OSPF Downward Option

PE1 router advertises intra area routes int MP-BGP. other PE2 redistributes the MP-BGP routes into OSPF as LSA type 3 (Summary address) as MPLS backbone acts as an OSPF superbackbone.

PE2 sets the Down bit on those routes to avoid routing loops in scenarios which a site is dual homed to the service provider network.

If a router receives a Type 3 LSA with its Down bit set on an interface that belongs to a VRF, it drops the LSA. In cases which the site is not dual home this loop prevention mechanism is undesirable.

To disable this behaviour the following command can be used under router ospf, which no longer perform PE specific checks:

R1(config-router)# capability vrf-lite

If the router doesn't support that command, different OSPF domain-ids should be set on the PE routers, which in turn changes the route types to External(LSA Type 5) instead of Summary (LSA Type3).

Resource Limitation in OSPF

Limit maximum number of LSAs in the database:
R1(config-router)#max-lsa <1-4294967294>

Limit maximum number of redistributed routes:
R1(config-router)#redistribute maximum-prefix <1-4294967294>

To specify the minimum percentage of CPU process time OSPF takes before the CPU should yield to a process with a higher priority:
R1(config-router)#process-min-time percent <1-100>

http://www.cisco.com/en/US/docs/ios/iproute_ospf/command/reference/iro_osp2.html

Sub-second Hello Interval in OSPF

R1(config-if)#ip ospf dead-interval minimal hello-multiplier <3-20>

OSPF Summary and Discard Route

R1(config-router)#area 0 range 100.0.0.0 255.255.192.0
R1(config-router)#no discard-route internal

Same as BGP and EIGRP, when summarising networks in OSPF, it automatically adds a route to Null0 interface. discard-route command stops OSPF from generating that route.

Conditional Default Route with IP SLA in OSPF

R1(config)#ip sla 1

R1(config-ip-sla)#icmp-echo x.x.x.x

R1(config-ip-sla)#frequency y

R1(config-ip-sla)#ip sla schedule 1 life forever start-time now

R1(config)#exit

R1(config)#track 1 ip sla 1

R1(config)# ip prefix-list TRACK_DUMMY permit 1.0.0.1/32

R1(config)#route-map TRACK IPSLA

R1(config-route-map)#match ip add prefix-list TRACK_DUMMY

R1(config)#router ospf 1

R1(config-router)#default-information originate always route-map TRACK_IPSLA

OSPF Type 7 to 5 Translation

• ABR with the highest router-id does the translation (avoid unnecessary and redundant LSA generation)
• Preserves forward address
• There is no type 4 ASBR summary (as forwarding address is preserved)
• Yet the calculation of the final forwarding path stays independent

OSPF Network Types Summary

Interface Type	uses DR/BDR?	Dynamic Neighbour Discovery	Default Hello Interval	Cisco Priprietary
broadcast	Yes	Yes	10	Yes
nonbroadcast	Yes	No	30	No
point-to-point	No	Yes	10	Yes
Loopback	No	-	-	Yes
point-to-multipoint	No	Yes	30	No
point-to-multipoint nonbroadcast	No	No	30	Yes

OSPF Frame-Relay Point-to-multipoint Host Routes

OSPF automatically adds a host(/32) route for neighbours on point to multipoint network. somehow serves as dlci map command.

R1:


router ospf 1
 log-adjacency-changes
 network 192.168.0.0 0.0.0.255 area 0

interface Serial1/0.1 multipoint

 ip address 192.168.0.1 255.255.255.0

 ip ospf network point-to-multipoint

 frame-relay interface-dlci 101

 frame-relay interface-dlci 102

 frame-relay interface-dlci 103

show ip route:


     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.2 [110/65] via 192.168.0.2, 00:09:13, Serial1/0.1
     20.0.0.0/24 is subnetted, 1 subnets
O       20.20.20.0 [110/65] via 192.168.0.2, 00:09:13, Serial1/0.1
     192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.0.0/24 is directly connected, Serial1/0.1
O       192.168.0.2/32 [110/64] via 192.168.0.2, 00:09:13, Serial1/0.1

OSPF Default Route Cost

set on ABR:
area [area-number] default-cost [cost]

OSPF Forwarding Address and How to Suppress It

If the forwarding address in an external LSA is specified, and this address is not reachable, the address contained in the LSA is not inserted into the route table. When NSSA ABR translates the type 7 NSSA LSA into they 5 LSA, by default the forwarding address is transferred from type 7 to type 5. The ABR can be configured to suppress the forwarding address during the translation, replacing the specified address with the address 0.0.0.0. When another router receives the type 5 external LSA with the forwarding address suppressed, instead of trying to direct traffic for the external address to the forwarding address the receiving router will attempt to direct the traffic to the toe 7 to type 5 translating ABR router.

area 10 nssa translate type 7 suppress-fa

Routing TCP/IP Volume 2, Second Edition

area range Command

area range command specifies the area to which the summary adress belongs, the summary address, and the address mask.

It also installs a route to the null interface automatically. The router can be configured to not install it in the route table using the command no discard-route.

no-advertise switch is used for filtering summary addresses to be advertise to backbone network.

Routing TCP/IP Volume 2, Second Edition

Which ABR Translates Type 7 to Type 5

In scenarios which there are more than one ABR connected to the NSSA area, only the ABR with the highest router-id may translate Type 7 LSA(s) to Type 5 LSA(s).

RFC 1587 - 3.4

http://gns3vault.com/OSPF/ospf-nssa-lsa-type-7-to-5-translator-election.html

OSPF External Type 1 and Type 2 Comparison

E1 Routes: Both the external cost and internal OSPF cost matters.
E2 Routes: Only the external cost matters (unless there is a tie).

Use E2 if the goal is to always send traffic through one ASBR.
Use E1 if the goal is to balance the traffic, and make each router pick the closes ASBR.

CCNP-TSHOOT 642-902 Official Certification Guide

Calculating the Cost of Type 2 External Routes-Inter-Area

1. Calculate the cost to rech the ABR, based on the local area's topology.
2. Add the cost from the ABR to the ASBR, as listed in a Type 4 LSA.

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Type 2 External Routes-Intra-Area

1. Find the advertising ASBR(s) as listed in the Type 5 LSA(s).
2. Calculate the lowest cost route to reach any of the ASBR(s) based on the area topology.

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Inter-Area Routes

1. Calculate the intra-area cost from the router to the ABR listed in the type 3 LSA.
2. Add the cost value listed in the Type 3 LSA

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Intra-Area Routes

1. find all subnets inside the area, based on LSA type 1 and 2.
2. Runs SPF and find possible paths.
3. Calculate the OSPF interface costs for all outgoing interfaces and picking the lowest total cost route for each subnet as the best route.

CCNP-ROUTE 642-902 Official Certification Guide

show ip ospf statistics

Provides information about how frequently a router is executing the SFP algorithm.This command also shows when the SPF algorithm last ran and is recommended as the first troubleshooting step for link-state advertisement (LSA) flapping.

Record of reasons causing SPF to be executed:

• N—A change in a network LSA (type 2) has occurred.
• R—A change in a router LSA (type 1) has occurred.
• SA—A change in a Summary autonomous system boundary router (ASBR) (SA) LSA has occurred.
• SN—A change in a Summary Network (SN) LSA has occurred.
• X—A change in an External Type-7 (X7) LSA has occurred.

CCNP-TSHOOT 642-832 Official Certification Guide and http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_s4gt.html#wp1167148

OSPF P-bit

When external routing information is imported into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated. Only those LSAs with the P-bit set are translated. When you redistribute information into the NSSA, the P-bit is automatically set. A possible workaround applies when the Autonomous System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR can then summarise with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml

OSPF Load Blance

If multiple equal-cost, equal-path-type exist, OSPF load balances over a maximum of 16.

R1(config-router)#maximum-path [1-16]