Disclaimer: at first I thought this was merely a question about the title, but I realised it's many questions. I'm sorry if this caused the post to be confusing. In an attempt to improve the situation, I include a short question list at the end.
I have a network setup like in the picture.
BGP speakers are R2, connected only to R6 (no i-BGP betwen R2 and R3), R6 connected to R2 and R5, R5 connected to R4 (i-BGP) and R6, R4 connected to R5 (i-BGP) and R3, and R3 connected to R4.
Inside AS 110 I have OSPF configured, with OSPF redistribution to BGP and BGP redistribution to OSPF (command redistribute bgp 110 in both R2 and R3), metric type 2.
I'm doing a traceroute from R1 to 222.130.10.6
I get this output:
R1#traceroute 222.130.10.6
Type escape sequence to abort.
Tracing the route to 222.130.10.6
1 222.110.20.3 24 msec
222.110.10.2 16 msec
222.110.20.3 16 msec
2 10.110.130.6 40 msec
10.110.120.4 24 msec
10.110.130.6 28 msec
This is not exactly what I expected. I understand from inspection with Wireshark and https://community.cisco.com/t5/routing/traceroute-with-multiple-next-hops/td-p/3404248 why this output appears "out of phase", meaning, why the two paths are scrambled, but I was hoping that the complete path through R1 was shown.
In Wireshark I confirmed that no UDP traceroute packets are being sent with a TTL bigger than 2 in either interface of R1. It seems like since it gets a response from one path with just 2 hops, it doesn't try to find the path with more hops.
Is this expected behaviour?
I have also noticed that ping 222.130.10.6 only uses the path through R2, i.e., the ICMP packets only appear in Wireshark in the interface between R2 and R1. In the interface between R1 and R3 nothing appears no matter how much I ping, even though both routes are valid according to the IP forwarding table of R1.
This is the IP forwarding table of R1:
R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 222.110.20.0/24 is directly connected, FastEthernet0/1
C 222.110.10.0/24 is directly connected, FastEthernet0/0
O E2 222.120.10.0/24 [110/1] via 222.110.20.3, 04:08:04, FastEthernet0/1
[110/1] via 222.110.10.2, 04:08:04, FastEthernet0/0
O E2 222.130.10.0/24 [110/1] via 222.110.20.3, 04:08:04, FastEthernet0/1
[110/1] via 222.110.10.2, 04:08:04, FastEthernet0/0
O 222.110.30.0/24 [110/20] via 222.110.20.3, 04:47:26, FastEthernet0/1
[110/20] via 222.110.10.2, 04:47:28, FastEthernet0/0
An analogous situation is happening with 222.120.10.4, the traceroute uses both paths but the longer one is not fully shown, and ping chooses only the shortest path.
What I was expecting would be that ping would choose a random path each time, and that traceroute would show both complete paths everytime. Aren't both paths, from the point of view of R1, of equal cost (even though one goes through more ASes)?
These are the external LSAs for the network of 222.130.10.6
R1#show ip ospf database external
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 1208
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 222.130.10.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000008
Checksum: 0x2E88
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 130
Routing Bit Set on this LSA
LS age: 1522
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 222.130.10.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000009
Checksum: 0x5962
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 120
Question list:
1- Is this standard traceroute behaviour, i.e., if there are multiple routes of different hop count traceroute doesn't fully show the longer routes? Is there a way to make it show the full route to 222.130.10.6 through R3?
2- Can R1 distinguish that one of the paths to 222.130.10.6 is longer in terms of number of ASes? Why/why not? I think not, but my intuition says it should. I realise R1 is not a BGP speaker, so for it to be aware of the path length in ASes, when each of R2 and R3 redistribute their BGP route to 222.130.10.0/24 into OSPF they would have to somehow give some metric that corresponds to path length. Analysing the External LSAs, it doesn't seem to have any such metric. I also checked the Router and Network LSAs, which as per my prediction didn't have any information about external networks.
3- As everything seems to indicate that R1 can't distinguish which path is longer, because that unknown metric I referenced seems to not exist, the only solution would be to close the BGP overlay loop and connect R2 to R3 via iBGP?
