ccnp1
EIGRP Split Horizon
A fundamental loop-prevention mechanism in EIGRP that prevents a router from advertising a route back out the same interface through which it was learned.
OSPF Adjacency Failure: Timer Mismatch
Unlike EIGRP, OSPF hello and dead timers must match exactly between neighboring routers for an adjacency to form. A mismatch will prevent the neighbor relationship.
BGP `summary-only` Keyword
When used with the `aggregate-address` command, this keyword suppresses the advertisement of more specific (component) network prefixes, ensuring only the single, broader aggregate route is advertised.
OSPF Administrative Distance (AD)
OSPF routes have an AD of 110. If a route to the same destination is learned from a source with a lower AD (e.g., static=1, EIGRP=90), the OSPF route will not be installed in the routing table.
DMVPN Phase 1 Spoke Configuration Requirement
A spoke router must have a static NHRP map (`ip nhrp map`) that maps the hub's tunnel IP address to its physical (NBMA) IP address, which is crucial for the spoke to find the hub.
BGP `as-set` Keyword
Used during route aggregation, this keyword preserves the AS_Path history of component routes. The collected ASNs are stored in a set that counts as only one AS hop for path length comparison.
OSPF Virtual Link
A logical connection used to bridge a discontiguous Area 0 or connect a non-backbone area to the backbone through a transit area. It is configured using the `area [transit_area_id] virtual-link [remote_router_id]` command.
DMVPN Tunnel State: NHRP
An intermediate state in tunnel establishment where an IKE session and IPsec SA exist, but the spoke router has not yet successfully registered with the Next Hop Server (NHS) hub.
OSPF `distribute-list` Application
A route filtering method that applies inbound to the routing table, preventing routes from the LSDB from being installed. It does not filter the exchange of LSAs between neighbors.
EIGRP Split Horizon in Hub-and-Spoke Topologies
A loop-prevention rule that can inadvertently block route propagation. If a hub router learns a route from one spoke, split horizon prevents it from advertising that route back out the same interface to other spokes.
BGP `as-set` Keyword
Used with the `aggregate-address` command to preserve the AS_Path history of component routes during summarization. The collected ASNs count as only one AS hop for path length comparison.
OSPF Virtual Link
A logical connection configured to bridge a discontiguous Area 0 or connect a non-backbone area to the backbone through a "transit area." It is created using the `area transit_area_id virtual-link remote_router_id` command.
BGP Atomic Aggregate Attribute
A flag indicating to receiving BGP speakers that a loss of path information has occurred during route aggregation. The aggregate route appears as if it originated from the aggregating AS.
OSPF `distribute-list` Application
A route filtering method that applies *inbound* to prevent routes from being installed into the routing table from the LSDB. It does not filter LSA exchange between neighbors.
Common OSPF Virtual Link Configuration Issues
Misconfiguring the transit area ID (using the destination area's ID instead of the transit area's) or using an interface IP address instead of the remote router's unique Router ID (RID).
DMVPN Tunnel States (in order of establishment)
1. INTF (line protocol down),
2. IKE (IPsec key exchange not established),
3. IPsec (Security Association not established),
4. NHRP (spoke not registered with hub),
5. Up (spoke registered successfully).
BGP `AS_SET` Hop Count Rule
When aggregating routes, the `as-set` attribute collects AS_Path attributes from component routes. For AS_Path length comparison, the entire `AS_SET` (even if it contains multiple ASNs) counts as only one AS hop.
`ip ospf mtu-ignore` Command
An OSPF interface command used as a solution to bypass the MTU comparison check, allowing neighbors with mismatched MTU values to form a FULL adjacency.
OSPF Adjacency Failure: Duplicate Router IDs
A cause for neighbor failure where two routers in an OSPF domain have the same Router ID (RID). This prevents adjacency and can lead to a syslog message like `%OSPF-4-DUP_RTRID_NBR`.
OSPF Passive Interface Feature
A feature that stops an interface from sending and receiving OSPF hello packets, preventing adjacency formation, but still allows the interface's network to be advertised into OSPF. It can be verified with `show ip protocols`.
BGP Route Filtering Limitation
A BGP neighbor cannot use both a `distribution list` and a `prefix list` simultaneously for either receiving or advertising routes; one must be chosen per direction.
OSPF LSA Type 7 (NSSA External LSA)
Sourced by an ASBR within a Not-So-Stubby Area (NSSA), it describes external routes. It exists only within the NSSA and is translated to a Type 5 LSA by the NSSA ABR to be flooded into Area 0.
In OSPF configuration, which command takes precedence if an interface is configured with both a `network` command and an `ip ospf` interface command?
The `ip ospf` interface command
When an OSPF router creates a summary route, what is automatically created locally to prevent routing loops for non-existent sub
null0 route
In an OSPF multi-access network, what is the recommended method to ensure a specific router (like a hub) becomes the Designated Router (DR)?
Set the OSPF priority to 0 on all other routers (spokes) on the segment.