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Next Hop Attribute on Multiaccess Media

http://img143.imageshack.us/img143/5610/nexthoponmultiaccessmediait0.jpg

Cisco says :

[[In Figure , RTA, RTB, and RTC share a common multiaccess medium. RTA and RTC are running EBGP, while RTC and RTB are running OSPF. RTC has learned network 11.11.11.0/24 from RTB by way of OSPF and is advertising it to RTA by way of EBGP. Since RTA and RTB are running different routing protocols, it would seem logical that RTA would consider RTC (10.10.10.2) as its next hop to reach 11.11.11.0/24. However, this would not happen. The correct behavior is for RTA to consider RTB, 10.10.10.3, as the next hop because RTB shares the same medium with RTC]].

1- What are these two different protocols (between these ip addresses 10.10.10.3 and 10.10.10.1)?

2- He has mentioned that: "RTB shares the same medium with RTC", doesn?t RTB share the same medium with RTA as well ?

3- I did not get why he said : ?this would not happen.? ?

3 REPLIES
Cisco Employee

Re: Next Hop Attribute on Multiaccess Media

This behavior is explained in RFC4271 section 5.1.3.

http://www.ietf.org/rfc/rfc4271.txt?number=4271

1- The reason RTC sends RTB address as the next-hop is that the three routers are on the same subnet.

2- The statement is a bit unclear. All three routers have to be on the same subnet for this behavior to happen.

3- What the author means is that RTC wouldn't pass its own IP address as the next hop. It would rather pass RTB's IP address.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Re: Next Hop Attribute on Multiaccess Media

1: RTA and RTB are not exchanging their routing tables directly with eachother. This is due to the fact that they are not running the same routing protocol.

2: Yes, all three routers share the same medium.

3:The less ideal forwarding would be to sent the data to RTC first. It would then cross the same medium twice. The optimal routing is to forward directly to the router that is nearest to the destination (RTB). That is in fact what is happening because the next-hop attribute is exchanged with the OSPF update from RTC.

Regards,

Leo

New Member

Re: Next Hop Attribute on Multiaccess Media

((This is due to the fact that they are not running the same routing protocol.))

Could you please tell me what are these two different protocols ? I meant to say what is routing protocol running on RTB's interace (10.10.10.3), because the author has not mentioned!!!! ,,,Is it OSPF as well ?

My understanding (correct me) to phrase "two different routing protocols" is two different IGPs routing protocol,,,,for this reason I wonder what are these two

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