ebgp-max-hop

Created: Nov 13, 2019 13:49:18Latest reply: Nov 13, 2019 14:53:37 122 2 0 0
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As the last post says, since the communication between loopback interfaces of two routers only needs to pass through one hop, Why the peer ebgp-max-hop 2 command must be configured when the loopback interfaces are used to establish an EBGP peer relationship?

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  • convention:

Featured Answers
Popeye_Wang
Admin Created Nov 13, 2019 14:02:49 Helpful(0) Helpful(0)

Hi Sprout,

To answer this question, we must review the conditions for establishing an eBGP peer relationship.

In the product documentation:

By default, an EBGP connection can be set up only on a directly-connected physical link.

In fact, there are two conditions here:

1. By default, the number of hops between EBGP neighbors is 1 hop.

2. The source addresses on both sides must be on the same network segment.

We are familiar with the first rule here, while the second rule is easily overlooked.

These two rules are more clearly seen on Cisco routers:

On Cisco routers, To establish an eBGP peer relationship with the loopback address when the router is directly connected, you do not need to change the maximum number of hops between eBGP peers on, you only need to remove the check on the same network segment. The command is as follows: disable-connected-check.

On Huawei routers, when ebgp-max-hop 2 is configured, why can an eBGP peer relationship be established? The explanation is as follows: When ebgp-max-hop 2 is configured, an eBGP peer relationship can be established between routers through two hops. In this case, the device does not check whether the eBGP peer addresses are in the same network segment.


  • x
  • convention:

All Answers
Popeye_Wang
Popeye_Wang Admin Created Nov 13, 2019 14:02:49 Helpful(0) Helpful(0)

Hi Sprout,

To answer this question, we must review the conditions for establishing an eBGP peer relationship.

In the product documentation:

By default, an EBGP connection can be set up only on a directly-connected physical link.

In fact, there are two conditions here:

1. By default, the number of hops between EBGP neighbors is 1 hop.

2. The source addresses on both sides must be on the same network segment.

We are familiar with the first rule here, while the second rule is easily overlooked.

These two rules are more clearly seen on Cisco routers:

On Cisco routers, To establish an eBGP peer relationship with the loopback address when the router is directly connected, you do not need to change the maximum number of hops between eBGP peers on, you only need to remove the check on the same network segment. The command is as follows: disable-connected-check.

On Huawei routers, when ebgp-max-hop 2 is configured, why can an eBGP peer relationship be established? The explanation is as follows: When ebgp-max-hop 2 is configured, an eBGP peer relationship can be established between routers through two hops. In this case, the device does not check whether the eBGP peer addresses are in the same network segment.


  • x
  • convention:

LuizPuppin
LuizPuppin MVE Created Nov 13, 2019 14:53:37 Helpful(0) Helpful(0)

By default all eBGP connections receive TTL 1 on their TCP packets. To connect loopback interfaces you need at least TTL 2 in TCP packets.
To do this change in eBGP connections you need to insert ebgp-max-hop parameter with the number of TTL field.
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  • convention:

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