Routing

Hello Experts

Can any body explain me what is accept-remote-nexthop in simple words? I really did not understand the concept from juniper documentation.

Sometimes it is hard to understand the Juniper documentation 😆

Let me give it a try, normally when you setup a (i) (e)BGP peering the next-hop is the IP of the otherside of the peering:

RouterA 10.1.0.1 <----eBGP----> RouterB 10.1.0.2. If you look from the Router A side the next-hop will be the peering IP on router B 10.1.0.2.

Overview

In some situations, it is necessary to configure a single-hop EBGP peer to accept a remote next hop with which it does not share a common subnet. The default behavior is for any next-hop address received from a single-hop EBGP peer that is not recognized as sharing a common subnet to be discarded. The ability to have a single-hop EBGP peer accept a remote next hop to which it is not directly connected also prevents you from having to configure the single-hop EBGP neighbor as a multihop session. When you configure a multihop session in this situation, all next-hop routes learned through this EBGP peer are labeled indirect even when they do share a common subnet. This situation breaks multipath functionality for routes that are recursively resolved over routes that include these next-hop addresses. Configuring the accept-remote-nexthop statement allows a single-hop EBGP peer to accept a remote next hop, which restores multipath functionality for routes that are resolved over these next-hop addresses.

BGP uses different tables to resolve protocol next-hop for different applications. In a normal BGP application like IPv4, the prefix is learned in the default table inet.0.  BGP will try to resolve its protocol next-hop in the table inet.3 first;  if fails, it will resolve in the table inet.0.

Understanding Indirect Next Hops

Junos OS supports the concept of an indirect next hop for all routing protocols that support indirectly connected next hops, also known as third-party next hops.

Because routing protocols such as internal BGP (IBGP) can send routing information about indirectly connected routes, Junos OS relies on routes from intra-AS routing protocols (OSPF, IS-IS, RIP, and static) to resolve the best directly connected next hop. The Routing Engine performs route resolution to determine the best directly connected next hop and installs the route to the Packet Forwarding Engine. By default, Junos OS does not maintain the route for indirect next hop to forwarding next-hop binding on the Packet Forwarding Engine forwarding table....so that is why special configuration is needed to accept those indirect nexthops.

Hope this helps a bit

Hello

But I am not able to understand. if protocol nexthop does not share the common subnet between EBGP peering then if we use multihop then why it will not work?

The documentation did not say configuring multihop will not work. In fact, I just copied the explanation from marc post.

"The ability to have a single-hop EBGP peer accept a remote next hop to which it is not directly connected also prevents you from having to configure the single-hop EBGP neighbor as a multihop session. When you configure a multihop session in this situation, all next-hop routes learned through this EBGP peer are labeled indirect even when they do share a common subnet. Configuring multihop works but it "breaks multipath functionality for routes that are recursively resolved over routes that include these next-hop addresses. Configuring the accept-remote-nexthop statement allows a single-hop EBGP peer to accept a remote next hop, which restores multipath functionality for routes that are resolved over these next-hop addresses."

So if you want to peer with a remote peer that does not share the same subnet, and you do not care about multipath, then you can use the multi-hop. If you want the functionality of multipath and multi-hop then you would configure accept-remote-nexthop. You cannot configure multihop and accept-remote-nexthop statements for the same EBGP peer.

Now this may inevitablly lead to the question of what is multipath?

https://www.juniper.net/documentation/en_US/junos14.2/topics/concept/bgp-multipath-understanding.html

In addition to the documentation on junipers website, there is also a classs offered that explains that concept (not any different, maybe the Instructor will have a better way of explaining it) and also allows you to configure both multi-hop and accept-remote and see how it works in a lab so you can get abetter understanding and explanation, for yourself firsthand.

AJER- This three-day course is designed to provide students with the tools required for implementing, monitoring, and troubleshooting Layer 3 components in an enterprise network. Detailed coverage of OSPF, BGP, class of service (CoS), and multicast is strongly emphasized. Through demonstrations and hands-on labs, students will gain experience in configuring and monitoring the Junos operating system and in monitoring device and protocol operations.