BGP attribute

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Jan 20th, 2008
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guys just a quick one......can someone please tell me why would you use as orgin attribute (like does it control traffic coming in or going out)??? and what abt MED.....i know abt local preference and weight is for going out???thnaks in advance guys

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s.arunkumar Sun, 01/20/2008 - 20:18
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"Origin code" is an attribute which used in path selection process along with other attributes.Three types of origin code as there

1.IGP(i)- normaly routes advertise by network cmd will have this.

2.EGP(e) - routes learned from EGP.

3.incomplete(?)- routes which are learned via redistribution..


MED is multi-exit-discriminator.It is condidered as a metric for BGP.It is normaly used to control incoming traffic,when multiple entry are there towards the AS.


arun

Istvan_Rabai Mon, 01/21/2008 - 13:45
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Hi,


The mentioned attributes are used in the BGP path selection process.


When a BGP router has more than one route in the BGP table, it has to decide which one is the best to offer to the routing table.


1. First it will choose a route with the highest weight.


2. If the routes are with the same weight, it will choose the route with the highest local preference value.


3. If the routes are with the same local preference, then it will chose the route originated by the local router (by itself). You can see this by the next-hop 0.0.0.0


4. If no paths can be selected in step 4. the it wiil choose the path with the shotest AS-path attribute.


5. If no path can be selected in step 4. it will choose the path with the lowest origin code.

To answer your question: the router prefers routes that were introduced into BGP with the "network" command over redistributed routes. Redistributed routes are considered less reliable or stable. So this controls outgoing traffic.

Just think with your common sense: you have two paths to the same destination: one is originated in BGP, the other is redistributed to BGP from RIP, OSPF, EIGRP or IS-IS.

Which is more likely to be the best route?


6. When no path can be selected in step 5., then the router will choose the path with the lowest MED value (this value may come from the neighboring autonomous system (AS)).

This way the neighboring AS can exert some degree of control on my outgoing traffic, which is the incoming traffic from the viewpoint of the neighboring AS.


....


and so on, there are more points to the list.


Cheers: Istvan


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