Routing table entries

borumas

I was wondering if anyone knew of any good places for documentation and examples of how and why some entries are added over others for the routing tables. Questions on them seem to be a weak area for me but I did not see very much info on them in the Cisco ICND book, an old Sybex CCNA book I have, or an old McGraw Hill ICND book I have laying around. One question I have is that static routes are always added to a routing table, this is correct right? Specifically I'm a bit confused when there's 2-3 devices that give the same route and appear to have the same metric as to which route gets added to the routing table.
I hope the questions make sense and thanks in advance for any help. I will be retaking the ICND soon so want to try to get this stuff cleared up.

tech-airman

borumas wrote:

I was wondering if anyone knew of any good places for documentation and examples of how and why some entries are added over others for the routing tables. Questions on them seem to be a weak area for me but I did not see very much info on them in the Cisco ICND book, an old Sybex CCNA book I have, or an old McGraw Hill ICND book I have laying around. One question I have is that static routes are always added to a routing table, this is correct right? Specifically I'm a bit confused when there's 2-3 devices that give the same route and appear to have the same metric as to which route gets added to the routing table.
I hope the questions make sense and thanks in advance for any help. I will be retaking the ICND soon so want to try to get this stuff cleared up.

borumas,

First, understand how each routing protocol by itself decides which routes are the best. Then, study the concept of "administrative distance."

Good luck.

Netstudent

if the adminstrative distance is the same and the metric is the same, it will load balance the equal paths.

tech-airman

Netstudent wrote:

if the adminstrative distance is the same and the metric is the same, it will load balance the equal paths.

Netstudent,

Which routing protocol does that?

Paul Boz

tech-airman wrote:

Netstudent wrote:

if the adminstrative distance is the same and the metric is the same, it will load balance the equal paths.

Netstudent,

Which routing protocol does that?

OSPF automatically load shares over up to four equal-cost links by default.

Netstudent

i believe RIP does and IGRP. IGRP can balance up to 6 unequal paths. With the right variance multiplier I might add.

borumas

Thanks for the replies, I will go back and reread the materials for each protocol to see how many equal paths each support, my other question about static routes is correct though right? I thought that the switch would assume that if someone manually entered in a static route that it should be an important enough path to add to the routing table, I believe I read something along those lines in my books.

tech-airman

borumas wrote:

Thanks for the replies, I will go back and reread the materials for each protocol to see how many equal paths each support, my other question about static routes is correct though right? I thought that the switch would assume that if someone manually entered in a static route that it should be an important enough path to add to the routing table, I believe I read something along those lines in my books.

borumas,

There is _one_ case where a static route would be overruled.

Source:

1. What is Administrative Distance? - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml

wait2dominate

tech-airman wrote:

borumas wrote:

Thanks for the replies, I will go back and reread the materials for each protocol to see how many equal paths each support, my other question about static routes is correct though right? I thought that the switch would assume that if someone manually entered in a static route that it should be an important enough path to add to the routing table, I believe I read something along those lines in my books.

borumas,

There is _one_ case where a static route would be overruled.

Source:

1. What is Administrative Distance? - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml

Isn't that when the administrative distance is set to higher than 1, or in reality, higher than whatever route you want to take precedence's Administrative distance?

I.E. Regular static route of 1 takes priority over all but connected routes....you want this to be a backup to eigrp(defualt distance of 90)

To achieve this, you make the regular route (ip route 0.0.0.0 0.0.0.0 Serial1), then add the administrative distance you want it to be(ip route 0.0.0.0 0.0.0.0 Serial1 250)

rjbarlow

Borumas, the speech is enough simple. Look the syntax for a static route:

ip route 110.9.5.0 255.0.0.0 137.24.43.9 130

that 130 is the parameter often omitted called administrative distance; if no value follow the three first values aftewr the command "ip route", a static route, like:

ip route 110.9.5.0 255.0.0.0 137.24.43.9

the administrative distance is the default; for static route is 1.

They are administrative distances for all routing protocols.

The rule is the best route is that with the lowest administrative distance and that will be written in the routing table.

Ah, not switch have routes, but routers.

Study administrative distance too.

dtlokee

Keep in mind the way that a router selects the route it installs in the routing table:

1. if there's only one known path to a prefix (route) install that prefix in the routing table
2. if there's more than one path to a prefix, select the one with the lowest administrative distance and install it in the routing table
3. if there's more than one path to the prefix with the same administrative distance, install the one with the lowest metric.
4. if there's more than one path to the prefix with the same administrative distance and metric, install up to the maximum number of paths (specified for each routeing protocol but is typically 4) into the routing table.

The administrative distance is only the second step in the process of selecting a route.

borumas

dtlokee wrote:

Keep in mind the way that a router selects the route it installs in the routing table:

1. if there's only one known path to a prefix (route) install that prefix in the routing table
2. if there's more than one path to a prefix, select the one with the lowest administrative distance and install it in the routing table
3. if there's more than one path to the prefix with the same administrative distance, install the one with the lowest metric.
4. if there's more than one path to the prefix with the same administrative distance and metric, install up to the maximum number of paths (specified for each routeing protocol but is typically 4) into the routing table.

The administrative distance is only the second step in the process of selecting a route.

That's awesome, basically everything I had questions on you answered in that post, now to make some flash cards with the max paths for each protocol. I had flash cards with the default administrative distances on them but not he # of paths. Thanks to everyone who responded.

dtlokee

That's awesome, basically everything I had questions on you answered in that post, now to make some flash cards with the max paths for each protocol. I had flash cards with the default administrative distances on them but not he # of paths. Thanks to everyone who responded.

Glad to help, another interesting note is the router will install the first paths to a destination up to the maximum paths that it receives. For example if you set the maximum paths to 2 for RIP the router will enter the first 2 routes it learns to the destination (with equal metrics of course) any more routes to the destination that it hears will be ignored. Now if you reload the router (or clear the routing table), it will learn the first 2 again but they may ot be the same 2 it learned before you reloaded the router. So the process is not deterministic.