EIGRP Next Hop

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Mar 29th, 2009
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Hi all, I came across a technical question regarding the Next Hop field in EIGRP packet and I really appreciate you to have some spare time to look into this.

According to technical references, the Next Hop field in the EIGRP IP Internal and External Routes TLVs identifies the next-hop router to send packets destined for a destination network.

I have performed some POC regarding the Next Hop field but it seems doesn't work as it intended to. I have tried a few IOSs but the results are still the same. Kindly refer to the attached network diagram for the discussion of the problem. The destination network in question - 172.16.1.0/24 is connected to RT3. RT2 has a static route to RT3. RT1 and RT2 are EIGRP neighbors and RT2 redistribute the static route to RT1 as IP external route. But the next hop is still pointing to RT2 instead of RT3 directly.

With OSPF, RT1 will point directly to RT3 when the static route is redistributed from RT2 to RT1. OSPF is making use of the Forwarding Address field in the AS External LSA to achieve this.


I have also attached the sh ip route on RT1 and RT2 when they are running in EIGRP and OSPF respectively.

I purposely not having the network statement in RT3 in order to observe how EIGRP on RT2 redistributed the static route to RT1. In real world, RT2 and RT3 would probably running BGP and RT2 is redistributing the BGP route into EIGRP.

With OSPF, the static route would be redistributed from RT2 to RT1 to directly forward the traffic to RT3 instead of going through RT2, which is an extra hop. Thanks.


Attachment:

http://rapidshare.com/files/214944023/eigrp-next-hop.zip.html

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Chin Hoong Yap Mon, 04/06/2009 - 22:37
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Thanks for your reply. I have tried that command and it doesn't really work.

Laurent Aubert Tue, 04/07/2009 - 11:10
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  • Cisco Employee,

Hi,


I made some more research and found out that this feature is not supported when the router is redistributing connected or static routes.


The main purpose of this feature is to allow spoke-2-spoke connection in DMVPN environment. In this case all the routes are native EIGRP


HTH


Laurent.



Chin Hoong Yap Tue, 04/07/2009 - 17:54
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I believe when Cisco invented EIGRP, no one ever though about DMVPN.


Below shows that some inefficient routing occurs due to IP redirection.

RT1#sh ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


172.16.0.0/24 is subnetted, 1 subnets

D EX 172.16.1.0 [170/30720] via 10.10.10.2, 00:01:56, FastEthernet0/0

10.0.0.0/24 is subnetted, 1 subnets

C 10.10.10.0 is directly connected, FastEthernet0/0

RT1#

RT1#debug ip icmp

ICMP packet debugging is on

RT1#

RT1#ping 172.16.1.1 repeat 1


Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:

!

Success rate is 100 percent (1/1), round-trip min/avg/max = 92/92/92 ms

RT1#

*Mar 1 00:06:47: ICMP: redirect rcvd from 10.10.10.2- for 172.16.1.1 use gw 10.

10.10.3

*Mar 1 00:06:47: ICMP: echo reply rcvd, src 172.16.1.1, dst 10.10.10.1

RT1#

RT1#ping 172.16.1.1 repeat 1


Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:

!

Success rate is 100 percent (1/1), round-trip min/avg/max = 36/36/36 ms

RT1#


So next time I have something to share for comparison between EIGRP and OSPF. :)

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