Lab Tip: Remembering IP Precedence Values

I’ve never bothered to memorize the Cisco descriptions of IP Precedence values (i.e. IP Precedence 4 is “flash-override”).  I’ve hit a couple of labs which ask you to manipulate traffic based on these descriptions.  For instance:

“Drop all flash traffic coming into r1 from VLAN 6.”

Some commands such as ‘random detect precedence’ only use the 0 – 7 values: 

r1(config-if)#random-detect precedence ?
  <0-7>  IP precedence
  rsvp   rsvp traffic

You could look this up in the DOC CD, but there’s a quick and dirty way to do this on the command line.

1) Create a class-map.
2) Type ‘match ip precedence ?’ to see the context-sensitive help.  This will give you a list of the IP Precedence values along with their descriptions.
3) Get what you need and remember to delete the class-map.

Rack13R6(config)#class-map FAKE
Rack13R6(config-cmap)#match ip precedence ?
  <0-7>           Enter up to 4 precedence values separated by white-spaces
  critical        Match packets with critical precedence (5)
  flash           Match packets with flash precedence (3)
  flash-override  Match packets with flash override precedence (4)
  immediate       Match packets with immediate precedence (2)
  internet        Match packets with internetwork control precedence (6)
  network         Match packets with network control precedence (7)
  priority        Match packets with priority precedence (1)
  routine         Match packets with routine precedence (0)
Rack13R6(config-cmap)#exit
Rack13R6(config)#no class-map FAKE

Question Of The Day: 15 May, 2008

Topic: OSPF

You are rolling out 10 Gigabit links across your core network.  You are running OSPF in your core.  You’ve noticed that the OSPF cost for the new 10 Gigabit links is 1.  This is the same as your FastEthernet and Gigabit links.  You are tasked with configuring the core routers so that the 10 Gigabit links will have a default OSPF cost of 10.

Click Here For The Answer

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Yesterday’s Question

Question Of The Day: 14 May, 2008 

Topic: OSPF

r1 and r2 are running OSPF in area 0.  They have established a neighbor adjacency over a single serial interface.  Configure LSAs on this adjacency to set the DoNotAge bit.

Answer:

r1 (or r2)
interface s0/0
 ip ospf demand-circuit

By default, an LSA will be refreshed every 30 minutes regardless if there are any changes in the network topology.  This is sometimes referred to as a ‘periodic’ or ‘paranoid’ update.  You can disable this update with the ‘ip ospf flood-reduction’ command.  Another way that you can accomplish this is by configuring the link as an OSPF demand circuit.

ip ospf demand-circuit

Usage Guidelines
On point-to-point interfaces, only one end of the demand circuit must be configured with this command. Periodic hello messages are suppressed and periodic refreshes of link-state advertisements (LSAs) do not flood the demand circuit. This command allows the underlying data link layer to be closed when the topology is stable. In point-to-multipoint topology, only the multipoint end must configured with this command.

Before:
r2#show ip ospf database router 1.1.1.1 | i LS
  LS age: 3
  LS Type: Router Links
  LS Seq Number: 8000000C

After:
r1(config)#int s0/0
r1(config-if)#ip ospf demand-circuit

Real World Warning: This will reset your OSPF neighbor adjacency:

*Mar  1 00:14:11.271: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on Serial1/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 00:14:15.051: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on Serial1/0 from LOADING to FULL, Loading Done

r2#show ip ospf database router 1.1.1.1 | i LS
  LS age: 5 (DoNotAge)
  LS Type: Router Links
  LS Seq Number: 8000000E
r2#show ip ospf interface s1/0

Serial1/0 is up, line protocol is up
  Internet Address 10.1.12.2/24, Area 0
  Process ID 100, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64
  Run as demand circuit.
  DoNotAge LSA allowed. 
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:09
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0×0(0)/0×0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 4 msec, maximum is 4 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1  (Hello suppressed)
  Suppress hello for 1 neighbor(s)

You only need to configure ‘ip ospf demand-circuit’ on one side of the link.  It will affect the DoNotAge bit on BOTH sides of the link (unlike the ‘ip ospf flood-reduction’ command):

r1#show ip ospf database router 2.2.2.2 | i LS
  LS age: 5 (DoNotAge)
  LS Type: Router Links
  LS Seq Number: 8000000C

This is good to keep in mind during a lab.  A task may ask you to set the DoNotAge bit on both sides on the link but restrict you to configuring only one side.