When BGP routers learn routes via BGP, they learn two key pieces of information; A
destination prefix, and a next hop address. Within an AS, BGP routers communicate
with IBGP. Within the AS however, the next hop address for each prefix is by default
not modified. That means that all IBGP routers within an AS will maintain a
consistent view of the external world.
However, consider the following.
R1 AS1 <-------> R2 AS2 <-------R2 AS2
EBGP IBGP
Consider that R1 advertises prefix 10/8 into AS2. R2 will learn this 10/8 address and
the corresponding next hop address. In this case, lets say that 11/8 is the subnet
between R1 and R2 and R1's uses 11.0.0.1 and R2 uses 11.0.0.2. Hence, R2 will
publish 10/8 with a next hop of 11.0.0.1 in its routing table.
R2 will then advertise 10/8 via IBGP to R2 with the next hop of 11.0.0.1. However, as
11/8 is an external point to point link and may not be a subnet that R2 is aware of.
If this is the case, R2 will not be able to resolve a route toward 11/8 and thus will
be unable to post the route due to the rules of BGP.
Two solutions exist to solve this problem (well two pop into my mind). The first way
would be to have R2 advertise the external link (11/8 in this case) into the AS so
that all IBGP routers will learn it. This however increases the size and complexity
of the IGP within the AS.
The second option is to have R2 replace the next hop address with his own address.
Naturally, his own address will be known throughout the AS (BGP depends on TCP so this
has to be the case) and thus when R2 receives the 10/8 advertisement, it will see R2's
interface as the next hop and thus be able to resolve the bgp next hop and post the
route.
I hope this makes sense :)
Pete
*********** REPLY SEPARATOR ***********
On 11/3/2000 at 11:58 AM suaveguru wrote:
>hi ,
>
>Anyone knows what does next-hop-self in bgp means
>please explain to me
>
>thanks
>
>suaveguru
>
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