Information Technology Reference
In-Depth Information
ISO/IEC 10589. The behavior of IS-IS cannot be modified to automatically advertise ES-IS
adjacencies in L2 LSPs without potentially creating other problems. The general operation
of IS-IS must remain compliant with the specification.
The practical implementation of this solution is for R1 to form an ES-IS adjacency with R2
and for R2 to form an ES-IS adjacency with R1. This is achieved through enabling CLNS
on the interface, not IS-IS, using the
clns enable
command. This means that R1 thinks it is
an IS and that R2 is an ES, whereas R2 thinks it is an IS and that R1 is an ES. An eBGP
peering session is formed between R1 and R2 using the 10.1.1.0/30 network to provide
reachability for the peering session.
The CLNS prefix information that R2 receives from R1 has a BGP next hop of R1's NSAP.
The next hop is reachable via an ES-IS adjacency, so the prefix passes the next-hop reach-
ability requirement in the BGP best-path algorithm. This allows the prefix to be installed
into the BGP RIB and, assuming it is the best path for that prefix, the routing table.
A complication appears when an eBGP learns that NSAP is advertised via iBGP. Figure A-4
shows an example.
BGP Next-Hop Complication with iBGP
Figure A-4
47.5678.2222.2222.2222.00
47.1234.1111.1111.1111.00
.2
.1
10.1.1.0/30
R1
R2
BGP AS 65101
BGP AS 65100
47.1235.1111.1111.1112.00
R3
If R1 advertises an NSAP via eBGP to R2, which in turn advertises it to R3 via iBGP, R3
receives the NSAP with a next hop of R1. R2 is unable to advertise the NSAP for R1, which
it knows via ES-IS, because it is L2-only and its adjacency with R3 is L2-only. This is seen
by R2 and R3 being in different areas—R2 in area 47.1234 and R3 in area 47.1235.
This complication is resolved by having R2 automatically include the prefix for R1 in its
L2 LSP if the following three conditions are met:
•
An ES-IS adjacency exists between the two eBGP peers.
•
A BGP Update message is received that contains an MP_REACH_NLRI with the
NSAP address family identifier.
•
The next-hop address in the MP_REACH_NLRI is identical to the NSAP of the eBGP
neighbor.
