BGP default-information originate

[jpl861]

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

[royalblues]

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]

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.

[jpl861]

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]

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.

[jpl861]

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?

[mohammedmahmoud · Accepted solution]

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.

[jpl861]

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]

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]

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.

[jpl861]

Now I get it. :) Thanks for all the help guys.

Regards,

John

[jambita27]

You're welcome hehe, brother :)

[CaiioJaks]

Hi Mohammed,
With respect to the way 2 -

network 

command.
I understand that besides having the 0.0.0.0/0 route is present in the Interior Gateway Protocol (IGP) routing table, rather having it

(the default route)

defined as a valid/functional

static route

at the router should meet the criteria for such method.
The scenario being:

default static route configured on the router + the network command

under the ipv4 address family within the

 BGP

configuration, should do the trick to inject it to the BGP neighbor(s)
Correct?

Thank you