Mpls ip ttlexpiration pop Command

One command can improve the use of the traceroute tool, at least in some cases. The command mpls ip ttl-expiration pop labels can make a difference. The value of labels is between 1 and 6. This command lets you control the behavior when an ICMP TTL expired message is generated. The default behaviour is to forward the ICMP message along the original LSP. With this command, you can specify the behavior when the TTL of a labeled packet expires, according to the number of labels in the label stack. If the configured number of labels is not exceeded, the default behavior is overridden. In that case, the generated ICMP message is forwarded after looking up the source IP address in the global routing table. If the configured number of labels is exceeded, the default forwarding behaviour is used. As an example, take a plain IPv4-over-MPLS network. This means that MPLS is enabled for all IGP prefixes in the global routing table. Traffic has only one label. If you perform a traceroute and configure the command mpls ip ttl-expiration pop 1 on all the LSRs, all generated ICMP messages are sent back directly to the source of the original packet.

Look at Figure 13-8. It shows the first probe of a traceroute from the left PE in a simple MPLS network. The command mpls ip ttl-expiration pop 1 is configured on all LSRs. The ICMP message is sent directly to the router that initiates the traceroute, if the packet has only one label. The probe has only one label because the destination IP address is in the global routing table.

Figure 13-8 Tracerouting in an MPLS Network: mpls ip ttl-expiration pop Command

Probe 1

london-ce CE

IGP Label

UDP

Dest IP Addr =

10.200.254.4

Source IP Addr

= 10.200.200.1

TTL = 1

Dest UDP Port

Loopback 0 10.1.2.1

10.200.203.2

ICMP

Loopback 0 10.1.2.1

10.200.203.2

IGP Label

ICMP

Dest IP Addr = 10.200.200.1 Source IP Addr = 10.200.200.2 TTL = 255 TTL Exceeded

The egress PE router receives packets with only one label with the default penultimate hop popping (PHP) behaviour in MPLS VPN networks. If the command mpls ip ttl-expiration pop 1 is configured on that PE router and a packet is received with its TTL expiring, the label is removed, and the IP lookup happens in the correct VRF table (the VRF table that is associated with the VPN label) to return the packet immediately.

Micro Expression Master

Micro Expression Master

If You Could Read Everyone Life A Book You Can Have Better Career, Great Relationships And Become Successful. This Book Is One Of The Most Valuable Resources In The World When It Comes To Reading the smallest and tiniest body Language and know what people are thinking about.

Get My Free Ebook


Post a comment