Next-Hop-Self; Figure 29: Next-Hop Behavior For Broadcast Multiaccess Media; Figure 30: Next-Hop Behavior For Nonbroadcast Multiaccess Media - Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual

JUNOSe 11.0.x BGP and MPLS Configuration Guide

Next-Hop-Self

In some circumstances, using a third-party next hop causes routing problems. These
configurations typically involve nonbroadcast multiaccess (NBMA) media. To better
understand this situation, first consider a broadcast multiaccess (BMA) media network,
as shown in Figure 29 on page 110.

Figure 29: Next-Hop Behavior for Broadcast Multiaccess Media

Routers Toledo, Madrid, and Barcelona are all on the same Ethernet network, which
has a prefix of 10.19.7.0/24. When router Toledo advertises prefix 192.168.22.0/23
to router Madrid, it sets the next-hop attribute to 10.19.7.5. Before router Madrid
advertises this prefix to router Barcelona, it sees that its own IP address, 10.19.7.7,
is on the same subnet as the next hop for the advertised prefix. If router Barcelona
can reach router Madrid, then it should be able to reach router Toledo. Router Madrid
therefore advertises 192.168.22.0/23 to router Barcelona with a next-hop attribute
of 10.19.7.5.
Now consider Figure 30 on page 110, which shows the same routers on a Frame
Relay NBMA network.

Figure 30: Next-Hop Behavior for Nonbroadcast Multiaccess Media

Routers Toledo and Madrid are EBGP peers, as are routers Madrid and Barcelona.
When router Toledo advertises prefix 192.168.22.0/23 to router Madrid, router Madrid
makes the same comparison as in the BMA example, and leaves the next-hop attribute
intact when it advertises the prefix to router Barcelona. However, router Barcelona
will not be able to forward traffic to 192.168.22.0/23, because it does not have a
direct PVC connection to router Toledo and cannot reach the next hop of 10.19.7.5.
110
Selecting the Best Path