network directly attached to the ES. The router
then searches for the destination address and forwards the
packet along the best route. If the destination ES is in the
same area, the local IS recognizes the location by listening to
End System Hello (ESH) packets and forwards the packet
appropriately. If the destination address is an ES in another
area, the Level 1 IS sends the packet to the nearest Level 12
IS. Forwarding through Level 2 ISs continues until the packet
reaches a Level 2 IS in the destination area. Within the
destination area, ISs forward the packet along the best path
until the destination ES is reached. Since each router makes
its own best-path decisions at every hop along the way, there
is a significant chance that paths will not be reciprocal. That
is, return traffic can take a different path than the outgoing
traffic. For this reason, it is important to know the traffic
patterns within your network and tune IS-IS for optimal path
selection if necessary.
Content 4.2 ISO
Addressing 4.2.9 OSI Addressing in Networks
Example To help understand OSI addressing in an IS-IS
network, consider traffic from router R7 to router R9 in Figure
: - R7 recognizes that the prefix of R9 (49.00CC) is
not the same as its prefix (49.00BB), so R7 passes the traffic
to the closest Level 12 router, which is R5. R7 uses its Level
1 topology database to find the best path to R5.
- R5
uses its Level 2 topology database to pick the best next hop to
reach the prefix 49.00CC, which is router R3. R5 does not use
the destination system ID in this decision.
- R3 uses
its Level 2 topology database to pick the best next hop to
reach the prefix 49.00CC, which is R1. R3 does not use the
destination system ID in this decision.
- R1 uses its
Level 2 topology database to pick the best next hop to reach
the prefix 49.00CC, which is R8. R1 does not use the
destination system ID in this decision.
- R8 recognizes
that the prefix of R9 (49.00CC) is the same as its prefix, so
R8 passes the traffic to R9 using its Level 1 topology database
to find the best path.
In the example displayed in
Figure , area 1 contains two routers: - One router
borders area 2 and is a Level 12 IS.
- The other
router is contained within the area and is a Level 1
only.
Area 2 has many routers: - A selection
of routers is specified as Level 1. The routers route either
internally to that area or to the exit points (Level 12
routers).
- Level 12 routers form a chain across the
area linking to the neighbor areas. Although the middle router
of the three Level 12 routers does not link directly to
another area, the middle router must support Level 2 routing to
ensure that the backbone is contiguous. If the middle router
fails, the other Level 1-only routers cannot perform the Level
2 function (despite providing a physical path across the area),
and the backbone is broken.
Area 3 contains one
router that borders areas 2 and 4, yet it has no intra-area
neighbors and is performing Level 2 functions only. If you add
another router to area 3, the border router reverts to Level
12 functions. As the figure shows, the border between the
areas in an IS-IS network is the link between Level 2 routers.
(This is in contrast to OSPF, where the border exists inside
the ABR itself.) In the figure, symmetric routing does not
occur because Level 2 details are hidden from Level 1 routers,
which recognize only a default route to the nearest Level 12
router. Traffic from router X to router Y flows from router X
to its closest Level 12 router. The Level 12 router then
forwards the traffic along the shortest path to the destination
area (area 2). When traffic flows into area 2, it is routed
along the shortest intra-area path to router Y. Router Y routes
return packets to router X via its nearest Level 12 router.
The Level 12 router recognizes the best route to area 1 via
area 4, based on the lowest cost Level 2 path. Because Level 1
and Level 2 computations are separate, the path taken from
router Y back to router X is not necessarily the least cost
path from router Y to router X.
Asymmetric routing (packets
taking different paths in different directions) is not
detrimental to the network. However, this type of routing can
make troubleshooting difficult and is sometimes a symptom of
suboptimal design. Like EIGRP and OSPF, a good IS-IS design is
generally hierarchical.
Content 4.2 ISO
Addressing 4.2.10 Route Leaking Route
leaking has been available since Cisco IOS Software Release
12.0. This feature allows selected Level 2 routes to leak in a
controlled manner to Level 1 routers, which helps avoid
asymmetric routing. Route leaking helps reduce suboptimal
routing by providing a mechanism for leaking, or
redistributing, Level 2 information into Level 1 areas. By
having more detail about interarea routes, a Level 1 router is
able to make a better choice about which Level 12 router to
forward the packet. Route leaking is defined in RFC 2966,
Domain-wide Prefix Distribution with Two-Level IS-IS, for use
with the narrow metric Type, Length and Value (TLV) types 128
and 130. The IETF has also defined route leaking for use with
the wide metric (using TLV type 135). To implement route
leaking, an up/down bit in the TLV indicates whether the route
identified in the TLV has been leaked. If the up/down bit is
set to 0, the route was originated within that Level 1 area. If
the up/down bit is set to 1, the route has been redistributed
into the area from Level 2. The up/down bit prevents routing
loops: a Level 12 router does not re-advertise into Level 2
any Level 1 routes that have the up/down bit set. Route leaking
should be planned and deployed carefully to avoid the situation
where a topology change in one area results in having to
recompute many routes in all other areas. Web Links
IS-IS Route Leaking Overview
http://www.cisco.com/en/US/tech/tk365/technologies
_tech_note09186a0080093f39.shtml
Content
4.3 IS-IS Operation 4.3.1
IS-IS Protocol Data Units The OSI stack defines a unit of
data as a PDU. OSI recognizes a frame as a data-link PDU and a
packet (or datagram, in the IP environment) as a network PDU.
Figure shows examples of three types of PDUs (all with IEEE
802.2 Logical Link Control [LLC] encapsulation). IS-IS and
ES-IS PDUs are encapsulated directly in a data-link PDU
(frame). There is no CLNP header and no IP header. This means
that IS-IS and ES-IS do not put routing information in IP or
CLNP packets. Instead, they encapsulate the routing information
directly in a data-link layer frame. CLNP packets contain a
full CLNP header between the data-link header and any higher
layer CLNS information. The IS-IS and ES-IS PDUs contain
variable-length fields, depending on the function of the PDU.
Each field contains a type code, a length, and the appropriate
values (TLVs). IS-IS defines four categories of PDUs:
- Hello (ESH, Intermediate System Hello [ISH], IS-IS Hello
[IIH]): Establishes and maintains adjacencies. Once the
neighbors have been discovered and have become adjacencies, the
hello PDUs act as keepalives to maintain the adjacencies and to
inform the neighbors of any changes in the topology.
- LSP: Distributes link-state information.
- Sequence Number: Informs other routers of LSPs that
may be outdated or missing from their own database to ensure
that all routers have the same information and are synchronized
(similar to an OSPF database description packet).
There are
two types of sequence number PDUs: - Complete sequence
number (CSNP): Describes the complete list of all LSPs in
the LSDB of a router
- Partial sequence number
(PSNP): Requests and acknowledges missing pieces of
link-state information
Content
4.3 IS-IS Operation 4.3.2
Link-State Packets In IS-IS, characteristics of a router
are defined by an LSP. The routers LSP contains an LSP header