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ospf forwarding address

swapansamanta
Level 1
Level 1

In this R1,R2 and R3 are running is ospf 1 area 0,and ,R3 is redistribute  point.if R3,R2and R4 are running rip when i traceroute from R1 to R4 it takes path R1-R2-R3-R4.but when R3,R2nad R4 are running eigrp.it takes path R1-R2-R4.R3 int f0/0 is not enable ospf...in both casses..

i am not able to understand why this is...

regards

swapan..

1 Accepted Solution

Accepted Solutions

Peter Paluch
Cisco Employee
Cisco Employee

Hi Swapan,

I think that the Administrative Distance (AD) is the answer here.

If you run OSPF and RIP, then on R2 and R3 that run both OSPF and RIP, OSPF-learned routes are preferred because the AD of OSPF is 110, lower than the AD of RIP which is 120. So, when R3 redistributes the RIP routes into OSPF, R2 will replace the RIP-learned route toward R4 with the OSPF-learned route through R3. Therefore, when you perform a traceroute from R1 to R4, R1 sends the packets to R2 (because it is on its path toward R3 as the ASBR), R2 forwards it to R3, and R3 then forwards the packets to R4 through the RIP-learned route.

However, when R2 and R3 run EIGRP, things get different because EIGRP has a lower AD than OSPF (90 vs. 110). Even though R3 performs redistribution from EIGRP to OSPF, for R2, this path is less trustworthy than the path learned via EIGRP from R4 directly, so R2 will keep the EIGRP-learned path in its routing table. While R1 learns about the path to R4 through OSPF (because the redistribution still takes place), as soon as its packets arrive at R2, R2 will forward the packets along the EIGRP-learned path to R4 directly.

One possible solution is to increase the administrative distance of EIGRP on R2 to a value higher than 110, say:

router eigrp 1
 distance eigrp 111 170

The first value is the AD for internal routes which we are going to increase above 110, and the second is the AD for external routes (those redistributed into EIGRP); we're going to keep that value at its defaults.

Best regards,
Peter

View solution in original post

2 Replies 2

Peter Paluch
Cisco Employee
Cisco Employee

Hi Swapan,

I think that the Administrative Distance (AD) is the answer here.

If you run OSPF and RIP, then on R2 and R3 that run both OSPF and RIP, OSPF-learned routes are preferred because the AD of OSPF is 110, lower than the AD of RIP which is 120. So, when R3 redistributes the RIP routes into OSPF, R2 will replace the RIP-learned route toward R4 with the OSPF-learned route through R3. Therefore, when you perform a traceroute from R1 to R4, R1 sends the packets to R2 (because it is on its path toward R3 as the ASBR), R2 forwards it to R3, and R3 then forwards the packets to R4 through the RIP-learned route.

However, when R2 and R3 run EIGRP, things get different because EIGRP has a lower AD than OSPF (90 vs. 110). Even though R3 performs redistribution from EIGRP to OSPF, for R2, this path is less trustworthy than the path learned via EIGRP from R4 directly, so R2 will keep the EIGRP-learned path in its routing table. While R1 learns about the path to R4 through OSPF (because the redistribution still takes place), as soon as its packets arrive at R2, R2 will forward the packets along the EIGRP-learned path to R4 directly.

One possible solution is to increase the administrative distance of EIGRP on R2 to a value higher than 110, say:

router eigrp 1
 distance eigrp 111 170

The first value is the AD for internal routes which we are going to increase above 110, and the second is the AD for external routes (those redistributed into EIGRP); we're going to keep that value at its defaults.

Best regards,
Peter

hello peter , thank you for your quick response...i have one more question.In this i am redistributing rip and ospf..

1.I am not enable ospf R3 f0/0 interface..That time when i traceroute from R1 to R4. Route take the R1-R2-R3-R4 path, advertising router is R3 router-id and forward address 0.0.0.0.

2.I am enable ospf R3 f0/0 interface.Route take R1-R2-R4 direct path,advertising router is same R3 router-id.but forward address R4 address(f0/0 interface address)

Explain in details...why this is happening...i have seen many blog...but i do not get appropriate answer...

regards

swapan