Optimization, convergence and scalability
1.3.f i Fast convergence requirements
General information on “EIGRP Fast convergence requirements”:
- Fast convergence requires a feasible successor route present in the topology table
- Without a feasible successor route a router must first send out a QUERY and wait for REPLY to decide whether there’s another path to a route or if it’s dead
- This slows down convergence
- Equal-Cost load-balancing is also important because up to 32 paths to a single destination can be installed in the routing table at the same time
- In case of a path failure on of the other links can be used
1.3.f ii Query propagation boundaries
General information on “EIGRP Query propagation boundaries”:
- There are several methods to limit the QUERY scope:
- Summarization: If router A sends a summarization route to router B, then any QUERY from router A to router B asking for a prefix within the summary route won’t find an exact match in router B’s topology table and QUERIES will be stopped right there.
- Route Filtering: When router A is filtering routes outbound to router B, then any QUERY from router A to router B asking for a filtered prefix won’t find an exact match in router B’s topology table and QUERIES will be stopped right there.
- Stub Routers: Stub routers are non-transit routers. Normal routers will never send QUERY messages to stub routers.
1.3.f iii IP FRR (single hop)
General information on “EIGRP IP FRR (single hop)":
- With FRR (Fast ReRoute) it’s possible to install an existing backup path in the FIB
- This can be done on a per-link or per-prefix basis
- This reduces the routing transition to less than 50ms
- Only available in EIGRP named mode
- EIGRP IP FRR tie-breakers:
- SRLG (Shared Risk Link Group) Disjoint: Don’t select a LFA of the same SRLG than the primary path.
- Interface Disjoint: Don’t select a LFA that uses the same outgoing interface.
- Lowest Backup Path Metric: Always select the LFA with the lowest metric.
- Linecard Disjoint: Don’t select a LFA that uses an interface on the same linecard.
- Important: The order of tie-breakers depends on the used IOS version!
- Example: Even if a Feasible Successor is in the topology table, it still takes some time to install it in the FIB and RIB. Although this process is fast, it takes longer than IP FRR.
“EIGRP IP FRR (single hop)” CLI configuration commands:
## Configuring EIGRP IP FRR (named mode) Router(config-router-af)# topology base Router(config-router-af-interface)# fast-reroute per-prefix [all | route-map [ROUTE-MAP-NAME]]
1.3.f iv Leak-map with summary routes
General information on “EIGRP Leak-map with summary routes”:
- EIGRP summarization automatically suppresses the advertisement of the individual summarized prefixes
- This behavior can be influenced by using leak-maps (= route-maps) (comparable to BGP unsuppress-maps)
- Example: Router A summarizes the subnets 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24 and 10.0.3.0/24 to 10.0.0.0/22 but additionally wants to send out the more specific route 10.0.2.0/24. This can be achieved using a summary-address together with a leak-map.
“EIGRP Leak-map with summary routes” CLI configuration commands:
## Configuring EIGRP summarization with leak map (classic mode) Router(config)# interface <if> Router(config-if)# [ip | ipv6] summary-address eigrp <asn> <network>/<mask> leak-map [ROUTE-MAP-NAME] ## Configuring EIGRP summarization with a leak map (named mode) Router(config-router-af)# af-interface <if> Router(config-router-af-interface)# summary-address <network>/<mask> leak-map [ROUTE-MAP-NAME]
1.3.f v EIGRP stub with leak map
General information on “EIGRP stub with leak map”:
- A stub router is also referred as “end of the line router” within a network where no more routers are behind
- Stub routers should not forward traffic between two remote EIGRP-learned networks (= no transit traffic)
- Query scope will be reduced because…
- Non-stub routers don’t send/forward queries to stub routers
- By default, EIGRP stub routers only advertise connected and summary routes
- This can be changed if needed (see below)
- Adjacency has to be rebuilt if stub is configured
- With a leak-map (= route-maps), a stub router can advertise additional routes he’s having but still doesn’t receive queries
- No additional arguments = Only connected and summary routes will be advertised (default behavior)
- connected = Connected routes for interfaces matched within a network command will be advertised
- summary = Auto/manual summary routes will be advertised
- static = Static routes will be advertised (these must be first redistributed into EIGRP on the stub router!)
- redistributed = Redistributed routes will be advertised (if configured)
- receive-only = No routes will be advertised. This option cannot be combined with any other option.
“EIGRP stub with leak map” CLI configuration commands:
## Configuring EIGRP stub with leak map (classic mode) Router(config-router)# eigrp stub [connected | summary | static | redistributed | receive-only] leak-map [ROUTE-MAP-NAME] ## Configuring EIGRP stub with leak map (named mode) Router(config-router-af)# eigrp stub [connected | summary | static | redistributed | receive-only] leak-map [ROUTE-MAP-NAME]