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New Member

How can routing protocol prevent network loop ?

I can't understand it.

STP can prevent loop by block loop port. How can routing protocol prevent network loop ?

thanks!

11 REPLIES
Green

Re: How can routing protocol prevent network loop ?

STP exists because there is no inherent control on a LAN segment - In the case of Ethernet, it is a bus-oriented (all devices connected in parallel) protocol, and MAC addresses are only significant to the segment/broadcast domain.

Routing protocols exist to control the potential for multiple paths; each routing protocol has some mechanism to look at all of the available paths and make a determination as to which path is most desirable.

Once that determination is made, then the local routing protocol will only use the most desirable path, as determined by the longest match with the destination address (network portion of the IP address).

Routing protocols all have some method to reduce or eliminate the chance of a routing loop unless the administrator overrides the protocol (by adding some bad static routes, face-to-face "Default Gateway" statements or similar config).

The bottom line is that IP adddressing provides enough information for the protocol to make a calculated decision for the best path, simple MAC addressing does not (there is no information to determine path, and it's only significant to the local segment / broadcast domain).

Good Luck

Scott

New Member

Re: How can routing protocol prevent network loop ?

Hi Scott,

Can you give more explain " Routing protocols all have some method to reduce or eliminate the chance of a routing loop " ?

What method is it?

thanks!

Re: How can routing protocol prevent network loop ?

hi Henrybb,

let say you are having multiple connection to the destination let say 2...and you are having routers in between two destination...now routing protocol select one best path on the basis of the metrics or cost its depend upon the routing protocol and router donot block the other path. so when ever source send the data it will travel through the best path.

then it will use the tools like splite horizon poision revers... hereby i am sending you one link which will help you to clear the issue how routing protocols prevent the loops????

http://www.certificationzone.com/newsletter/SL/IE-RIP-WP1-F03_RLP.html

remeber to rate the post if it helps

regards

Devang

Re: How can routing protocol prevent network loop ?

Hello Devang,

that's a nice link for loop prevention mechanism on distance vector routing protocols.

I'd also suggest him to research link-state routing protocols where loop prevention is inherent to the routing protocol.

OSPF is a good example.

Vlad

New Member

Re: How can routing protocol prevent network loop ?

Hi Vlad,

Can u give more detail about "link-state routing protocols where loop prevention is inherent to the routing protocol. " ?

I can't find these document.

thanks!

Re: How can routing protocol prevent network loop ?

hi,

you can have more materials on the following links

http://www.certificationzone.com/cisco/studyguides/?module=studyguides&action=showov&id=38

hope it will help you

regards

Devang

New Member

Re: How can routing protocol prevent network loop ?

Hi Devang,

I read the document which you provide.But I can't find any information about loop prevention.

Can you give other link ?

thanks!

Re: How can routing protocol prevent network loop ?

you can have more information on link state routing protocol on this link

http://www.cisco.com/en/US/tech/tk365/tk480/tsd_technology_support_sub-protocol_home.html

you can also search on cisco web site and during reading if you have any questions then you can post it in discussion form.

regards

Devang

New Member

Re: How can routing protocol prevent network loop ?

Hi Devang,

thanks for your reply.

I read some documention about OSPF but I still can't understand how link state routing protocol do loop prevention.

Does it do by know whole network topology ?

Re: How can routing protocol prevent network loop ?

Hello,

yes...

OSPF uses a Shortest Path First (SPF) algorithm to compute routes in the routing table. The SPF algorithm computes the shortest (least cost) path between the router and all the networks of the internetwork. SPF-calculated routes are always loop-free.

Instead of exchanging routing table entries like RIP routers, OSPF routers maintain a map of the internetwork that is updated after any change to the network topology. This map, called the link state database, is synchronized between all the OSPF routers and is used to compute the routes in the routing table. Neighboring OSPF routers form an adjacency, which is a logical relationship between routers to synchronize the link state database.

btw, EIGRP too has a loop prevetion algorithm.

Check:

http://www.cisco.com/warp/public/103/eigrp-toc.html#loopfree

Please rate any helpful posts.

vlad

Re: How can routing protocol prevent network loop ?

hi,

okay now i will explain you...

as per document i think you must have read about shortest path algo.

normally in OSPF each and ever router is having the clear idea about how each and every router connected with each other...right

normally ospf maintain three table...

topology table

neighbour table

routing table

each and every router having the same topology table...now each and every router run the SPF algorithm by trating it self as a ROOT and using that algorithm it calculate the best path towards the same destination...and install the best path towards the each destination in the routing table...and this process will run on all the router and find out the loop free best path to the destination...

hope this helps you

rate this post if it helps

regards

Devang

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