A. Although EIGRP can propagate a default route using the default
network method, it is not required. EIGRP redistributes default routes
directly.
Q. Should
I always use the eigrp log-neighbor-changes command when I configure EIGRP?
A. Yes, this command makes it easy to determine why an EIGRP neighbor
was reset. This reduces troubleshooting time.
Q. Does
EIGRP support secondary addresses?
A. EIGRP does support secondary addresses. Since EIGRP always sources
data packets from the primary address, Cisco recommends that you configure all
routers on a particular subnet with primary addresses that belong to the same
subnet. Routers do not form EIGRP neighbors over secondary networks. Therefore,
if all of the primary IP addresses of routers do not agree, problems can arise
with neighbor adjacencies.
Q. What
debugging capabilities does EIGRP have?
A. There are protocol-independent and -dependent debug commands.
There is also a suite of show commands that display neighbor
table status, topology table status, and EIGRP traffic statistics. Some of
these commands are:
- show ip eigrp neighbors
- show ip eigrp interfaces
- show ip eigrp topology
- show ip eigrp traffic
Q. What
does the word serno mean on the end of an EIGRP topology entry when you issue
the show ip eigrp topology command?
A. For example:
show ip
eigrp topology
P
172.22.71.208/29, 2 successors, FD is 46163456
via
172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via
172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266
Serno stands
for serial number. When DRDBs are threaded to be sent, they are assigned a
serial number. If you display the topology table at the time an entry is
threaded, it shows you the serial number associated with the DRDB.
Threading is
the technique used inside the router to queue items up for transmission to
neighbors. The updates are not created until it is time for them to go out the
interface. Before that, a linked list of pointers to items to send is created
(for example, the thread).
These sernos
are local to the router and are not passed with the routing update.
Q. What
percent of bandwidth and processor resources does EIGRP use?
A. EIGRP version 1 introduced a feature that prevents any single EIGRP
process from using more than fifty percent of the configured bandwidth on any
link during periods of network convergence. Each AS or protocol (for instance,
IP, IPX, or Appletalk) serviced by EIGRP is a separate process. You can use
the ip bandwidth-percent eigrpinterface configuration command in
order to properly configure the bandwidth percentage on each WAN interface.
Refer to the EIGRP White Paper for
more information on how this feature works.
In addition, the implementation of partial
and incremental updates means that EIGRP sends routing information only when a
topology change occurs. This feature significantly reduces bandwidth use.
The feasible successor feature of EIGRP
reduces the amount of processor resources used by an autonomous system (AS). It
requires only the routers affected by a topology change to perform route
re-computation. The route re-computation only occurs for routes that were
affected, which reduces search time in complex data structures.
Q. Does
EIGRP support aggregation and variable length subnet masks?
A. Yes, EIGRP supports aggregation and variable length subnet masks
(VLSM). Unlike Open Shortest Path First (OSPF), EIGRP allows summarization and
aggregation at any point in the network. EIGRP supports aggregation to any bit.
This allows properly designed EIGRP networks to scale exceptionally well
without the use of areas. EIGRP also supports automatic summarization of
network addresses at major network borders.
Q. Does
EIGRP support areas?
A. No, a single EIGRP process is analogous to an area of a link-state
protocol. However, within the process, information can be filtered and
aggregated at any interface boundary. In order to bound the propagation of
routing information, you can use summarization to create a hierarchy.
Q. Can
I configure more than one EIGRP autonomous system on the same router?
A. Yes, you can configure more than one EIGRP autonomous system on the
same router. This is typically done at a redistribution point where two EIGRP
autonomous systems are interconnected. Individual router interfaces should only
be included within a single EIGRP autonomous system.
Cisco does
not recommend running multiple EIGRP autonomous systems on the same set of
interfaces on the router. If multiple EIGRP autonomous systems are used with
multiple points of mutual redistribution, it can cause discrepancies in the
EIGRP topology table if correct filtering is not performed at the
redistribution points. If possible, Cisco recommends you configure only one
EIGRP autonomous system in any single autonomous system. You can also use
another protocol, such as Border Gateway Protocol (BGP), in order to connect
the two EIGRP autonomous systems.
Q. If
there are two EIGRP processes that run and two equal paths are learned, one by
each EIGRP process, do both routes get installed?
A. No, only one route is installed. The router installs the route that
was learned through the EIGRP process with the lower Autonomous System (AS)
number. In Cisco IOS Software Releases earlier than 12.2(7)T, the router
installed the path with the latest timestamp received from either of the EIGRP
processes. The change in behavior is tracked by Cisco bug ID CSCdm47037.
Q. What
does the EIGRP stuck in active message mean?
A. When EIGRP returns a stuck in active (SIA) message, it means that it
has not received a reply to a query. EIGRP sends a query when a route is lost
and another feasible route does not exist in the topology table. The SIA is
caused by two sequential events:
- The route reported by the SIA has gone away.
- An EIGRP neighbor (or neighbors) have not replied to the query for that route.
When the
SIA occurs, the router clears the neighbor that did not reply to the query.
When this happens, determine which neighbor has been cleared. Keep in mind that
this router can be many hops away. Refer to What Does the EIGRP DUAL-3-SIA Error Message Mean? for more
information.
Q. What
does the neighbor statement in the EIGRP configuration section do?
A. The neighbor command is used in EIGRP in order to
define a neighboring router with which to exchange routing information. Due to
the current behavior of this command, EIGRP exchanges routing information with
the neighbors in the form of unicast packets whenever the neighbor command
is configured for an interface. EIGRP stops processing all multicast packets
that come inbound on that interface. Also, EIGRP stops sending multicast
packets on that interface.
The ideal
behavior of this command is for EIGRP to start sending EIGRP packets as unicast
packets to the specified neighbor, but not stop sending and receiving multicast
packets on that interface. Since the command does not behave as intended,
the neighbor command should be used carefully, understanding
the impact of the command on the network.
Q. Why
does the EIGRP passive-interface command remove all neighbors for an interface?
A. The passive-interface command disables the
transmission and receipt of EIGRP hello packets on an interface. Unlike IGRP or
RIP, EIGRP sends hello packets in order to form and sustain neighbor adjacencies.
Without a neighbor adjacency, EIGRP cannot exchange routes with a neighbor.
Therefore, the passive-interface command prevents the exchange
of routes on the interface. Although EIGRP does not send or receive routing
updates on an interface configured with the passive-interface command,
it still includes the address of the interface in routing updates sent out of
other non-passive interfaces. Refer to How
Does the Passive Interface Feature Work in EIGRP? For more information.
Q. Why
are routes received from one neighbor on a point-to-multipoint interface that
runs EIGRP not propagated to another neighbor on the same point-to-multipoint
interface?
A. The split horizon rule prohibits a router from advertising a route
through an interface that the router itself uses to reach the destination. In
order to disable the split horizon behavior, use the no ip
split-horizon eigrp as-numberinterface command. Some important
points to remember about EIGRP split horizon are:
- Split horizon behavior is turned on by default.
- When you change the EIGRP split horizon setting on an interface, it resets all adjacencies with EIGRP neighbors reachable over that interface.
- Split horizon should only be disabled on a hub site in a hub-and-spoke network.
- Disabling split horizon on the spokes radically increases EIGRP memory consumption on the hub router, as well as the amount of traffic generated on the spoke routers.
- The EIGRP split horizon behavior is not controlled or influenced by the ip split-horizon command.
For more
information on split horizon and poison reverse, refer to Split Horizon and Poison Reverse. For more information on commands,
refer to EIGRP Commands.
Q.
When I configure EIGRP, how can I configure a network statement with a mask?
A. The optional network-mask argument was first added to the network
statement in Cisco IOS Software Release 12.0(4)T. The mask argument can be
configured in any format (such as in a network mask or in wild card bits). For
example, you can use network 10.10.10.0 255.255.255.252 or network
10.10.10.0 0.0.0.3.
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