Ii. Principles and Experimental Verification of mpls ttl values

I. MPLS IN BGP Theory

No mpls ip propagate-ttl [forwarded | local]

Note: you only need to configure this command on the entry PE!

If you want to hide the P router in the public network, and the PE router entry PE at the egress, you can use this command if it is not tracked by traceroute in the private network.

By default, when an IP packet arrives at the entry PE, the TTL value in the IP packet header is reduced by 1 and then copied to the new tag TTL. If you use this command, the TTL value in the IP address header is no longer copied. Instead, the TTL value in the tag is set to 255. In this way, the public network P and the outbound PE are hidden.

Command: no mpls ip propagate-ttl forwarded only applies to the traffic flowing into the PE

No mpls ip propagate-ttl local takes effect only for the traffic generated by this PE

No mpls ip propagate-ttl takes effect for all traffic.

No mpls ip Propagate-original TTLDiagram:

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II:Theoretical Experiment verification of MPLS IN BGP

1. Configure no mpld ippropagate-ttl on PE1 before comparison:

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It can be concluded that in the actual working process, configuring this command only hides the IP router in the public network and does not hide the exported PE, which is different from the theory.

2. packet capture for the outbound interface:

Description: For TRACEROUTE and PING

1. Traceroute uses UDP to send packets. ICMP is used to receive packets.

2. In the MPLS environment, the default Traceroute value is 1.

On R1, traceroute 10.1.2.1 (CE2 loose)

1. Capture packets at CE1 S0/0:

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Conclusion:

1. Because UDP is unreliable, each packet is sent three times;

2. The traceroute UDP packet is used only as a kind of probing. It explores the TTL value required to reach the destination CIDR block. Obviously, because hidden commands are configured, PE1, PE2, and CE2 can reach the network segment 10.1.2.1/32, with 3 hops in total;

3. The starting TTL value of UDP is 1. The returned packet through ICMP cannot be reached. The TTL value is accumulated until the TTL value reaches the destination CIDR block.

2. Capture packets at S0/0 of PE1 Port:

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3. Capture packets at s0/0 port of R1:

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4. Capture packets at s0/0 port of R2:

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5. Capture packets at the s0/0 port of PE2:

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Conclusion:

1. on a public ip router that does not include an egress PE, the TTL value of MPLS is decreased from 255, and the ip ttl value remains unchanged;

2. On the egress PE, the TTL value of MPLS disappears and the ip ttl does not change;

3. On the entry PE, the ip ttl value changes, minus one.

This article is from the "Man Yu Tian" blog, please be sure to keep this source http://nanjingzhonglei.blog.51cto.com/5890022/1294804