why need to add ebgp-multihop command when i using loopback interface?

huangzhy · ‎01-21-2003

i used two routers as ebgp peers.And the two router used loopback interface to bulit neighbor adjacency. But the loopback interfaace is not in the same subnet.So i need to use the neighbor ebgp-multihop 2 command to built ebgp adjacency. But the ebgp-multihop is used to set the ttl to 2,but the two routers is directly connected, and if the ttl is 1,the other router can still receive the packets,so why still need to use the multihop command?

a.manosca · ‎01-21-2003

Using a loopback interface to define neighbors is common with iBGP, but not with eBGP. Normally the loopback interface is used to make sure the IP address of the neighbor stays up and is independent of hardware functioning properly. In the case of eBGP, peer routers are frequently directly connected and loopback does not apply.

So I guess you're using the loopback interfaces for load-balancing along with ebgp-multihop. If not, better use the point-to-point interface's address to avoid using ebgp-multihop .

If you only have a single connection between the two eBGP peers, what benefit do you get in using a loopback if one of the side of the point-to-point connection goes down.

Hope this makes sense.

huangzhy · ‎01-21-2003

i have two routers interconnect by two links by using ebgp. I just want to built one ebgp peer among the two routers ,so i use loopback interface to build the peer. If i use interface address to built the peer,it will need two peers. And my questions is why the two router directed connect ,why still need to use the multihop command to set the ttl? As far as i know , when the router receives a packet ,it decrease the ttl by one ,if the ttl reaches zero,the router discard the packet . So if the two routers interconnect by two links by using ebgp,i used the loopback interface ,and why do i still need to use the multi-hop command ?The other router can receive the packet so if i do not use the command ,i think it will still work.

a.manosca · ‎01-21-2003

Hi, besides using the loopback interface to build a single eBGP peering,

it can also be used to enable load-balancing in the twolinks.

The use of loopback interfaces, update-source and ebgp-multihop

is a workaround in order to achieve load balancing between two

eBGP speakers over parallel serial lines. In normal situations,

BGP picks one of the lines to send packets on, and load balancing

wouldn't happen. By introducing loopback interfaces, the next hop

for eBGP is the loopback interface.

Static routes or an IGP can be used to introduce two equal cost

paths to reach the destination.

The link below shows a common example:

http://www.cisco.com/warp/customer/459/40.html#1

Without the neighbor ebgp-multihop command on each router,

BGP would not perform load balancing. The neighbor ebgp-multihop

and neighbor update-source router configuration commands have the

effect of making the loopback interface the next hop for EBGP,

which allows load balancing to occur.

Now with the ttl matter, you are right. The TTL should be decreased

as is passes through a router to avoid looping or having the packet

remain too long in the network. But I believe that once the packet

has reached the destination, it will not be forwarded anymore.

Regards.

maneesh · ‎01-30-2003

Hi,

EBGP sessions use TCP connections. Please understand that this has to go through the normal routing process on the router, which means that ttl value is decremented when routing to the loopback interface. Since the default ttl is 1, the packet will be dropped when routing to the loopback interface. So you use the ebgp multihop command to set the ttl to a suitable value. This primarily becuase the loopback is not in the same subnet.

Best Regards

Maneesh