Archive for category OSPF
MTU issues, OSPF and 3550’s
Posted by juecker in Cisco, Networking, OSPF on November 20th, 2008
I ran into a problem where the MTU difference was giving OSPF problems. One of the parameters that must match for an OSPF adjacency to form is the MTU. As is documented here this can be an issue. It looks like the default MTU for a Cisco 3550 is 1503 unless someone knows otherwise (I don’t have a 3550 handy).
Changing how OSPF trusts other OSPF routes
Posted by juecker in Cisco, Networking, OSPF on November 16th, 2008
That’s a convoluted way of saying that you can selectively change the administrative distance of learned OSPF routes on a router. Check it out:
R6 is learning OSPF routes from R5 (amongst other things). Of course by default, OSPF’s “trust level” or administrative distance is 110. So all learned OSPF routes are going to have an administrative distance of 110:
Gateway of last resort is not set
145.55.0.0/16 is variably subnetted, 6 subnets, 5 masks
O 145.55.136.5/32 [110/64] via 145.55.156.5, 00:00:05, Serial0/1
[110/64] via 145.55.136.5, 00:00:05, Serial0/0
O 145.55.136.3/32 [110/128] via 145.55.156.5, 00:00:05, Serial0/1
[110/128] via 145.55.136.5, 00:00:05, Serial0/0
C 145.55.136.0/29 is directly connected, Serial0/0
C 145.55.156.0/22 is directly connected, Serial0/1
O 145.55.25.0/24 [110/65] via 145.55.156.5, 00:00:05, Serial0/1
[110/65] via 145.55.136.5, 00:00:05
[110/65] via 145.55.136.5, 00:00:05, Serial0/0
C 145.55.64.0/19 is directly connected, FastEthernet0/0
O 192.168.5.0/24 [110/65] via 145.55.156.5, 00:00:05, Serial0/0
[110/65] via 145.55.136.5, 00:00:05, Serial0/0
C 192.168.6.0/24 is directly connected, Loopback1
O 192.168.2.0/24 [110/66] via 145.55.156.5, 00:00:05, Serial0/1
[110/66] via 145.55.136.5, 00:00:05
[110/66] via 145.55.136.5, 00:00:05, Serial0/0
O 192.168.3.0/24 [110/129] via 145.55.156.5, 00:00:05, Serial0/1
[110/129] via 145.55.136.5, 00:00:05, Serial0/0
R6#
To change the administrative distance of one route, let’s choose the 192.168.5.0/24 route, you can apply the distance router command (this command isn’t OSPF specific, it’s for all IP routing protocols):
distance {ip-address {wildcard-mask}} [ip-standard-list] [ip-extended-list]
So by applying the following commands, we can change the administrative distance to 50:
R6(config)#ip access-list standard AD_Change
R6(config-std-nacl)#permit 192.168.5.0 0.0.0.255
R6(config-std-nacl)#
R6(config-std-nacl)#router ospf 67
R6(config-router)#distance 50 5.5.5.5 0.0.0.0 AD_Change
5.5.5.5 is the router which is sourcing the routes that we’d like to change and 0.0.0.0 means that we want to only match 5.5.5.5. Now check it out:
Gateway of last resort is not set
145.55.0.0/16 is variably subnetted, 6 subnets, 5 masks
O 145.55.136.5/32 [110/64] via 145.55.156.5, 00:01:04, Serial0/1
[110/64] via 145.55.136.5, 00:01:04, Serial0/0
O 145.55.136.3/32 [110/128] via 145.55.156.5, 00:01:04, Serial0/1
[110/128] via 145.55.136.5, 00:01:04, Serial0/0
C 145.55.136.0/29 is directly connected, Serial0/0
C 145.55.156.0/22 is directly connected, Serial0/1
O 145.55.25.0/24 [110/65] via 145.55.156.5, 00:01:04, Serial0/1
[110/65] via 145.55.136.5, 00:01:04
[110/65] via 145.55.136.5, 00:01:04, Serial0/0
C 145.55.64.0/19 is directly connected, FastEthernet0/0
O 192.168.5.0/24 [50/65] via 145.55.156.5, 00:01:04, Serial0/1
[50/65] via 145.55.136.5, 00:01:04, Serial0/0
C 192.168.6.0/24 is directly connected, Loopback1
O 192.168.2.0/24 [110/66] via 145.55.156.5, 00:01:05, Serial0/1
[110/66] via 145.55.136.5, 00:01:05
[110/66] via 145.55.136.5, 00:01:05, Serial0/0
O 192.168.3.0/24 [110/129] via 145.55.156.5, 00:01:05, Serial0/1
[110/129] via 145.55.136.5, 00:01:05, Serial0/0
R6#