes thanks! So if performing a show ip route and I get your table above, which route would not be entered into the routing table of the receiving router? Would it be safe to say then that R 192.168.3.0 [120/15] wouldn't because of the administrative distance of 15? Hope I am not making this more difficult than it has to be. | |
| cyscoguru 2002-07-27, 9:32 pm |
| thats ok. you have it mixed up. [120/15] 120 is the administrative distance. the 15 is the metric and as follows the routing protocol judge the metrics by.
hop count rip
bandwith and delay igrp
bandwith and delay eigrp
bandwith ospf
bandwith isis
hope this helps. | |
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| Thank you so much cyscoguru! Helped me out alot. | |
| cyscoguru 2002-07-27, 9:37 pm |
| no problem. anytime! |
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