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| 9.6 |  | BGP Attributes |
| 9.6.2 |  | The Next Hop attribute |

The next hop attribute is a well-known mandatory attribute, type code 3. In terms of an IGP, such as RIP, the "next hop" to reach a route is the IP address of the router that has announced the route.

The next hop concept with BGP is more complex and takes one of the following three forms:

- For EBGP sessions, the next hop is the IP address of the neighbor that announced the route.
- For IBGP sessions, where routes originated inside the AS, the next hop is the IP address of the neighbor that announced the route. For routes injected into the AS by way of EBGP, the next hop learned from EBGP is carried unaltered into IBGP. The next hop is the IP address of the EBGP neighbor from which the route was learned.
- When the route is advertised on a multiaccess medium, such as Ethernet or Frame Relay, the next hop is usually the IP address of the interface of the router. This will be the interface connected to the media that originated the route.

Figure illustrates the BGP Next Hop attribute. RTC is running an EBGP session with router RTZ and an IBGP session with router RTA. RTC is learning route 128.213.1.0/24 from RTZ. In turn, RTC router is injecting the local route 128.212.1.0/24 into BGP.

RTA learns route 128.212.1.0/24 by way of 2.2.2.2, the IP address of the IBGP peer announcing the route. Therefore, according to the definition, 2.2.2.2 is the next hop for RTA to reach 128.212.1.0/24. Similarly, RTC sees 128.213.1.0/24 coming from RTZ by way of Next Hop 1.1.1.1. When it passes this route update to RTA by way of IBGP, RTC includes the next hop information, unaltered. Therefore, RTA receives the BGP update about 128.213.1.0/24 with Next Hop 1.1.1.1. This is an example of the EBGP next hop being carried into IBGP.

As can be seen, the next hop is not necessarily reachable by way of a direct connection. The next hop for RTA for 128.213.1.0/24 is 1.1.1.1, but reaching it requires a pathway through 3.3.3.3.Therefore, the next hop behavior mandates a recursive IP routing table lookup for a router to know where to send the packet. To reach the Next Hop 1.1.1.1, RTA will consult its IGP routing table to see if and how 1.1.1.1 is reachable. This recursive search continues until the router associates destination 1.1.1.1 with an outgoing interface. The same recursive behavior is performed to reach Next Hop 2.2.2.2. If a hop is not reachable by way of an IGP, BGP considers the route as being inaccessible.

| Web Links Configuring BGP http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm