eBGP miltihop

Answered Question
May 1st, 2007
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Hi all,


I have some confuse about when to use this command "ebgp-multihop" , Is it use when two different brand router.

(e.g. Cisco Router connect to Juniper Router via non directly connection). Then cisco router

need to issue "neighbor x.x.x. ebgp-multihp" to form the bgp neighbor

Correct Answer by mohammedmahmoud about 10 years 1 month ago

Hi,


No, ebgp multihop is used when having eBGP peering with non direct connected eBGP peer ip address (whatever the vendor is), by default eBGP messages are send with a TTL of 1, and accordingly if the eBGP neighbor ip is not directly connected the eBGP session won't get up.


Commonly used when peering using loopbacks, since it is not directly connected we must use ?ebgp-multihop 2? command to set TTL to 2 for the peer connection to be formed (by routing the packets to the loopback, instead of dropping it if the TTL was sent with the default value of 1 ? can?t be routed).


HTH, please rate if it does help,

Mohammed Mahmoud.

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Correct Answer
mohammedmahmoud Tue, 05/01/2007 - 05:17
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Hi,


No, ebgp multihop is used when having eBGP peering with non direct connected eBGP peer ip address (whatever the vendor is), by default eBGP messages are send with a TTL of 1, and accordingly if the eBGP neighbor ip is not directly connected the eBGP session won't get up.


Commonly used when peering using loopbacks, since it is not directly connected we must use ?ebgp-multihop 2? command to set TTL to 2 for the peer connection to be formed (by routing the packets to the loopback, instead of dropping it if the TTL was sent with the default value of 1 ? can?t be routed).


HTH, please rate if it does help,

Mohammed Mahmoud.

acbenny Tue, 05/01/2007 - 05:48
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Then how about using "neighbor [neighbor ip] update-source loopback 0"?


Is it mean

For non directly ebgp, use "ebgp-multihop"?

For non directly ibgp, use "neighbor [neighbor ip] update-source loopback 0"


royalblues Tue, 05/01/2007 - 05:56
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Friend,


Actually you would need both the commands.


As said earlier, the EBGP multihop command is required when the EBGP peers are not directly connected. Peering with loopback Ips is not considered to be a direct connection.


BGP by default sources the BGP packets from with the source IP of the outgoing interface

The update source on the other hand tells the router to source the BGP packets with the IP mentioned in the command.


say for eg.. you have a topology like R1--R2--R3 and you are doing a BGP peering with R1 and R3. In this scenario you would require the ebgp multihop command. The peering is done on the serial IPs


Now if you plan to Peer R1 & R3 over loopbacks you would actually require both the commands as they are not directly connected and the packets are also not sourced via the outgoing interface IP.


HTH, rate if it does

Narayan


Jon Marshall Tue, 05/01/2007 - 05:59
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Hi


If you are using update-source loopback 0 under your BGP confgiuration then yes you would need ebgp-multihop because your EBGP peering is not using the physical interfaces and as Mohammed said EBGP asssumes a TTL of 1 unless told otherwise.


HTH


Jon

mohammedmahmoud Tue, 05/01/2007 - 06:01
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Hi,


You'll need the update-source loopback command whether it is eBGP or iBGP if you wish to peer with loopbacks, it tells the router to use which IP address when exchanging BGP messages.



HTH, please rate if it does help,

Mohammed Mahmoud.

kcgeorge1218 Tue, 05/01/2007 - 11:11
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Hello -


A segway to the main question, but are there any serious design/performance implications by having multiple multihop neighbors ?


Thx

George

mohammedmahmoud Tue, 05/01/2007 - 14:39
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Hi,


No, i don't think so, having multiple multihop neighbors has no side effects, its a normal scenario for many providers/customers.



HTH, please rate if it does help,

Mohammed Mahmoud.

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