ORIGIN Attribute E

Unanswered Question
Feb 16th, 2009

Hello,

when I run a command to check the bgp routing table I see couple of routes

coming in via with the attribute as origin E.

Could you please tell me why do we have E as origin when EGP is not used as a routing protocol any where ... Instead its EBGP

Regards

Ajai

I have this problem too.
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Paolo Bevilacqua Mon, 02/16/2009 - 02:53

Connect to the router that originates the route, and you will find the reason with it's EBGP.

You cannot be sure on anything until you do so.

Paolo Bevilacqua Mon, 02/16/2009 - 04:15

Actually, EGP is the 'official' term for "external origin" in BGP specification, see rfc1771 4.3

Since it cannot be excluded that BGP is the only existing External Gateway Protocol, the more generic acronym is used.

Jon Marshall Mon, 02/16/2009 - 04:17

Paolo

Agreed, i just think Ajai may be getting confused because he is seeing EGP and in 99.9% of cases that will be EBGP.

I think the label EGP is just misleading nowadays.

Jon

Harold Ritter Mon, 02/16/2009 - 07:10

Jon,

What is being stated by the document you are referring to is incorrect. EGP in this context really means EGP as defined by RFC904.

I will make sure it gets fixed.

Regards

Jon Marshall Mon, 02/16/2009 - 04:25

Ajai

Just to add to previous posts.

IGP = Interior Gateway Protocol

IGP examples are RIP, RIPv2, IGRP, EIGRP, OSPF

EGP = Exterior Gateway Protocol

EGP examples are EGP, EBGP

And this is where the confusion arises i think. EGP is a general term for all Exterior Gateway Protocols but it is also a specific implementation of a routing protocol.

When you see EGP in relation to origin with BGP it is referring to the general term rather than the specific implementation.

Jon

athambi Mon, 02/16/2009 - 06:00

Let me make it a little bit more clear .Lets Say we have router A in AS 100 and Router B in AS 200. They have an EBGP peering .Now router A is advertising the route 192.168.10.0/24 to router B. Question is that when I do the command “show ip bgp”

In router B how will the ORIGIN attribute appear in the BGP table will it be E or I.

Hope I am clear .

Harold Ritter Mon, 02/16/2009 - 07:07

Ajai,

Just to clarify, EGP in the BGP origin attribute context really means EGP, as in the protocol that was used before BGP and that is defined by RFC904.

http://www.ietf.org/rfc/rfc0904.txt?number=904

This was not clear in RFC1771 but it is now in RFC4271.

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

The origin attribute will be set to "e" under two conditions. The first one being if the route is redistributed into BGP from EGP, which is very unlikely these days, given that no customer that I know of is still running EGP. EGP has also been pulled from recent IOS versions. The second one is if it is being explicitly set using a route-map as follow:

route-map test permit 10

set origin egp

Regards

athambi Mon, 02/16/2009 - 19:19

Hello Hritter,

I understood the second condition where we use the route-map to set the origin and that was always in my mind .

Could you please get me an exmaple of the first condition of redistributing an EGP into BGP

Cheers

Ajai

Harold Ritter Tue, 02/17/2009 - 07:27

Ajai,

I would just like to reiterate that EGP has, as far as I know, disappeared from the face of the earth. As I also mentioned, it has been pulled from all recent IOS releases as it is not needed anymore. So you are probably more likely to get hit by lightning twice in the same day than seeing that first condition ;-)

As for the example, It would look something like this:

autonomous-system 2

!

router egp 1

redistribute bgp 2

network 2.0.0.0

neighbor 192.168.12.1

!

router bgp 2

bgp log-neighbor-changes

redistribute egp 1

neighbor 192.168.23.3 remote-as 3

!

The prefix received from EGP and redistributed into BGP would look like this in BGP:

BGP routing table entry for 1.0.0.0/8, version 2

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

Not advertised to any peer

2 1

192.168.23.2 from 192.168.23.2 (2.2.2.2)

Origin EGP, metric 1, localpref 100, valid, external, best

R3#

Regards

Mohamed Sobair Mon, 02/16/2009 - 04:34

Hi,

I think you didnt understand the meaning of the term "EGP" correctly.

"EGP" means Exterior Gateway Protocol and since BGP is the Only Exterior Protocol used to connect Autonomous System boundaries, thats why Border Gateway Protocol is being represented as EGP.

The Origins in BGP are 3:

1- IGP

2- EGP

3- Incomplete

The lowest Origin is more perfered as described in the BGP best path Seletion process.

The term "EGP" in the Origin means the route is learned via EBGP.

HTH

Mohamed

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