EIGRP summarisation, stub routing, stuck in active and default routes

Manual summarisation

Unlike OSPF, EIGRP can summarise on any interface of any router, because it is a distance-vector protocol and each router decides what it tells its neighbours. A summary replaces many specific routes with one, keeping routing tables small and hiding instability.

! Classic mode, on the interface towards the core
interface GigabitEthernet0/0
 ip summary-address eigrp 100 10.20.0.0 255.255.0.0
!
! Named mode
router eigrp CAMPUS
 address-family ipv4 unicast autonomous-system 100
  af-interface GigabitEthernet0/0
   summary-address 10.20.0.0 255.255.0.0

The summarising router installs a route for the summary to Null0 with administrative distance 5. That discard route drops traffic for unused addresses in the range instead of sending it back out along a default route, which would loop. The summary's metric is the best metric of the specific routes inside it.

Sometimes one router behind the summary must still see a specific route, for example a server subnet that has a second exit. A leak map lets selected specific routes through alongside the summary: ip summary-address eigrp 100 10.20.0.0 255.255.0.0 leak-map LEAK-SERVERS, where the route-map matches the routes to leak.

Auto-summary

Older IOS releases summarised automatically at classful network boundaries, so all of 10.0.0.0/8 might be advertised as a single route. That breaks discontiguous networks, where parts of the same major network sit on both sides of a different network: both sides advertise 10.0.0.0/8 and traffic goes to the wrong place. From IOS 15 auto-summary is off by default. If you meet it in an old configuration, turn it off with no auto-summary.

Queries, stuck in active and the DUAL state machine

When the successor for a route fails and there is a feasible successor, the router switches to it locally; the route stays passive. If there is no feasible successor, the route goes active and the router sends queries to all neighbours. Each neighbour either answers from its own table or, if it has lost the route too, queries its own neighbours. This chain is the diffusing computation managed by DUAL's finite state machine. The route becomes passive again only when every reply is in.

In a large flat network queries can travel to every router. If one neighbour does not reply, the active timer (3 minutes by default) runs. Halfway through, the router sends an SIA-query; if the neighbour still does not reply, the router resets that neighbour. This is the stuck in active (SIA) condition, and it usually points to an overloaded router or a poor link somewhere far from where the original failure happened.

Stub routing

A remote site with one or two uplinks can never be a useful transit path, so there is no point querying it. Configure it as an EIGRP stub. Its neighbours stop sending it queries, and it advertises only the route types you allow.

router eigrp 100
 eigrp stub connected summary

Options are connected, summary (the default pair), static, redistributed, receive-only and leak-map. A stub router does not advertise routes it learned from one neighbour to another, which also stops a dual-homed branch accidentally becoming a transit path between hubs. A stub leak map relaxes that for selected routes. Combine stubs at the edge with summaries at distribution routers, and queries stay small.

Default routes and a real-world example

To give the network a default route through EIGRP you can redistribute a static default (redistribute static with ip route 0.0.0.0 0.0.0.0 ...), or advertise a summary of 0.0.0.0 0.0.0.0 towards branches, which replaces every other route they would have learned through that interface. The old ip default-network command, which flags a classful network as the candidate default, still exists but is legacy.

Real-world example: a retail chain's hub routers logged stuck-in-active resets every few weeks, each time dropping dozens of store neighbours. The stores were dual-homed to two hubs, and none was configured as a stub, so every lost route caused queries to every store over slow broadband links. The team made every store router eigrp stub connected summary and summarised each region at the distribution routers. Queries no longer reached the stores, the SIA resets stopped, and store routers no longer appeared as transit paths between the two hubs.