BGP origin types ( a quick explaination )

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If you ever worked with BGP, you will see the route information contain; EGP/IGP/INCOMPLETE. What does this mean?

·       IGP  =  means the prefix was originated from  routing information learned from an interior gateway protocol such as  OSPF/EIGRP/RIP

·       EGP = means the prefix originated from routing information obtained from a  the EGP protocol such as some other  eBGP router

·       INCOMPLETE = means the prefix originate mainly from  aggregate statement or via redistribution of a static route.

Take a look at this prefix as seen on a route-server at host dot net ( telnet route-server.host.net );

route-server> show ip bgp 27.89.0.0/16

BGP routing table entry for 27.89.0.0/16

Paths: (1 available, best #1, table Default-IP-Routing-Table)

  Not advertised to any peer

  13645 19151 2516

    64.135.0.1 from 64.135.0.1 (64.135.0.1)

      Origin IGP, localpref 100, valid, external, best

      Community: 13645:3111

      Last update: Mon Jan 21 13:40:18 2013

 

The above shows the originate  AS == 2516 and via IGP

Now look at the following;

route-server> show ip bgp 1.229.138.0/24

BGP routing table entry for 1.229.138.0/24

Paths: (1 available, best #1, table Default-IP-Routing-Table)

  Not advertised to any peer

  13645 19151 9318 38388

    64.135.0.1 from 64.135.0.1 (64.135.0.1)

      Origin incomplete, localpref 100, valid, external, best

      Community: 13645:3111

      Last update: Mon Jan 21 13:40:14 2013

The above originated via AS38388 but the origin information was incomplete. So we can guess it was via aggregation or a redistribution process.

And now the  following;

show ip bgp ipv4 unicast  172.16.29.0/24

BGP routing table entry for 172.16.29.0/24, version 124

Paths: (1 available, best #1, table default, not advertised to EBGP peer)

Multipath: eBGP

  Advertised to update-groups:

     65001   

  Local

    10.0.0.1 (metric 91) from 10.11.0.1 (10.11.0.1)

      Origin EGP, metric 0, localpref 220, valid, internal, best

      Community: 65001:777 no-export

      Originator: 10.11.1.26, Cluster list: 10.23.44.1

Okay the last  route was something that I created in my local LAN.  Notice in all samples, how the origin source are all unique?

Okay what does this mean in  bgp path selection? 

Will part of the tie breaker  when we have paths for the same destination, but with different origin type is to prefer lowest origin code.

IGP

  Than 

EGP

  Than

Incomplete

So we select paths that are from a IGP process,  over those from a EGP process and like-wise if the origin type is incomplete, is least preferred.

Think of it in this fashion; Who would be more trusted  if you want to call some one directly and had their  phone #?

1: the person that has his/her number and gives it to you directly?

or

2: the a phone number found in the white pages of the phonebook ?

or

3: some number found laying on the ground and we have no clue as to who/what/where that number originated from?

No this means very little in real-life, because we can enforce or control the origin type via route-map

i.e

Enter configuration commands, one per line.  End with CNTL/Z.

rtr1(config)#route-map setorigin permit 10

rtr1(config-route-map)#set origin?

  egp         remote EGP
  igp         local IGP
  incomplete  unknown heritage

So we can change any of the originate information or attribute.  I hope this helps you in your understanding of BGP

origin-types

Ken Felix

Freelance Network/Security Engineer

Kfelix     at hyperfeed d-o-t com