OSPF
Overview
Areas
- Backbone area (0) is required any time you have more than a single area
Router's Role/Type
Internal router- All interfaces in same area
Backbone router- Has an interface in area 0
Area border router (ABR)- Has an interface in area 0 and another area. Must also be a backbone router
- Use
show ip ospf borderto determine ABR/ASBR Autonomous System Border Router (ASBR)- Performs redistribution to/from another routing domain
- Originates LSA Type-5 or LSA Type-7
Virtual-links and GRE Tunnels DA#1
- Area 0 cannot be discontiguous
- Use virtual-link to allow backbone area transit over another area
- Is not data plane, just allows establishment of control plane
- i.e. packets are not moving over the virtual link
Virtual link configuration:
router ospf <pid>
area <transit-area> virtual-link <target-rid>
Show virtual links: show ip ospf virtual-links
GRE tunnel vs virtual-link
- Virtual link: control plane only, not found in routing table
- "GRE tunnel for only OSPF traffic"
- GRE tunnel: control and data plane
- By default, OSPF cost 1000
OSPF and BGP RID. BGP synchornization DA#2
OSPF RID- 32-bit dotted-decimal value, unique in entire routing domain. Is not an IP address
- Numerically highest IP address will be chosen if not defined
BGP synchronization- Requires a route to be seen via IGP to be valid in BGP
- OSPF and BGP RID must also match at point of EGP/IGP redistribution
Split-Horizon Rule
Areas must use the backbone area to communicate between one another
Capability Transit DA#3
- Enabled by default
- Overrides the split horizon rule, so data plane traffic may transit directly between non-zero areas
- If disabled, traffic would try to transit through area-0
To turn off:
router ospf <pid>
no capability transit
LSAs
Advertisements regarting the states of links
LSA Types
| LSA | Advertising Rtr | Rt Tbl | Database |
|---|---|---|---|
| 1 | All routers | O |
show ip ospf database router |
| 2 | DR | O |
show ip ospf database network |
| 3 | ABR | O IA |
show ip ospf database summary |
| 4 | ABR | show ip ospf database asbr-summary |
|
| 5 | ASBR | O E2 O E1 |
show ip ospf database external |
| 7 | ASBR | O N2 O N1 |
show ip ospf database nssa-external |
Type-1- Router LSA intra-area routers
Type-2- Network LSA intra-area multi-access networks
Type-3- Summary LSA inter-area routes
Type-4-
Summary ASBR LSA Reachability to the ASBR
- "If you want to connect to the router in the LSID, go to the router in the Adv_R"
- LSID = RID of external router
- Adv_R =
- Metric = cost from my area to redistributed route
Type-5-
External routes (from another routing domain)
- Contains an LSID (route I distribute)
- Contains an Adv_R (route originating LSA)
- Contains an FA (next hop)=0.0.0.0 suppressed - found by looking up Type-1 LSA for Adv_R
Type-7-
NSSA LSA
- LSID=network
- Adv_R=RID of advertiser
- FA=IP of ASBR not suppressed in NSSA
Type-7 to Type-5 Conversion
N2 route converted to E2 route Type-7 routes converted into Type-5 going to area 0 FA is not suppressed, no Type-4 is advertised
External route metrics
E2/N2- Cost of network from advertising ASBR (cost=20)
E1/N1- Cost of network from advertising router + cost of me to ASBR
Bit types
E-bit- I have capability of having external routes in my table. Normal area E is set, Stub area E is cleared
V-bit- I termimnate a virtual link.
B-bit- I am an ABR
Traffic types
intra-area- Traffic within same area
inter-area- Traffic that enters one area from another area (same routing domain)
external- Traffic that was redistributed into my routing domain
OSPF DR/BDR
DR/BDR is elected on multi-access network.
- Prevents full-mesh of adjacencies
- each router establishes adjaceny with only DR
- DR floods updates to
DROthers
- OSPF priority=1 by default, 0=don't participate
- Tie-breaker: highest OSPF RID
RFC5185
DA#4
Note: intra-area routes (stay in same area) preferred over inter-area routers
Allows a link to be configured in multiple areas
- Must be a point to point link from OSPF's perspective
ip ospf multi-area <area-id>
Must configure link as point-to-point:
R2#show ip ospf multi-area
OSPF_MA0 is down, line protocol is down
Primary Interface GigabitEthernet0/1, Area 1
Interface ID 3
MTU is 1500 bytes
Interface DOWN as link is not P2P
Multi-area established w/ 1 neighbor:
R2#show ip ospf multi-area
OSPF_MA0 is up, line protocol is up
Primary Interface GigabitEthernet0/1, Area 1
Interface ID 3
MTU is 1500 bytes
Neighbor Count is 1
Two neighborships shown:
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
0.0.0.1 0 FULL/ - 00:00:30 12.1.1.1 GigabitEthernet0/1
0.0.0.1 0 FULL/ - 00:00:32 12.1.1.1 OSPF_MA0
Virtual interface is created:
R2#show ip ospf int br
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Gi0/1 1 0 12.1.1.2/24 1 P2P 1/1
MA0 1 1 Unnumbered Gi0/1 1 P2P 1/1
DA#5
Use RFC5185 multi-area to give the ASBR a link in another area
- Routes are not redistributed into a given area, just into the OSPF routing domain
ASBR cannot exist in a stub area:
*Dec 17 17:26:15.631: %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area
OSPF Recursion Process (DA#6)
R1#show ip ospf database external
OSPF Router with ID (0.0.0.1) (Process ID 1)
Type-5 AS External Link States
LS age: 90
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 0.0.0.4
LS Seq Number: 80000001
Checksum: 0x3F51
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
Look for Adv_R:
R1#show ip ospf database router adv-router 0.0.0.4
OSPF Router with ID (0.0.0.1) (Process ID 1)
Go look for LSA Type-4:
R1#show ip ospf database asbr-summary 0.0.0.4
OSPF Router with ID (0.0.0.1) (Process ID 1)
Summary ASB Link States (Area 1)
LS age: 212
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 0.0.0.4 (AS Boundary Router address)
Advertising Router: 0.0.0.2
LS Seq Number: 80000001
Checksum: 0x4C69
Length: 28
Network Mask: /0
MTID: 0 Metric: 128
Go look for this Adv_R
R1#show ip ospf database router adv-router 0.0.0.2
OSPF Router with ID (0.0.0.1) (Process ID 1)
Router Link States (Area 1)
LS age: 294
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 0.0.0.2
Advertising Router: 0.0.0.2
LS Seq Number: 80000002
Checksum: 0x725B
Length: 36
Area Border Router
Number of Links: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 12.1.1.2
(Link Data) Router Interface address: 12.1.1.2
Number of MTID metrics: 0
TOS 0 Metrics: 64
How do I get to 12.1.1.2?
R1#show ip route 12.1.1.2
Routing entry for 12.1.1.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via GigabitEthernet0/2
Route metric is 0, traffic share count is 1
We are directly connected, so just go directly to 12.1.1.2.
R1#show ip route 4.4.4.0
Routing entry for 4.4.4.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 192
Last update from 12.1.1.2 on GigabitEthernet0/2, 00:05:14 ago
Routing Descriptor Blocks:
* 12.1.1.2, from 0.0.0.4, 00:05:14 ago, via GigabitEthernet0/2
Route metric is 20, traffic share count is 1
Same thing in an NSSA area
R4#show ip ospf database nssa-external
OSPF Router with ID (0.0.0.4) (Process ID 1)
Type-7 AS External Link States (Area 2)
LS age: 49
Options: (No TOS-capability, Type 7/5 translation, DC, Upward)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 0.0.0.4
LS Seq Number: 80000001
Checksum: 0x73EA
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 34.1.1.4
External Route Tag: 0
Type 7/5 translation means P-bit is set (propagate bit)
When LSA is converted from type7/5 on 0.0.0.3, it becomes the Adv_R, Fwd_Addr is propagated:
R1#show ip ospf database external
OSPF Router with ID (0.0.0.1) (Process ID 1)
Type-5 AS External Link States
LS age: 127
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 0.0.0.3
LS Seq Number: 80000001
Checksum: 0xE5B
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 34.1.1.4
External Route Tag: 0
Example The 34.1.1.4 network (Fwd_Addr) is suppressed upstream. 4.4.4.0 network is removed from the routing table since ASBR is not accessible to R1.
Solution: suppress the FA when type7 is converted to type4. Now normal recursion will be used on R1 to find Type-4 LSA to provide reachability to the ASBR (R3). R3#show run | sec router ospf router ospf 1 router-id 0.0.0.3 area 2 nssa translate type7 suppress-fa area 2 range 34.1.1.0 255.255.255.0 not-advertise
Multiple ABR in NSSA (DA#7)
In NSSA with multiple ABR's, there is an election and the highest RID becomes the advertising router (and performs type7 to type5)
Override higher RID with:
R3(config-router)#area 1 nssa translate type7 always
Do this on multiple ABR's and both will be the advertising router.
Network type impact on redistribution (DA#8)
Show all ABR/ASBR and cost to routers
R5#show ip ospf border-routers
OSPF Router with ID (0.0.0.5) (Process ID 1)
Base Topology (MTID 0)
Internal Router Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 0.0.0.3 [1] via 35.1.1.3, GigabitEthernet0/3, ASBR, Area 0, SPF 3
i 0.0.0.4 [1] via 45.1.1.4, GigabitEthernet0/4, ASBR, Area 0, SPF 3
When redistributing, a P2P network type will result in a supressed FA. Broadcast network type will set the FA to the specific address (since multiple routers could exist on the segment)
(DA#9)
Fwd_Addr must be an IA route in OSPF, otherwise route will be ignored.
RFC2328 (DA#10)
RFC2328 "When the network numbers (in an LSID) are identical, there must be a way to differentiate one network from another."
- Least specific route: gets the normal LSID
- More specific routes: uses broadcast address of that network
R2#show ip ospf d external
OSPF Router with ID (0.0.0.2) (Process ID 1)
Type-5 AS External Link States
LS age: 232
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 2.0.0.0 (External Network Number )
Advertising Router: 0.0.0.2
LS Seq Number: 80000001
Checksum: 0xC1DA
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
LS age: 232
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 2.0.255.255 (External Network Number )
Advertising Router: 0.0.0.2
LS Seq Number: 80000001
Checksum: 0xC1DA
Length: 36
Network Mask: /16
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
RFC1583 & RFC2328 (DA#11)
By default RFC1583 is used
RFC1583- "The cost of a summary should be based on the lowest cost of a specific route within that summary."
RFC2328- "The cost of a summary should be based on the highest cost of a specific route within that summary."
To implement RFC2328:
router ospf <pid>
no compatible rfc 1583
Route precedence (DA#12)
- Default: N is preferred over E (RFC3101)
- RFC1587 E is preferred over N
- OSPF lower PID is preferred when running two PIDs
Preference of routes:
- Intra area
Oroutes- More than one entry? Use lower OSPF PID
- Inter area
O IAroutes - External NSSA Type 1
O N1- E1 and N1 route competing? If Fwd_Addr is different, cost will be referenced before RFC3101
- External Type 1
O E1 - External NSSA Type 2
O N2 - External Type 2
O E2