PATH MANIPULATION AND CONTROL

Route Manipulation and Control

Route Tags

To create a route tag list, use the global configuration mode command: route-tag list list_name

route-tag list TO_CAT2 seq 10 permit 0.0.0.2 0.0.0.0


A route-tag can be referenced inside of a route map using the keyword match.

route-map ADV_TO_R4 permit 10
match tag list TO_CAT2

To filter advertisements:

router eigrp 10 distribute-list route-map ADV_TO_R4 out eth0/0

Note: that distribute-lists have an impact on high-availability and fail-over.

Distribute List

Allow the filtering of network prefixes on a neighbor-by-neighbor basis using standard or extended access control lists.

Prefix List

Allow the filtering of network prefixes on a neighbor-by-neighbor basis using a prefix list.

Filter-List

Used in OSPF and BGP. In OSPF filter lists can be used to filter prefixes between OSPF areas.

Preventing Loops with Tags

Split Horizon

Split horizon is a method of preventing routing loops in distance-vector routing protocols such as RIP and EIGRP by prohibiting a router from advertising a route back onto the interface from which it was learned. Split horizon follows the principle that; Never advertise a route out of the interface through which you learned it.

Split horizon prevents the advertisement of reverse routes. EIGRP and BGP enable split horizon by default. DMVPN is one of the technologies that will require the disabling of split horizon on the hub.

To disable split-horizon on an interface, use the interface configuration command: no ip split-horizon {eigrp }

TODO:

1. View split horizon status of an interface.
2. Disable split horizon on interface
3. Disable split horizon for EIGRP. Note: EIGRP resyncs after split horizon change.

Route Poisoning

Advertises a prefix with an infinity metric; in RIP, this is metric of 16., OSPF sets the LSA age to 3600

Maximum Hops

The maximum hops in EIGRP can be modified using the EIGRP router configuration command: metric maximum-hops number where number is the maximum number of hops beyond which a route will be considered unreachable. The modification of the maximum-hops results in a new adjacency being created with existing neighbors.

Note: Modification of the maximum-hops results in the deletion of all EIGRP sourced routes in the RIB.

The maximum-hops can be configured using the command: show ip protocols

In EIGRP named mode, the maximum-hops can be configured under the topology base address-family configuration mode.

Filtering

EIGRP

Configuration of a distribution-list results in a resynchronization. Modification of the referenced access lists also results in a resynchronization.

Access-lists additionally affect the high-availability when interfaces do down.

Offset Lists

When configuring offset-lists, configuring an non-existent ACL or configuring zero (0) results in all network prefixes being included in their offset-lists metric modification.

Offset-lists provide some form of route poisoning.