Route Filtering with distribute-list

3 ways to filter routes for redistribution :

- Standard ACL
access-list permit ip 10.0.0.0 0.0.15.255

Match 10.0.0.0/20 but also 10.0.0.0/21 .. 21 etc any smaller mask.

To match also the mask use Extended ACL

-Extended ACL
access-list 101 permit ip 10.0.0.0 0.0.0.0 255.255.248.0 0.0.0.0

Match only 10.0.0.0/20

-Prefix-list
ip prefix-list TEST 10.0.0.0/20

But permit to limit a range of valid mask
ip prefix-list TEST 10.0.0.0/20 le 24
It match 10.0.0.0/24 and also 10.0.4.0/22

ip prefix-list TEST 10.0.0.0/20 ge 24
It match 10.0.0.0/24 but also 10.0.0.0/25 ...

OSPF Fast Hello

Normal OSPF timers could be configured defining the hello-interval and dead-interval, as hello interval is 1 sec, detect a dead neighbor within 1 second means at first loss of a hello.

ip ospf hello-interval 1-65535s
ip ospf dead-interval 1-65535s


Fast hello permit a hello-interval below 1sec. The following command means hello of 333ms and a dead-interval of 1sec :

ip ospf dead-interval minimal hello-multiplier 3

Split Horizon on Frame-Relay

Split horizon must be take into account for DV protocols :

By default split-horizon is enabled on all cisco router interface.

Except for frame-relay interface, and particulary on mutilpoint Frame-Relay interface (ex on the Hub router). It acts differently for RIP and EIGRP

RIP : by default split-horizon is disabled on FR interface

EIGRP : Split-horizon must be manually disabled on FR interface

int s0/1
no ip split-horizon eigrp 100

OSPF Network Types

A review of the different network types and behaviour of ospf :

NBMA : default for frame-relay physical and point-to-multipoint subinterface
Broadcast :default for ethernet
Point-to-Point : default for point-to-point subinterface


Remark : When using Pt-to-Mpt the next hop is the adjacent router and not the originating router as in Nbma or Broadcast networks

Tips :
Point-to-x : no DR/BDR
Non-broadcast : manual neighbor