Compatible RFC1583 behavior

EdTheLadEdTheLad Posts: 2,112Member
My curiosity got the better of me and i wanted to know what this command did. After some research i discovered that it's enabled by default on Cisco ios and it has two functions.

1) By default when multiple type 3 routes are summarized as the abr, the advertised summary route will inherit the lowest cost from its child routes. When the "no compatiblity rfc1583" command is issued, this behavior is changed so the highest child cost is associated with the summary. This works as expected on 12.4 ios.

2) Loop avoidance, by default when a Cisco router receives a type 5 route from multiple ASBRs, it will choose the lowest path cost to the ABR regardless of whether it's intra or inter area, both intra and inter path costs are compared and the lowest wins.

As per the RFC 2328, when rfc1583 compatibility is disabled, the behavior changes and now intra area routes are preferred over inter-area.


This section only applies when RFC1583Compatibility is set
to "disabled".

The path preference rules, stated from highest to lowest
preference, are as follows. Note that as a result of these
rules, there may still be multiple paths of the highest
preference. In this case, the path to use must be determined
based on cost, as described in Section 16.4.

o Intra-area paths using non-backbone areas are always the
most preferred.

o The other paths, intra-area backbone paths and inter-
area paths, are of equal preference.


R4
R3
R1
R5

R4 and R5 are ASBRs advertising ext route 19.19.19.19/32, R1 is the ABR, R4 & R3 are in Area 1; R5 is in Area 0.

(no compatibility configured)
R3#sh runn | sec router ospf
router ospf 1
router-id 3.3.3.3
network 10.0.13.3 0.0.0.0 area 1
network 10.0.34.3 0.0.0.0 area 1


R3#sh ip ospf border-routers | i ASBR
i 4.4.4.4 [6] via 10.0.34.4, Ethernet0/0, ASBR, Area 1, SPF 12
I 5.5.5.5 [4] via 10.0.13.1, Ethernet0/1, ASBR, Area 1, SPF 12
R3#

R3#sh ip ospf database external 19.19.19.19

OSPF Router with ID (3.3.3.3) (Process ID 1)

Type-5 AS External Link States

LS age: 1910
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 19.19.19.19 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000016
Checksum: 0xD757
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 240
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 19.19.19.19 (External Network Number )
Advertising Router: 5.5.5.5
LS Seq Number: 80000018
Checksum: 0xB573
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0


R3#sh ip route 19.19.19.19
Routing entry for 19.19.19.19/32
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 4
Last update from 10.0.13.1 on Ethernet0/1, 00:01:12 ago
Routing Descriptor Blocks:
* 10.0.13.1, from 5.5.5.5, 00:01:12 ago, via Ethernet0/1
Route metric is 20, traffic share count is 1
R3#


As shown above, 19.19.19.19 is known via 2 ASBR's, 4.4.4.4 and 5.5.5.5; 4.4.4.4 is known via intra area route, 5.5.5.5 is known via inter area route. The cost to 5.5.5.5 is lower and hence 5.5.5.5 is used even though it's an inter-area route.

That's whats expected in regards to the RFC, now i will disable the rfc1583 compatibility, this should then cause R3 to give preference to the intra area ASBR 4.4.4.4

router ospf 1
router-id 3.3.3.3
no compatible rfc1583
network 10.0.13.3 0.0.0.0 area 1
network 10.0.34.3 0.0.0.0 area 1
R3(config-router)#

R3(config-router)#do sh ip ospf bord | i ASBR
i 4.4.4.4 [6] via 10.0.34.4, Ethernet0/0, ASBR, Area 1, SPF 13
I 5.5.5.5 [4] via 10.0.13.1, Ethernet0/1, ASBR, Area 1, SPF 13
R3(config-router)#

R3#sh ip ospf database external 19.19.19.19

OSPF Router with ID (3.3.3.3) (Process ID 1)

Type-5 AS External Link States

LS age: 285
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 19.19.19.19 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000017
Checksum: 0xD558
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 594
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 19.19.19.19 (External Network Number )
Advertising Router: 5.5.5.5
LS Seq Number: 80000018
Checksum: 0xB573
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

R3#sh ip route 19.19.19.19
Routing entry for 19.19.19.19/32
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 4
Last update from 10.0.13.1 on Ethernet0/1, 00:06:46 ago
Routing Descriptor Blocks:
* 10.0.13.1, from 5.5.5.5, 00:06:46 ago, via Ethernet0/1
Route metric is 20, traffic share count is 1


Interesting nothing changes, so it looks like Cisco haven't implemented that feature.
Networking, sometimes i love it, mostly i hate it.Its all about the $$$$
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