OSPF
Latest 10 Articles
FRRouting loopback interfaces and OSPF costs. [ipSpace]
OSPF summarisation and split areas. [ipSpace]
OSPF blog posts. [ipSpace]
OSPF ECMP with unnumbered IPv4 interfaces. [ipSpace]
OSPF and ARP on unnumbered IPv4 interfaces. [ipSpace]
Unnumbered links in OSPF. [Lost In Transit]
OSPF convergence in a hub-and-spoke topology. [Lost In Transit]
In defense of OSPF in the underlay -- in some situations. [DC Overlords]
OSPF external routes (Type-5 LSA) mysteries. [ipSpace]
Older Articles
Understanding OSPF Router ID assignment. [Packet Pushers]
Do OSPF router IDs actually have to be unique? [Packet Pushers]
Running OSPF over unnumbered Ethernet interfaces. [ipSpace]
LSA/LSP flooding in OSPF and IS-IS. [ipSpace]
OSPF inter-process route selection. [ipSpace]
Why is OSPF not using TCP? [ipSpace]
What I've learned about scaling OSPF in DCs. [Elegant Network]
OSPF: graphs, LSAs, and the LSDB [Packet Pushers]
Running OSPF in a single non-backbone area. [ipSpace]
Is OSPF unpredictable or just unexpected? [ipSpace]
OSPF topology transparent zones. [Russ White]
Five ways OSPF is broken. [Packet Pushers]
OSPFv3 configuration example on Cisco IOS. [ipCisco]
Synchronising BGP and OSPF. [ipSpace]
What you were never told about OSPF. [Network Collective, YouTube] [Russ White]
The OSPF two part metric. [Russ White]
Five functional facts about OSPF. [Packet Mischief]
The unintended consequences of NSSA kludges. [ipSpace]
OSPF TLVs: taking advantage of improvements in computing power. [Russ White]
OSPF forwarding address: yet another kludge. [ipSpace]
OSPF forwarding address take 2. [ipSpace]
Why OSPF needs forwarding address with NSSA areas. [ipSpace] [Russ White]
More thoughts on OSPF FA. [ipSpace]
NSSA saga continues. [ipSpace]
OSPF areas and summarisation: theory and reality. [ipSpace]
Do we still need OSPF areas and summarisation? [ipSpace] [Russ White]
The new strange behavior for OSPF ‘redistribute subnets’. [PacketU]
Don't run OSPF with your customers. [ipSpace]
OSPF path selection challenge. [INE]
OSPF design considerations. [Lost In Transit]
OSPF enhancements in recent IOS versions. [MellowD]
Demystifying the OSPFv3 database. [MellowD: part 1, part 2, part 3, part 4]
SPF delay -- CCDE. [Mellowd]
OSPF data center design. [StackExchange]
How does OSPF decide that a network is a stub network? [StackExchange]
OSPF manual neighbor configuration requirement on NBMA network. [StackExchange]
Hub & spoke OSPF DR and BDR. [StackExchange]
How bad is the OSPF vulnerability exposed by Black Hat? [EtherealMind] [Routing Freak]
Redistribute command in an all OSPF network. [Reddit]
Dual stack OSPF redistribution in MP-BGP. [StackExchange]
Network not participating in OSPF. Does not appear in the routing table. [StackExchange]
OSPF: inconsistent mask for area range? [StackExchange]
Summarised OSPF routes advertised? [StackExchange]
What is the advantage of an “area” in OSPF configuration? [StackExchange]
T2 LSA's in OSPF, why are they necessary? [StackExchange]
Juniper: why is OSPF adjacency breaking when I enable FBF on an OSPF interface? [StackExchange]
3 subnets, 2 OSPF areas - will this work? [StackExchange]
Using latency to calculate OSPF metrics. [StackExchange]
Inter-process OSPF route selection rules. [ipSpace]
OSPF -- setting MTU values for Cisco and Juniper. [Network Sherpa]
OSPF -- sequence numbers are so negative. [Network Sherpa]
OSPF -- immediate hellos. [Network Sherpa]
OSPF -- broadcast interfaces and Type-2 LSAs. [Network Sherpa]
OSPF -- lingering LSAs from unreachable routers. [Network Sherpa]
OSPF -- routing bit set on this LSA. [Network Sherpa]
OSPF Type-5 E1 path cost calculation. [Network Sherpa]
Change in OSPF DR creates extra network LSAs. [ipSpace]
OSPF -- what really triggers Type-4 LSA origination? [Network Sherpa]
OSPF -- why have a Type-4 LSA? [Network Sherpa]
OSPF LSInfinity != LSInfinity. [Network Sherpa]
How OSPF max-metric works. [Network Sherpa]
Loop-free alternate: OSPF meets EIGRP. [ipSpace]
Black hat OSPF vulnerabilities. [Juniper] [Network World]
OSPF designated router election. [PacketLife]
OSPF and connected networks: to redistribute or not? [IOS Hints]
Understanding OSPF external route path selection. [CCIE Blog]
OSPF virtual links. [PacketLife]
OSPF and MTU mismatch. [CCIE Blog]
OSPF route selection rules. [IOS Hints]
Understanding inter-area loop prevention caveats in OSPF. [CCIE Blog] [PDF attached below]
OSPF on the move? Include a forwarding address. [CCIE Blog]
Cycling OSPF authentication without adjacency downtime. [PacketLife]
OSPF fast convergence. [CCIE Blog]
OSPFv2 authentication confusion. [PacketLife]
Conditional OSPF default route origination based on classless IP prefixes. [NIL Wiki] [IOS Hints]
OSPF sham links. [CCIE Blog]
OSPFv2 vs OSPFv3. [PacketLife]
OSPF filtering -- inter-area vs. intra-area. [PacketLife]
Overview
Link-state protocol.
Uses Dijkstra which calculates the shortest path tree in each area.
OSPF v2 for IPv4 and OSPFv3 for IPv6.
LSDB = link-state database.
Area 0 = backbone, all inter-area traffic goes via area 0.
Directly attached routers (i.e. in the same broadcast domain) form adjacencies using hello packets:
DR and BDR controls traffic between neighbours
224.0.0.5 = all SPF/link-state routers
224.0.0.6 = all DR routers
Does not use TCP/UDP, but IP protocol 0x89.
Area Types
Backbone: all other areas connect to it, so it must be (logically) contiguous.
Stub: contains intra-, inter-area and default routes, but no external routes.
Totally stubby: contains intra-area and default routes only.
NSSA: contains intra-, default and external routes:
within NSSA, ASBR sends type 7 LSAs.
at ABR, sends type 5 LSAs to rest of domain.
Router Types
ABR: connects one or more areas to area 0.
ASBR: connected to more than one AS, and typically runs BGP as well.
IR: internal router (inside an area).
BR: backbone router (inside area 0).
DR/BDR
If priority = 0 then never DR/BDR.
Highest priority wins, then highest logical IP address, then highest active interface IP address.
When DR fails, BDR becomes new DR and another BDR is elected.
New routers with higher priority do not take over DR/BDR unless there is a DR/BDR failure.
LSA Types
1 – Router – Intra-area (generated by all routers)
2 – Network – Intra-area (generated by DR on NBMA networks)
3 –Summary – Routes to networks (generated by ABR for intra-AS routes)
4 – Summary – Routes to ASBRs (generated by ABR for intra-AS routes)
5 – AS External – Routes to external destinations (generated by ASBR)
7 – NSSA external link state
8 – External attributes for BGP
9, 10, 11 – Opaque LSA
Path Preference
Path cost/metric is usually associated with link speed
4 metric types:
intra-area
inter-area
external type 1 = external + internal path cost
external type 2 = external path cost
OSPF-TE
Extension to OSPF.
Additional properties:
max reservable bandwidth
unreserved bandwidth
available bandwidth
IOS Configuration Example
router ospf
router-id x.x.x.x
auto-cost reference-bandwidth <...>
area 0.0.0.0 authentication message-digest
passive-interface default
no passive-interface <...>
network x.x.x.x y.y.y.y area z.z.z.z
JUNOS Configuration Example
protocol
ospf
reference-bandwidth <x>
area 0.0.0.0
authentication type md5
interface <i/f>
metric
authentication
md5 <...>
passive
policy-options
prefix-list <name>
<...>
policy-statement <name>
term <name>
from <...>
then [accept|reject]
firewall
filter <name>
term <name>
from [src/dst address|src/dst port]
then
count <name>
[accept|reject|discard]
© Robert Larsen. All rights reserved.