We've looked at the routing table and the summary of active routing protocols in previous posts. As mentioned before, the routing table is built from the best routes learned from different routing sources. EIGRP is one such source, and the EIGRP topology table is where EIGRP stores routes learned via EIGRP. The show ip eigrp topology command displays the contents of the EIGRP topology table. Example:
Device# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status, s - sia status
P 192.168.2.0/24, 1 successors, FD is 131153920
via 10.1.1.2 (131153920/163840), Ethernet0/0
via 10.3.3.3 (137625600/6635520), Ethernet0/1
P 192.168.1.0/24, 2 successors, FD is 137625600
via 10.4.4.4 (137625600/6635520), Ethernet0/2
via 10.3.3.3 (137625600/6635520), Ethernet0/1
P 10.0.0.0/8, 1 successors, FD is 409600
via 192.0.2.1 (409600/128256), Ethernet0/0
P 172.16.1.0/24, 1 successors, FD is 409600
via 192.0.2.1 (409600/128256), Ethernet0/0
P 10.0.0.0/8, 1 successors, FD is 281600
via Summary (281600/0), Null0
P 10.0.1.0/24, 1 successors, FD is 281600
via Connected, Ethernet0/0
The very first line EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1) indicates that this is the EIGRP topology table for IPv4, the autonomous system number is 1 and the router ID is 10.0.0.1. Next, the first important piece of information for the route entries is the status code, which is listed first for each entry. The status code has the following possible values:
P (Passive) state is what you want. It means the network is stable and the router is forwarding packets to the destination. A (Active) means that the router is actively trying to compute a new path to the destination. Therefore, P 192.168.2.0/24, 1 successors, FD is 131153920 indicates that the route is good. In addition, this line displays the remote destination (192.168.2.0/24), how many best paths this route has that are installed into the routing table (1 successors), and the feasible distance or the total metric to reach the network (FD). via 10.1.1.2 (131153920/163840), Ethernet0/0 indicates who the route was learned from (via 10.1.1.2), the feasible/calculated distance and the advertised/reported distance of the neighbor (131153920/163840), and the interface to reach the network (Ethernet0/0). The details of EIGRP operation are beyond the scope of this post but the feasible distance and reported distance are used by EIGRP to find loop-free paths.
The show ip eigrp topology command can accept optional keywords, but without specifying them, only topology entries with feasible successors are displayed and only feasible paths are shown. The show ip eigrp topology all-links displays all entries in the EIGRP topolgoy table, including non-feasible succesor routes. Example:
Device# show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 172.16.1.0/24, 1 successors, FD is 409600, serno 14
via 10.10.1.2 (409600/128256), Ethernet0/0
via 10.3.3.3 (6635520/281600), Ethernet0/1
via 10.1.4.3 (2586111744/2585599744), Serial3/0, serno 18
Briefly, the route must pass the feasibility condition to become a feasible successor. The feasibility condition states that the reported distance from the neighbor must be lower than the feasible distance or the best metric for that route. From the above output, 2585599744 is not lower than 409600 (FD) and, as a result, it is not a feasible successor. On the contrary, via 10.3.3.3 meets the feasibility condition (281600 < 409600) and it is a feasible successor.
Finally, the show ip eigrp topology prefix command displays detailed information about a single prefix. Example:
Device# show ip eigrp topology 10.0.0.0/8
EIGRP-IPv4 VR(vr1) Topology Entry for AS(1)/ID(10.1.1.2) for 10.0.0.0/8
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 82329600, RIB is 643200
Descriptor Blocks:
10.1.1.1 (Ethernet2/0), from 10.1.1.1, Send flag is 0x0
Composite metric is (82329600/163840), route is Internal
Vector metric:
Minimum bandwidth is 16000 Kbit
Total delay is 631250000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
Originating router is 10.1.1.1
The first part of the command output shows basic information including the EIGRP AS number (1) and router ID (10.1.1.2), the state of the route (Passive), the type of route (Successor) and the feasible distance (FD is 82329600). The Composite metric indicates both the calculated distance and the reported distance (82329600/163840) for the route entry and Internal means the route was learned within the same EIGRP AS and not redistributed from an external source. The vector metric part is interesting because it displays the individual components used in EIGRP metric calculation. By default, only bandwidth and delay are used in the calculation but all components are still advertised in the route. Hop count and MTU are not considered in the calculation, but they are still listed in the output. See: EIGRP Wide Metrics White Paper
Cisco documentation: show ip eigrp topology
Device# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status, s - sia status
P 192.168.2.0/24, 1 successors, FD is 131153920
via 10.1.1.2 (131153920/163840), Ethernet0/0
via 10.3.3.3 (137625600/6635520), Ethernet0/1
P 192.168.1.0/24, 2 successors, FD is 137625600
via 10.4.4.4 (137625600/6635520), Ethernet0/2
via 10.3.3.3 (137625600/6635520), Ethernet0/1
P 10.0.0.0/8, 1 successors, FD is 409600
via 192.0.2.1 (409600/128256), Ethernet0/0
P 172.16.1.0/24, 1 successors, FD is 409600
via 192.0.2.1 (409600/128256), Ethernet0/0
P 10.0.0.0/8, 1 successors, FD is 281600
via Summary (281600/0), Null0
P 10.0.1.0/24, 1 successors, FD is 281600
via Connected, Ethernet0/0
The very first line EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1) indicates that this is the EIGRP topology table for IPv4, the autonomous system number is 1 and the router ID is 10.0.0.1. Next, the first important piece of information for the route entries is the status code, which is listed first for each entry. The status code has the following possible values:
- P - Passive—Indicates that no EIGRP computations are being performed for this route. (good)
- A - Active—Indicates that EIGRP computations are being performed for this route. (bad)
- U - Update—Indicates that a pending update packet is waiting to be sent for this route.
- Q - Query—Indicates that a pending query packet is waiting to be sent for this route.
- R - Reply—Indicates that a pending reply packet is waiting to be sent for this route.
- r - Reply status—Indicates that EIGRP has sent a query for the route and is waiting for a reply from the specified path.
- s - sia status—Indicates that the EIGRP query packet is in stuck-in-active (SIA) status.
P (Passive) state is what you want. It means the network is stable and the router is forwarding packets to the destination. A (Active) means that the router is actively trying to compute a new path to the destination. Therefore, P 192.168.2.0/24, 1 successors, FD is 131153920 indicates that the route is good. In addition, this line displays the remote destination (192.168.2.0/24), how many best paths this route has that are installed into the routing table (1 successors), and the feasible distance or the total metric to reach the network (FD). via 10.1.1.2 (131153920/163840), Ethernet0/0 indicates who the route was learned from (via 10.1.1.2), the feasible/calculated distance and the advertised/reported distance of the neighbor (131153920/163840), and the interface to reach the network (Ethernet0/0). The details of EIGRP operation are beyond the scope of this post but the feasible distance and reported distance are used by EIGRP to find loop-free paths.
The show ip eigrp topology command can accept optional keywords, but without specifying them, only topology entries with feasible successors are displayed and only feasible paths are shown. The show ip eigrp topology all-links displays all entries in the EIGRP topolgoy table, including non-feasible succesor routes. Example:
Device# show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(1)/ID(10.0.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 172.16.1.0/24, 1 successors, FD is 409600, serno 14
via 10.10.1.2 (409600/128256), Ethernet0/0
via 10.3.3.3 (6635520/281600), Ethernet0/1
via 10.1.4.3 (2586111744/2585599744), Serial3/0, serno 18
Briefly, the route must pass the feasibility condition to become a feasible successor. The feasibility condition states that the reported distance from the neighbor must be lower than the feasible distance or the best metric for that route. From the above output, 2585599744 is not lower than 409600 (FD) and, as a result, it is not a feasible successor. On the contrary, via 10.3.3.3 meets the feasibility condition (281600 < 409600) and it is a feasible successor.
Finally, the show ip eigrp topology prefix command displays detailed information about a single prefix. Example:
Device# show ip eigrp topology 10.0.0.0/8
EIGRP-IPv4 VR(vr1) Topology Entry for AS(1)/ID(10.1.1.2) for 10.0.0.0/8
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 82329600, RIB is 643200
Descriptor Blocks:
10.1.1.1 (Ethernet2/0), from 10.1.1.1, Send flag is 0x0
Composite metric is (82329600/163840), route is Internal
Vector metric:
Minimum bandwidth is 16000 Kbit
Total delay is 631250000 picoseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
Originating router is 10.1.1.1
The first part of the command output shows basic information including the EIGRP AS number (1) and router ID (10.1.1.2), the state of the route (Passive), the type of route (Successor) and the feasible distance (FD is 82329600). The Composite metric indicates both the calculated distance and the reported distance (82329600/163840) for the route entry and Internal means the route was learned within the same EIGRP AS and not redistributed from an external source. The vector metric part is interesting because it displays the individual components used in EIGRP metric calculation. By default, only bandwidth and delay are used in the calculation but all components are still advertised in the route. Hop count and MTU are not considered in the calculation, but they are still listed in the output. See: EIGRP Wide Metrics White Paper
Cisco documentation: show ip eigrp topology
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