EBGP Peer

[prashant]

Team,

Can anyone let me know why EBGP connected directly?

Regards

Prashant

[cadet alain]

Hi,

You can do an eBGP peering with a non directly connected IP but you'll have to use multihop or ttl security to set the outgoing TTL higher than 1( default for eBGP peering on Cisco IOS) or if peering between loopbacks you may use the disable-connected check command.

Regards

Alain

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[prashant]

Hi,

Will you elaborate me pls. I little bit confuse. I have some query here.

Multihop: I think we use Multihop when we have two link connection otherwise Mutihop is not required. Pls let me know if i m worng.

Suppose we have single connection then what ?

You mention: TTL higher than 1( default for eBGP peering on Cisco IOS)

Then what about other vendor. What is TTL value for BGP.

And how Its define TTL value 1 in BGP. When i m testing i am getting more then 1.

Hope you understand my question. Pls clear me briefly.

Regards

Prashant

[Harold Ritter]

Hi Prashant,

You are correct that ebgp multihop is generally not required. It is sometimes required when running the eBGP session over an intermediary L3 device such as a firewall. I also know service providers who will place a L3 device at the customer premise as demarcation equipment, which makes it necessary to use multihop.

Regards

[cadet alain]

Hi,

R1-R2 are 2 eBGP peers and I do the peering on the directly connected interface, as you see below the outgoing TTL is 1

R1(config-router)#do sh ip bgp nei 10.0.12.2 | i link|TTL
BGP neighbor is 10.0.12.2,  remote AS 2, external link
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1

Now if I do an iBGP peering:

R1(config-router)#do sh ip bgp nei 10.0.12.2 | i link|TTL
BGP neighbor is 10.0.12.2,  remote AS 1, internal link
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 255

It seems Juniper does the same(TTL of 1 for eBGP) and I'm not sure about other vendors.

Now I peer on the loopbacks and use the ebgp-multihop so:

R1(config-router)#do sh ip bgp nei 2.2.2.2 | i link|TTL
BGP neighbor is 2.2.2.2,  remote AS 2, external link
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 255

Regards

Alain

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[prashant]

Hi Alain,

Thanks for your help. Now i got it the answer. I have tested in GNS3 lab and found it.

But when i try in our L3 4500 Switch, it is not showing. Even though i have tried in 2600 router but no luck.There is any reson to not showing.

And one more question i have.

There is any perticular reson to use TTL vlaue 1 in EBGP?

Regards

Prashant

[milan.kulik]

Hi,

ad your first question

"

Multihop: I think we use Multihop when we have two link connection otherwise Mutihop is not required. Pls let me know if i m worng.

Suppose we have single connection then what ?"

I'm afraid you are mixin Multihop amd Maximum-paths features here.

You might use  maximum-paths 2 command if you have two BGP sessions and you want to run load balancing over them.

See

http://www.cisco.com/en/US/customer/docs/ios/12_2/iproute/command/reference/1rfbgp1.html#wp1257092

for details.

Best regards,

Milan

[Harold Ritter]

Hi Milan,

I believe the original poster was referring to eBGP peering to the loopback address of the neighbor and have multiple static routes over different physical interfaces between the two routers. This is one use case of ebgp multihop.

Regards 

[prashant]

Hi Milan,

I got my answer. i have tried to open your link but its not working. Can you share me the that document.

And can you share any document which have full document of BGP including MPLS also from A to Z.

Regards

Prashant.