Explain the Routing table behaviour

[libra_ali786]

Hi i am new and preparing the CCNA exam, I just ceated one topology and running the Rip version 2 and Eigrp , When i look the routing table R0 and R3 i seen the Rip learned the 4 routes and Eigrp learned 3 Routes

[Deepak Kumar]

Hi,

I think you asked about the highlited part of the routing table. Which is learning from R (RIP) routing protocol and showing "Possibly Down.

Every dynamic routing protocol is having some timer settings for the update, invalid, Hold, flush the routing table if required. Same RIP process level timers for the update, invalid, hold down and flush.

Default timer as, all in seconds:

Update: (30) how often to send updates in seconds
Invalid: (180) how many seconds, since seeing a valid update, to consider the route invalid, and placing the route into hold down
Hold Down: (180) Once in hold down, how long (in seconds) to “not believe” any equal or less impressive (worse) route updates for routes that are in hold down
Flush: (240) how many seconds, since the last valid update, until we throw that route in the trash (garbage collection for un-loved non-updated routes)

When the router not received any routing update till to Invalid time and put it in Hold down. While the routes are in hold down, the router still forwards packets to those networks, based on the last information that it last learned about how to reach those networks. The routes will show up as “possibly down”.

more practical and study, visit below link:

http://blog.ine.com/2010/04/15/how-basic-are-rip-timers-test-your-knowledge-now/

Regards,

Deepak Kumar

[libra_ali786]

OK. Also explain why rip learned the 4 routes and eigrp 3?

[Deepak Kumar]

Hi,

I am not sure about your configuration. It is difficult to give a correct answer of your new query. But I am assuming that EIGRP is much faster protocol compare to RIP. So It's 4th route has been flushed from routing table. 

If you need a correct answer, share your topology with all configuration. 

Regards,

Deepak Kumar

[libra_ali786]

Please see the attached topology. Change the file extension into .pkt     , it is a packet tracer topology.

Because they are not allowed upload packet tracer file.

[Deepak Kumar · Accepted solution]

Hi,

I got your point and I will explain the complete story:

Q1. Why is EIGRP learning only 3 routes?

Ans: In the attached topology, you have 6 networks. In any of the router suppose on R3 router you have 3 connected network and 3 remote networks. As you know that routing table is depending on some factors as Prefix, AD, Metric etc. Now Three connected network is having lowest AD value 0 so that three remote networks are installed in the routing table and rest three networks are on the remote network, which is learned from EIGRP AD value 90.

Q2. Why is RIP route learning if you have EIGRP too?

Ans: You configured RIP2, which is supporting to Subnetting and VLSM but by default RIP v2 is not a classless routing protocol. You have to run a command as "No Auto-Summery" on the router which is default enabled on the Cisco router.  So In the attached topology RIP2 is still working as a classful routing protocol and its learning  like 

10.0.0.0/8 [120/2] via 50.50.50.1, 00:00:18, Serial0/0/1

As I mentioned earlier that routing table depends on many factors and first factors as Prefix length. Here RIP is working as classful protocol show it is learning different subnet compare to EIRP protocol. So this is the reason that RIP (120) is having more AD value then EIGRP (90) but it installed in the routing table. Here, a router is understanding that it is learning different subnets from another routing protocol. 

Q3. Why is rip learning 4 routes?

Ans: I feel that this is a Packet Tracer bug because It must be flush the route which is more than 15 (0-15) hops count. I checked all debug on packet tracer and found that two routes:

R 50.0.0.0/8 is possibly down, routing via 40.40.40.1, Serial0/0/0

R 20.0.0.0/8 is possibly down, routing via 50.50.50.1, Serial0/0/1

are learned from 16 hops away. Its making loop in the routing table. It looks like as packet tracer bug.

Regards,

Deepak Kumar

[libra_ali786]

Thanks you Deepak.