The Topology Table
EIGRP uses its topology table to store all the information it needs
to calculate a set of distances and vectors to all reachable
destinations. EIGRP maintains a separate topology table for each
routed protocol. A sample EIGRP topology table is shown in Figure .
The topology table is made up of all
the EIGRP routing tables in the autonomous system. By tracking this
information, EIGRP routers can find alternate routes quickly. The
topology table includes the following fields:
- Feasible distance (FD is xxxx)
- The feasible distance (FD) is the lowest calculated metric to
each destination. For example, in Figure
,
the feasible distance to 32.0.0.0 is 2195456 as indicated by FD
are 2195456.
- Route source (via
xxx.xxx.xxx.xxx) - The source
of the route is the identification number of the router that
originally advertised that route. This field is populated only
for routes learned externally from the EIGRP network. Route
tagging can be particularly useful with policy-based routing.
For example, in Figure
,
the route source to 32.0.0.0 is 200.10.10.10 via
200.10.10.10.
- Reported distance (FD/RD) -
The reported distance (RD) of the path is the distance reported
by an adjacent neighbor to a specific destination. For example,
in Figure
,
the reported distance to 32.0.0.0 is 281600 as indicated by
(2195456/281600).
In addition to these fields, each
entry includes the interface through which the destination is
reachable.
EIGRP sorts the topology table so
that the successor routes are at the top, followed by feasible
successors. At the bottom, EIGRP lists routes that DUAL believes to
be loops in the topology table.
How does an EIGRP router determine
which routers are successors and which routers are feasible
successors? Assume that RTA's routing table includes a route to
Network Z via RTB (see Figure ).
From RTA's point of view, RTB is the current successor for Network
Z; RTA will forward packets destined for Network Z to RTB. RTA must
have at least one successor for Network Z for DUAL to place it in
the routing table.
Can RTA have more than one successor
for Network Z? If RTC claims to have a route to Network Z with the
exact same metric as RTB, then RTA also considers RTC a successor,
and DUAL will install a second route to Network Z via RTC (see
Figure ).
Any of RTA's other neighbors that
advertise a loop-free route to Network Z (but with a RD higher than
the best-route metric and lower than the FD) will be identified as
feasible successors in the topology table, as shown in Figure .
A router views its feasible
successors as neighbors that are downstream, or closer, to the
destination than it is. If something goes wrong with the successor,
DUAL can quickly identify a feasible successor from the topology
table and install a new route to the destination. If no feasible
successors to the destination exist, DUAL places the route in the
active state. Entries in the topology table can be in one of two
states: active or passive. These states identify the status of the
route indicated by the entry rather than the status of the entry
itself.
A passive route is one that is stable
and available for use. An active route is a route in the process of
being recomputed by DUAL. Recomputation happens if a route becomes
unavailable and DUAL can not find any feasible successors. When this
occurs, the router must ask neighbors for help in finding a new,
loop-free path to the destination. Neighbor routers are compelled to
reply to this query. If a neighbor has a route, it will reply with
information about the successor(s). If not, the neighbor notifies
the sender that it does not have a route to the destination either.
Excess recomputation is a symptom of
network instability and results in poor performance. To prevent
convergence problems, DUAL always tries to find a feasible successor
before resorting to a recomputation. If a feasible successor is
available, DUAL can quickly install the new route and avoid
recomputation.
"Stuck in Active" Routes
If one or more routers to which a query is sent do not respond with
a reply within the active time of 180 seconds (3 minutes), the
route, or routes, in question are placed in the "stuck in
active" state. When this happens, EIGRP clears the neighbors
that did not send a reply and logs a "stuck in active"
error message for the route(s) that went active.
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