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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 via 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.
The figure illustrates the BGP Next
Hop attribute. RTC is running an EBGP session with RTZ
router and an IBGP session with the RTA. RTC is learning route
128.213.1.0/24 from the 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 via
2.2.2.2, the IP address of the IBGP peer announcing the route. Thus,
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 via Next Hop 1.1.1.1. When it passes this route update to RTA
via IBGP, RTC includes the Next Hop information, unaltered. Thus,
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 you can see, the Next Hop is not
necessarily reachable via a direct connection. RTA's next hop for
128.213.1.0/24 is 1.1.1.1, but reaching it requires a pathway
through 3.3.3.3. Thus, 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 via an IGP, BGP
would consider the route as being inaccessible.
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