Hi All,
I have a doubt related to OSPF route selection : (Please refer to the
topology below)
R1--------Ethernet-------R2
\ /
\ /
\ /
Gig0 \___________/Gig1
|____R3____|
The router to be considered is R3. This R3 receives multiple External
Type-1 LSAs due to "default-originate" being used on remote routers
which lie beyond R1 & R2. Router R3 establishes adjacency with R1 and R2
on two different broadcast segments.
Now the ospf database on R3 shows that it has learnt 0.0.0.0 from six
(6) different Advertising routers and also each of there Forwarding
Address Field is NON-zero address. The detailed display of the LSA shows
that all are of the same Metric Type i.e. E1 and of same Metric Value
i.e.6
Out of 6, three of Advertising routers and their corresponding
Forwarding Address are reachable via Gig0 while the rest of Advertising
Routers and their corresponding Forwarding Address are reachable via
Gig1. OSPF interface cost on both Gig1 and Gig0 are equal (1)
Now the questions are:
1. In the above situation, when 0.0.0.0 needs to be installed in
the Routing Table as OE2 route, then will it appear with two next-hops
(both Gig0 & Gig1) or it shall appear only on one next-hop (either Gig1
or Gig0) ? The behavior that I am seeing in router R3 is that ==> Route
Table has installed 0.0.0.0 as OE2 with nexthop on Gig1. Is this a
correct behavior?
2. Moreover I want to know whether Section 12.4.4 & Page 140-141
(RFC 2328), which says "The following rule is thereby established: if
two routers, both reachable from one another, originate functionally
equivalent AS-external-LSAs (i.e., same destination, cost and non-zero
forwarding address), then the LSA originated by the router having the
highest OSPF Router ID is used." is applicable in this situation or not
?
3. If #2 is applicable to the above situation, then correct my
deduction if I am wrong, as mentioned below:
I found that Advertising Router with highest router-id and its
forwarding address are ONLY REACHABLE via Gig1. Thus the 0.0.0.0 gets
installed with nexthop as Gig1 in the routing table
I would really appreciate your help.
Regards,
Jishnu
FYI, I am just adding the database log: (from router R3)
show ip ospf database external 0.0.0.0
OSPF Router with ID (202.87.37.225) (Process ID 10)
Type-5 AS External Link States
LS age: 1355
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 123.108.37.253
LS Seq Number: 80000082
Checksum: 0x1B87
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 123.108.37.17
External Route Tag: 0
LS age: 1294
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 123.108.37.254
LS Seq Number: 80000083
Checksum: 0x327E
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 123.108.37.1
External Route Tag: 0
LS age: 1284
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 124.153.97.253
LS Seq Number: 80000084
Checksum: 0x8E49
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 124.153.97.5
External Route Tag: 0
LS age: 1183
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 124.153.97.254
LS Seq Number: 80000082
Checksum: 0xC410
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 124.153.97.9
External Route Tag: 0
LS age: 1217
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 202.87.37.230
LS Seq Number: 80000696
Checksum: 0x5DE8
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 59.163.24.70
External Route Tag: 0
LS age: 1358
Options: (No TOS-capability, No DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 202.87.37.231
LS Seq Number: 80000085
Checksum: 0xF858
Length: 36
Network Mask: /0
Metric Type: 1
TOS: 0
Metric: 6
Forward Address: 219.64.42.2
External Route Tag: 0
show ip ospf neighbor
Process ID 10
Neighbor ID Pri State Dead Time Address
Interface
202.87.37.231 1 FULL/DR 00:00:31 202.87.37.165
gigabitethernet1
202.87.37.230 1 FULL/DR 00:00:39 202.87.37.161
gigabitethernet0
show ip ospf interface
gigabitethernet1 is up, line protocol is up
Internet Address 202.87.37.166/30, Area 1
Process ID 10, Router ID 202.87.37.225, Network Type BROADCAST, Cost:
1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 202.87.37.231, Intf address 202.87.37.165
Backup Designated router (ID) 202.87.37.225, Intf address
202.87.37.166
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 202.87.37.231 (Designated Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
gigabitethernet0 is up, line protocol is up
Internet Address 202.87.37.162/30, Area 1
Process ID 10, Router ID 202.87.37.225, Network Type BROADCAST, Cost:
1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 202.87.37.230, Intf address 202.87.37.161
Backup Designated router (ID) 202.87.37.225, Intf address
202.87.37.162
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 202.87.37.230 (Designated Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
loopback1 is up, line protocol is up
Internet Address 202.87.37.225/32, Area 1
Process ID 10, Router ID 202.87.37.225, Network Type LOOPBACK, Cost:
1000
Loopback interface is treated as a stub Host
show ip route ospf
Codes: R - RIP, O - OSPF, C - connected, S - static
IA - OSPF inter area route, E1 - OSPF external type 1 route,
E2 - OSPF external type 2 route, N1 - OSPF NSSA external type 1
route,
N2 - OSPF NSSA external type 2 route
* - candidate default route
O*E1 0.0.0.0/0 [110/8][10] via 202.87.37.165, gigabitethernet1
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