BGP default-information originate

Answered Question
Apr 19th, 2007

Hi BGP experts,

How does default-information originate advertise default route? We have lots ISPs and we are only asking those ISPs to advertise default routes to our network. We are connected to ISPs using eBGP.

One of our ISP router advertise 0.0.0.0 network to our internal network via iBGP by using default-information originate command. Actually, not only one ISP router was configured this way. All of them except one.

However, we have one ISP router also in a different site that advertises 0.0.0.0 to its iBGP peers without the default-information originate command. How did this happen?

Take note that 0.0.0.0 is always being advertised by the ISP. We don't have static routes to 0.0.0.0 configured in any of our routers.

What if I remove the default-information originate to all of the ISP routers? What will happen? Since one of our ISP router was configured this way.

Need clarification. Thanks Guys.

-John

I have this problem too.
0 votes
Correct Answer by mohammedmahmoud about 6 years 12 months ago

Hi John,

I am glade that you are building it up right, but please note that BGP never advertise a default route (iBGP or eBGP) unless one of the 3 actions explained above is taken.

HTH,

Mohammed Mahmoud.

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Average Rating: 4.8 (11 ratings)
royalblues Fri, 04/20/2007 - 00:10

John,

The default-information originate command is used to configure a BGP routing process to advertise a default route (network 0.0.0.0).

A redistribution statement must also be configured to complete this configuration or the default route will not be advertised.

The configuration of the default-information originate command in BGP is similar to the configuration of the network (BGP) command. The default-information originate command, however, requires explicit redistribution of the route 0.0.0.0. The network command requires only that the route 0.0.0.0 is present in the Interior Gateway Protocol (IGP) routing table. For this reason, the network command is preferred.

if u remove the default-information originate command, the routers which were receiving this prefix via this router would purge this entry and can cause an outage as well

HTH, rate if it does

Narayan

mohammedmahmoud Fri, 04/20/2007 - 00:57

Hi John,

A default route can be originated from a BGP neighbor in 3 ways:

1. default-information originate + redistribute static (or any dynamic routing protocol having the default route - you may filter only the default route)

2. network command but must make sure the default route is present in the routing table

3. Another way of advertising a default route to a specific BGP neighbor is by issuing the neighbor default-originate command. This method does not require the presence of the 0.0.0.0/0 network in the routing table of the advertising router.

The configuration of the default-information originate command in BGP is similar to the configuration of the network (BGP) command. The default-information originate command, however, requires explicit redistribution of the route 0.0.0.0. The network command requires only that the route 0.0.0.0 is present in the Interior Gateway Protocol (IGP) routing table. For this reason, the network command is preferred.

Check which method did this ISP router use in order to take the right decision, before removing or changing anything.

HTH, Please rate if it does,

Mohammed Mahmoud.

John Patrick Lopez Sun, 04/22/2007 - 19:20

Thanks for the reply guys. We don't have any redistribution in any of our ISP routers. I just want to ask what is the difference between the two? Here's a config of one of our edge routers.

router bgp 2456

no synchronization

bgp log-neighbor-changes

network 203.167.7.0

timers bgp 15 45

neighbor 10.199.2.2 remote-as 2456

neighbor 10.199.2.2 update-source FastEthernet0/0

neighbor 10.199.2.2 next-hop-self

neighbor 10.199.2.3 remote-as 2456

neighbor 10.199.2.3 update-source FastEthernet0/0

neighbor 10.199.2.3 next-hop-self

neighbor 10.199.2.112 remote-as 2456

neighbor 10.199.2.112 update-source FastEthernet0/0

neighbor 10.199.2.112 next-hop-self

neighbor 10.199.2.113 remote-as 2456

neighbor 10.199.2.113 update-source FastEthernet0/0

neighbor 10.199.2.113 next-hop-self

neighbor 203.167.2.242 remote-as 968

neighbor 203.167.2.242 ebgp-multihop 255

neighbor 203.167.2.242 update-source FastEthernet0/1

neighbor 203.167.2.242 version 4

default-information originate

no auto-summary

This configuration advertises default-route to 10.199.2.2 and 10.199.2.3. These are internal routers.

Here's another configuration.

router bgp 2456

no synchronization

bgp log-neighbor-changes

network 121.97.1.0 mask 255.255.255.0

neighbor 10.19.0.2 remote-as 2456

neighbor 10.19.0.2 update-source GigabitEthernet0/0

neighbor 10.19.0.2 next-hop-self

neighbor 10.19.0.3 remote-as 2456

neighbor 10.19.0.3 update-source GigabitEthernet0/0

neighbor 10.19.0.3 next-hop-self

neighbor 121.97.7.93 remote-as 6648

neighbor 121.97.7.93 soft-reconfiguration inbound

This also advertises default-route to 10.19.0.2 and 10.19.0.3 which are internal routers. But this was configured without the default-information originate command. What's the difference between the two? We only accept default-routes from the ISP. Thanks Guys.

mohammedmahmoud Sun, 04/22/2007 - 23:36

Hi John,

The "default-information originate" command in BGP doesn't work without redistribution (search for it under the address-family ipv4 under the BGP process), or else search for the "network 0.0.0.0" or finally Check for this command under bgp process under the address-family ipv4:

address-family ipv4

neighbor x.x.x.x default-originate

HTH, please rate if it does help

Mohammed Mahmoud.

John Patrick Lopez Tue, 04/24/2007 - 04:09

So the second configuration is really enough to advertise default routes to iBGP peers since I'm receiving default routes from the ISP?

So what you mean is that, the purpose of default-information originate command is to advertise default-route if the default-route was received from IGP like OSPF/EIGRP or if it was statically configued?

But if the the default-route was received from an eBGP peer then the edge router will automatically advertise this to internal network?

Correct Answer
mohammedmahmoud Tue, 04/24/2007 - 04:41

Hi John,

I am glade that you are building it up right, but please note that BGP never advertise a default route (iBGP or eBGP) unless one of the 3 actions explained above is taken.

HTH,

Mohammed Mahmoud.

John Patrick Lopez Tue, 04/24/2007 - 04:47

So BGP will not advertise default-route to iBGP or eBGP peer if I don't have a route to 0.0.0.0? For example, my edge router is not running eBGP with the ISP. I can't make my edge router advertise 0.0.0.0 to iBGP peers if I don't use a static route pointing to ISP then redistribute it to BGP? Then I need to issue default-information originate command to advertise 0.0.0.0 to iBGP peers.

Because I was thinking of the default-information originate being used by OSPF. Correct me if I'm wrong, in OSPF, if I use "default-information originate always" then I'll be able to advertise a default route to the network even if I don't have a route to 0.0.0.0?

Harold Ritter Tue, 04/24/2007 - 04:59

the only purpose of the "default-information originate" command in BGP is to allow the redistribution of the default route in BGP. It doesn't generate the default route as it is the case for OSPF.

The "neighbor default-originate" command on the other hand, does advertise the default route to the specific neighbor whether it is present in BGP or not.

Hope this helps,

mohammedmahmoud Tue, 04/24/2007 - 05:14

Hi John,

In the case of BGP, if you are using the "default-information originate" the default route must be in the routing table, but if using the "neighbor x.x.x.x default-originate" in front of a neighbor the default route doesn't need to be in the routing table.

While OSPF has the always keyword, BGP doesn't.

HTH,

Mohammed Mahmoud.

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Posted April 19, 2007 at 11:51 PM
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