OSPF’s Maximum Metric
There are some situations where you just don’t want any traffic flowing through a particular router unless there are ABSOLUTELY NO other alternatives. How can we do that? Tell a particular router to advertise his/her LSAs with the maximum metric possible (0xFFFF).
There are several good reasons to make use of this feature listed here. I’m particularly fond of the third option, gracefully removing a router. This could also apply to maintenance windows. If I know I’m going to be knocking out a transit link, I could force neighboring routers to recalculate a next-hop before I knock out the link. This way no packets are black-holed while SPF calculations are performed.
Assume I need to get from R0 to R3 (1.2.3.4) and my only transit routers are R1 and R2. R0 is currently using CEF to load-balance over the two paths:
R0#sh ip route 1.2.3.4
Routing entry for 1.2.3.4/32
Known via "ospf 1", distance 110, metric 21, type intra area
Last update from 1.1.1.1 on FastEthernet0/0, 00:00:27 ago
Routing Descriptor Blocks:
* 1.1.1.3, from 1.1.1.5, 00:00:27 ago, via FastEthernet0/1
Route metric is 21, traffic share count is 1
1.1.1.1, from 1.1.1.5, 00:00:27 ago, via FastEthernet0/0
Route metric is 21, traffic share count is 1
R0# |
If I were to unplug R1 without warning, a packet or two might be on its way from R0 to R1 to R3, only to die at R1 due to a power loss. Instead, I’ll tell R1 to advertise the maximum possible metric:
R1(config)#router ospf 1 R1(config-router)#max-m R1(config-router)#max-metric ? router-lsa Maximum metric in self-originated router-LSAs R1(config-router)#max-metric ro R1(config-router)#max-metric router-lsa ? external-lsa Override external-lsa metric with max-metric value include-stub Set maximum metric for stub links in router-LSAs on-startup Set maximum metric temporarily after reboot summary-lsa Override summary-lsa metric with max-metric value <cr> R1(config-router)#max-metric router-lsa |
Now on R0 there’s only one route to 1.2.3.4 because the metric in R1’s LSA is 65535:
R0#sh ip route 1.2.3.4
Routing entry for 1.2.3.4/32
Known via "ospf 1", distance 110, metric 21, type intra area
Last update from 1.1.1.3 on FastEthernet0/1, 00:02:58 ago
Routing Descriptor Blocks:
* 1.1.1.3, from 1.1.1.5, 00:02:58 ago, via FastEthernet0/1
Route metric is 21, traffic share count is 1
R0#
R0#sh ip o d ro 5.5.5.5
OSPF Router with ID (3.3.3.3) (Process ID 1)
Router Link States (Area 0)
Routing Bit Set on this LSA
LS age: 211
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 5.5.5.5
Advertising Router: 5.5.5.5
LS Seq Number: 80000012
Checksum: 0x109D
Length: 60
Area Border Router
Number of Links: 3
Link connected to: a Transit Network
(Link ID) Designated Router address: 1.1.1.6
(Link Data) Router Interface address: 1.1.1.6
Number of TOS metrics: 0
TOS 0 Metrics: 65535
<!-- output omitted -->
R0# |
This will be handy.
Where are the updates to this site? I miss the dry wit and precise, quantitative solutions to inefficiency that it offered.