Consider the below topology for further reading,
With the above topology, Normal ICMP Ping tool works as below,
When normal ICMP echo is originated from R5 to 126.96.36.199, the ICMP packet will have the following details,
With the above details, even if the MPLS LSP is not established, we can reach the destination, as all intermittent routers will be aware of the internal network addresses.
If normal ICMP ping is originated when LSP is broken between R4 and R6, R4 will have to look into the destination address of the packet and perform RIB lookup and will be IP Forwarded. So with normal ICMP ping, we don’t have any way to detect the data plane failure in end to end LSP.
How MPLS Ping works?
MPLS Ping doesn’t rely on ICMP echo messages. Instead it uses UDP protocol with both source and destination port as 3503 and relies on MPLS Echo request and MPLS Echo reply.
When MPLS ping is triggered from any MPLS router, it will generate UDP segment with source/destination port as 3503. The source address will be selected as usual while the destination address will be 127.0.0.1. The IP TTL will be set to 1.
Below is a sample IP format when MPLS Ping is originated from R5 to 188.8.131.52/32,
Now the originating LSR will look into the LFIB and populate the label header with respective labels to reach the FEC, in our case 184.108.40.206/32.
R4 on receiving the MPLS packet will be able to send to the actual destination only if the LSP is end to end. If the LSP is broken between R4 and R6, R4 will look into the destination IP address which will be 127.0.0.1 and won’t be able to perform IP forwarding. As per RFC 1812, a router should not forward any packet that has destination address of 127.0.0.0/8
Below is few simple testcase examples which will help us better understand how MPLS Ping works.
End to end LSP is established from R5 to R6 and is capable of Label switching packets. Now when MPLS Ping is originated from R5 to 220.127.116.11/32, it will be sent out with respective label(s) as per LFIB. R6 on receiving it, will respond back with MPLS echo reply (Message Type=02) with UDP source/destination port as 3503.
LSP is fine between R5 and R4, But LDP is disabled in R4 resulting in broken LSP. Now R4 will have to perform IP lookup as the LFIB will not have any outgoing label. Assuming R4 is loaded with IOS version which supports MPLS ping, it will understand that the packet is MPLS ping which needs to be label switched while the outgoing interface is not enabled with LDP. So it will send an error message to ultimate source with Return Code=09 and subcode=01 (See error code details in RFC 4379).
Above same testcase is tried with R4 loaded with IOS version which doesn’t support MPLS Ping. So R4 will not understand the MPLS Ping traffic and will drop on looking into the destination IP address. So no error messages will be received on R5,
Points to remember:
- MPLS Ping will use UDP protocol of source and destination as 3503. It will not use the normal ICMP messages.
- Destination IP address of the MPLS ping packet will be from 127.0.0.0/8 network
- IP TTL will be set to 1.
- It is better to have all routers loaded with IOS which supports MPLS ping. If the destination doesn’t support MPLS ping, we may not receive the MPLS Echo reply and may assume that the LSP is broken.