networks from other routers inside the autonomous
system. The external distance (administrative distance 170)
applies to networks introduced to EIGRP from outside this
autonomous system through redistribution.
Content
2.3 Implementing and Verifying EIGRP
2.3.8 The show ip eigrp interfaces Command The
show ip eigrp interfaces command displays information
about interfaces configured for EIGRP. Figure displays sample
output generated by the command. This output includes the
following key elements: - Interface: Interface
over which EIGRP is configured.
- Peers: Number
of directly connected EIGRP neighbors.
- Xmit Queue
Un/Reliable: Number of packets remaining in the Unreliable
and Reliable transmit queues.
- Mean SRTT: Mean
SRTT interval, in milliseconds.
- Pacing Time
Un/Reliable: Pacing time used to determine when EIGRP
packets should be sent out the interface (unreliable and
reliable packets).
- Multicast Flow Timer:
Maximum number of seconds in which the router sends multicast
EIGRP packets.
- Pending Routes: Number of routes
in the packets in the transmit queue waiting to be sent.
Content 2.3 Implementing and
Verifying EIGRP 2.3.9 The show ip eigrp
topology Command Another command used to verify EIGRP
operations is the show ip eigrp topology command. For
example, Figure illustrates that router R1 has an ID of
192.168.1.101 and is in AS 100—the EIGRP ID is the highest IP
address on an active interface for this router. The command
output lists the networks known by the router through the EIGRP
routing process. The output uses the following codes:
- P (Passive): Network is available, and installation
can occur in the routing table. Passive is the correct state
for a stable network.
- A (Active): Network is
currently unavailable, and installation cannot occur in the
routing table. Active means that there are outstanding queries
for this network.
- U (Update): Network is being
updated (placed in an update packet). This code also applies if
the router is waiting for an acknowledgment for this update
packet.
- Q (Query): Outstanding query packet for
this network. This code also applies if the router is waiting
for an acknowledgment for a query packet. Basically, this code
indicates that the router has sent a query packet to a neighbor
router.
- R (Reply status): Router is generating
a reply for this network or is waiting for an acknowledgment
for the reply packet.
- S (Stuck-in-active
status): EIGRP convergence problem for the network with
which it is associated.
The number of successors
available for a route is indicated in the command output. In
this example, all networks have one successor. If there were
equal-cost paths to the same network, a maximum of six paths
would be shown. The number of successors corresponds to the
number of best routes with equal cost. For each network, the FD
is displayed, followed by the next-hop address, which is
followed by a field similar to (40514560/28160) in the figure.
The first number in this field is the FD for that network
through this next-hop router, and the second number is the
advertised distance from the next-hop router to the destination
network.
Content 2.3 Implementing and
Verifying EIGRP 2.3.10 The show ip eigrp
traffic Command To display the number of various EIGRP
packets sent and received, use the show ip eigrp
traffic command, as illustrated in Figure . For example, in
this network, router R1 has sent 429 hello messages and
received 192 hello messages.
Content 2.4
Implementing Advanced EIGRP Features 2.4.1
Route Summarization Automatic summarization of routes at
the major classful boundary is a characteristic of distance
vector operations. Traditional distance vector protocols, such
as RIPv1, are classful routing protocols and cannot assume the
mask for networks that are not directly connected, because
routing updates do not exchange masks. Automatic summarization
is enabled by default for EIGRP. Summarizing routes at major
classful boundaries creates smaller routing tables, which makes
the routing update process less bandwidth-intensive. However, a
drawback to RIPv1 is the inability to create summary routes at
arbitrary boundaries within a major network.With EIGRP you can
disable automatic summarization and create one or more summary
routes within the network on any bit boundary as long as a more
specific route exists in the routing table. When a more
specific route no longer exists the summary route is removed
from the routing table.The minimum metric of the specific
routes is used as the metric of the summary route.In the
routing table, summary routes are automatically assigned to
interface null0 to prevent routing loops. For the same reason,
a summary route to interface null0 is created when manual
summarization is configured. For example, if the summarizing
router receives a packet to an unknown subnet that is part of
the summarized range, the packet matches the summary route
based on the longest match. The packet is forwarded to the
null0 interface (in other words, it is dropped), which prevents
the router from forwarding the packet to a default route and
possibly creating a routing loop. For manual summarization to
be effective, blocks of contiguous addresses (subnets) must
come together at a common router so that the router can
advertise a single summary route. The number of subnets that
can be represented by a summary route is directly related to
the difference in the number of bits between the subnet mask
and the summary mask. The formula 2n, where n equals the
difference in the number of bits between the summary and subnet
mask, indicates how many subnets can be represented by a single
summary route. For example, if the summary mask contains three
fewer bits than the subnet mask, eight (23 = 8) subnets can be
aggregated into one advertisement.For example, if network
10.0.0.0 is divided into /24 subnets and is summarized to the
summarization block 10.1.8.0/21, the difference between the /24
networks and the /21 summarizations is 3 bits; therefore, 23 =
8 subnets can be aggregated. The summarized subnets range from
10.1.8.0/24 through 10.1.15.0/24.When configuring summary
routes, the IP address of the summary route and the summary
mask needs to be specified. EIGRP handles many of the details
that surround proper implementation, including metrics, loop
prevention, and removing the summary route from the routing
table if none of the more specific routes are valid.
Content 2.4 Implementing Advanced EIGRP
Features 2.4.2 Configuring Manual Route
Summarization EIGRP automatically summarizes routes at the
classful boundary—the boundary where the network address ends
as defined by class-based addressing. In most cases, auto
summarization is beneficial, because it keeps the routing
tables as compact as possible. For example, Figure displays
router RTC advertising its routes to RTD. Even though RTC is
connected only to subnet 2.1.1.0, it will advertise that it is
connected to the entire Class A network, 2.0.0.0.However, in
some cases, you may not want automatic summarization to occur.
For example, if you have discontiguous networks, you need to
disable automatic summarization to minimize router confusion.
Figure displays the problem associated with discontiguous
networks and automatic summarization. Router RTD ignores the
route advertisement from RTC, since it is already directly
connected to a network in the classful 2.0.0.0/8 range.To
disable automatic summarization, use the no
auto-summary command, as shown in Figure . Figure
illustrates what happens when automatic summarization is
disabled: Router RTD now accepts the route advertisement from
RTC.It may sometimes be advantageous to manually create a
summary route at an arbitrary bit boundary. You can use the